Journal of Human Evolution xxx (2014) 1e9
Contents lists available at ScienceDirect
Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol
Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example Shu Pei Wang a, Hao Yang a, b, Jiang Wei Wu a, Nicolas Gauthier a, Toshiyuki Fukao c, Grant A. Mitchell a, * ^te Sainte-Catherine, Montreal H3T 1C5, QC, Division of Medical Genetics, Department of Pediatrics, Universit e de Montr eal and CHU Sainte-Justine, 3175 Co Canada b Laboratory of Animal Fat Deposition and Muscle Development, College of Animal Science and Technology, Northwest A&F University, Yangling 712100, Shaanxi, China c Department of Pediatrics, Gifu University School of Medicine, Gifu 500, Japan a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2013 Accepted 18 June 2014 Available online xxx
Genes and the environment both influence the metabolic processes that determine fitness. To illustrate the importance of metabolism for human brain evolution and health, we use the example of lipid energy metabolism, i.e. the use of fat (lipid) to produce energy and the advantages that this metabolic pathway provides for the brain during environmental energy shortage. We briefly describe some features of metabolism in ancestral organisms, which provided a molecular toolkit for later development. In modern humans, lipid energy metabolism is a regulated multi-organ pathway that links triglycerides in fat tissue to the mitochondria of many tissues including the brain. Three important control points are each suppressed by insulin. (1) Lipid reserves in adipose tissue are released by lipolysis during fasting and stress, producing fatty acids (FAs) which circulate in the blood and are taken up by cells. (2) FA oxidation. Mitochondrial entry is controlled by carnitine palmitoyl transferase 1 (CPT1). Inside the mitochondria, FAs undergo beta oxidation and energy production in the Krebs cycle and respiratory chain. (3) In liver mitochondria, the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) pathway produces ketone bodies for the brain and other organs. Unlike most tissues, the brain does not capture and metabolize circulating FAs for energy production. However, the brain can use ketone bodies for energy. We discuss two examples of genetic metabolic traits that may be advantageous under most conditions but deleterious in others. (1) A CPT1A variant prevalent in Inuit people may allow increased FA oxidation under nonfasting conditions but also predispose to hypoglycemic episodes. (2) The thrifty genotype theory, which holds that energy expenditure is efficient so as to maximize energy stores, predicts that these adaptations may enhance survival in periods of famine but predispose to obesity in modern dietary environments. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Triglycerides Fatty acids Ketone bodies Brain evolution Thrifty genotype Inuit Carnitine palmitoyl transferase
Introduction Biochemistry provides a link between two pillars of current studies of evolution, the genome and studies of the environmental conditions under which humans and their ancestors lived and evolved. We illustrate this using the model of lipid energy metabolism, the process by which energy is derived from fat stores.
Abbreviations: AcAc, acetoacetate; CoA, coenzyme A; CPT, carnitine palmitoyl transferase; HMG, 3-hydroxy-3-methylglutaric acid; mHS, mitochondrial HMG-CoA synthase; SCOT, succinyl-CoA: 3-oxoacid transferase; TG, triglycerides. * Corresponding author. E-mail address: [email protected] (G.A. Mitchell).
Energy metabolism is important for human brain evolution: the adult human brain weighs only 2% of body mass but accounts for 20% of whole body resting energy expenditure; in human newborns, the corresponding figures are ~11% of body weight and >50% of energy consumption (Kennedy and Sokoloff, 1957; Sokoloff, 1989). Energy use is thought to be an important determinant of brain size (Laughlin and Sejnowski, 2003). Mammals have developed complex strategies for storing and releasing energy. After fasting longer than a few hours, mammals derive energy principally from stores of fat. Lipid energy metabolism involves white adipose tissue, which stores and releases fatty acids, and fatty acid metabolizing tissues, which use fatty acids or their byproducts as fuel.
http://dx.doi.org/10.1016/j.jhevol.2014.06.013 0047-2484/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, S.P., et al., Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example, Journal of Human Evolution (2014), http://dx.doi.org/10.1016/j.jhevol.2014.06.013
2
S.P. Wang et al. / Journal of Human Evolution xxx (2014) 1e9
We begin this article with discussion of ancient metabolic pathways. Then we describe the general structure and control of lipid energy metabolism in humans today and how current metabolic pathways may have been pieced together from the prehistoric genomes. Finally, we note some examples of gene-environment combinations of current interest, which may be advantageous or cause disease depending upon environmental conditions, suggesting that the refinement of lipid energy metabolism is an ongoing process. Ancient metabolism: the wellsprings of life Oxygen, lipids and energy metabolism Fascinating hypotheses have been advanced concerning (1) how organic compounds and chemical reactions resembling those used in metabolic pathways of modern humans may have arisen on earth before cellular life emerged (Bada and Lazcano, 2002; MelendezHevia et al., 2008; Fani and Fondi, 2009; Brown, 2012; Cleaves et al., 2012; Danger et al., 2012) and (2) how the first lipid-like molecules may have arisen (Segre et al., 2001). These subjects are beyond the scope of this review. The first cells did not use oxygen, i.e., they were anaerobic. Most anaerobic energy metabolism proceeds by partial degradation of molecules (fermentation), accompanied by the production of small amounts of energy and the release of the partially degraded, energy-rich molecules from the cell. One such metabolic pathway, that is found in nearly all organisms, is the partial degradation of glucose to pyruvate (glycolysis) in the cytoplasm, with two branches leading from pyruvate (Fig. 1). These core reactions permit energy production and provide many of the substrates for synthesis of amino acids and other essential compounds (Bada and Lazcano, 2002; Melendez-Hevia et al., 2008; Cleaves et al., 2012; Danger et al., 2012). In aerobic organisms, oxygen is consumed to produce energy. Oxygen, produced by photosynthesis in plants, began to accumulate in the atmosphere about 500 million years ago (Gould, 1994). Oxidative metabolism offered a new, much more efficient way than simple glycolysis to obtain energy from food. Fatty acids are degraded by oxidation. The oxidation of fatty acids can be viewed in three steps, beta oxidation, the Krebs cycle and the respiratory chain. Fatty acids are long molecules. Each cycle of beta oxidation shortens the fatty acid and produces a molecule of acetyl-coenzyme A (CoA), plus energy-rich reduced nucleotides. The shortened fatty acyl-CoA that is produced can undergo further beta oxidation. Eventually most fatty acids are completely converted to acetyl-CoA. In the Krebs cycle, acetyl-CoA enters carbon dioxide is released (Fig. 2a). In contrast to glycolysis, the Krebs cycle is located in the
Glycerol-P Glycine Ribose
ATP
Glutamate Porphyrins AcCoA
Glucose
Glutamine
FAs
Serine
Glycolysis 1
Pyruvate
succinyl-CoA citrate 2-KG [H] [H] [H] [H]
[H]
oxaloacetate
( )
succinate
2
Figure 1. The core pathways of metabolism. The earliest cells are thought to have possessed a central core of anaerobic metabolism from which limited amounts of energy were derived and that furnished the precursors for synthesizing essential cell components such as ribonucleic acids, amino acids, FAs, porphyrins and reducing equivalents (here indicated as [H]). The Krebs cycle can be created by the addition of a single reaction (succinyl coenzyme A synthetase, shown between parentheses on the far right of the figure). Dashed lines indicate segments composed of more than one enzymatic reaction. Based on (Melendez-Hevia et al., 2008). Numbers in circles: 1 ¼ glycolysis; 2 ¼ Krebs cycle-related reactions.
mitochondrion. The Krebs cycle could have arisen by the acquisition of a single enzyme, by which the end compounds of the two branches of the pathways of metabolism from pyruvate were connected (e.g., Melendez-Hevia et al., 2008), thus creating a cycle (Fig. 1). In the Krebs cycle, nucleotides such as nicotinamide adenine dinucleotide (NAD) undergo reduction and their reduced forms (e.g., NADH) are rich in energy. The third step is terminal oxidation by the electron transport chain (also known as the respiratory chain). The electron transport chain is a marvel of complexity (DiMauro and Schon, 2003; Vafai and Mootha, 2012). Situated in the inner membrane of mitochondria, it receives high energy hydrogen atoms from NADH and from reduced flavin nucleotides. These high energy compounds are produced mainly by the Krebs cycle and by beta oxidation of fatty acids. In the course of oxidative metabolism, the hydrogen atom is split into its component proton and electron. The chemical energy derived from the sequential reactions along the electron transport chain is used to expel protons from the matrix space inside the mitochondrion. Three of the enzyme complexes of the electron transport chain, complexes I, III and IV, each expel protons from the matrix. This creates a proton (pH) gradient between the mitochondrial matrix and the extramitochondrial space, with a lower concentration of protons (higher pH) inside the mitochondrial matrix than outside. Complex V of the electron transport chain harnesses the proton gradient, and couples the reentry of protons into the mitochondrial matrix to the synthesis of adenosine triphosphate (ATP). ATP is the main fuel molecule of the cell. There is great interest in understanding how this elaborate system evolved from simpler elements (Albert et al., 2002; Muller and Gruber, 2003; Vafai and Mootha, 2012) but we will not pursue the question further in this review. Mitochondria: the cellular furnace Mitochondria are thought to be descendants of an aerobic monocellular organism that was engulfed by another cell but that survived. This relationship between two organisms, or endosymbiosis (Margulis and Chapman, 1998; Dyall et al., 2004), would have provided the host cell with the major advantage of deriving over ten-fold more energy from available fuels than from glycolysis alone. Arguably, endosymbiosis was one of the most important events in eukaryotic evolution. Mitochondria retain many general features of prokaryotes, which differ from those of the rest of the cell (Gray, 2012; Vafai and Mootha, 2012). This includes the presence in mitochondria of multiple copies of a small circular chromosome that resembles a bacterial plasmid, of mitochondria-specific systems of DNA replication and RNA synthesis, of a genetic code that differs from that of nuclear DNA, and of distinct mitochondrial protein translation machinery. Also of note, mitochondria are bounded by two membranes, an outer membrane in which the lipid content is similar to that of other eukaryotic cell membranes, and an inner membrane with a distinct lipid composition and a specific set of mitochondrial transporters that ensure efficient movement of proteins and metabolites. Thus, the earliest cells evolved a system for extracting a high fraction of the chemical energy of foods for the synthesis of the basic chemicals of life (Figs. 1 and 2a). This system provided the framework on which energy metabolism in multicellular organisms was constructed. The mechanisms of metabolic evolution Metabolic evolution has occurred by several mechanisms, leading to the metabolic state of today's higher organisms including humans. Lipid energy metabolism provides examples of at least
Please cite this article in press as: Wang, S.P., et al., Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example, Journal of Human Evolution (2014), http://dx.doi.org/10.1016/j.jhevol.2014.06.013
S.P. Wang et al. / Journal of Human Evolution xxx (2014) 1e9
a (Glucose
1
Pyr)
AcCoA
2
AcCoA
[H]
Energy
(Leucine) AcCoA
CoA
HMG-CoA
5 CoA
3
3
7?
AcAc 8
(Isoprenoids)
FA-CoA
4
AcAcCoA
AcAc
6 Succ
Succ-CoA
b
Figure 2. Lipid energy metabolism (a) in early prokaryotic cells and (b) in humans. Glycolysis, the Krebs cycle, the respiratory chain and FA beta oxidation are shown. (a) Hypothetical early energy metabolism. SCOT may have been present, and would have provided a means to cleave excess acetoacetyl-CoA or a similar acetyl-CoA, allowing it to exit from the cell. If the cell was not partitioned, as is the case in most prokaryotic cells, it is possible that HMG-CoA synthase may potentially have allowed for cholesterol synthesis and for ketogenesis. In this scenario, ketogenesis can be seen as a fermentation by which excess short chain products of FA degradation are released in order to free coenzyme A for other tasks. Elements not involved in lipid energy metabolism are shown in parentheses. Numbers in circles: 1 ¼ glycolysis; 2 ¼ Krebs cycle; 3 ¼ respiratory chain; 4 ¼ beta oxidation of fatty acids; 5 ¼ short chain 3-ketothiolase; 6 ¼ SCOT; 7 ¼ HMG-CoA synthase; 8 ¼ HMG-CoA lyase. (b) Human lipid energy metabolism. Human lipid energy metabolism is distributed among several specialized organs. TGs are the source of fuel. They are synthesized and stored in adipose tissue (Ad). Other sources of TGs are the gut, in which absorbed FAs are synthesized to TGs and exported as chylomicrons (CHYL) and the liver, in which cytoplasmic FA-CoAs are used to synthesize TGs that are exported as very low density lipoprotein (VLDL). In the liver and most tissues other than the brain, FAs are internalized and degraded as follows: activation by esterification with CoA, import to mitochondria via the carnitine shuttle, and oxidation to acetyl-CoA by beta oxidation (b-Ox). A branch of this pathway, that in adult mammals is essentially confined to the liver, is the HMG-CoA pathway of ketogenesis. In the liver, SCOT activity is suppressed and ketogenesis proceeds via mitochondrial HMG-CoA synthase. The following reaction, HMG-CoA lyase, is irreversible, conferring directionality to ketogenesis in liver. HMG-CoA lyase is multifunctional, serving also in leucine catabolism. Finally, extrahepatic organs, including the brain, can take up ketone bodies and use them for energy or synthesis. Numbers in circles: 1e8 as in Fig. 2aeb; 9 ¼ 3-hydroxybutyrate dehydrogenase; 10 ¼ the carnitine shuttle (CPT1 ¼ black square; carnitine/acylcarnitine transporter ¼ gray circle; CPT2 ¼ gray square); 11 ¼ cell membrane FA entry and exit; 12 ¼ adipocyte lipolysis; 13 ¼ intravascular hydrolysis of TGs in lipoproteins by lipoprotein lipase; 14 ¼ cytoplasmic TG synthesis.
four such mechanisms. First, specific steps in energy metabolism became concentrated in specialized tissues, such as liver and adipose tissue. Second, hormonal and neurological control of cell functions developed, which alter gene expression and the concentrations of signaling metabolites. A third mechanism is gene duplication. One copy of a gene suffices to carry out the original function. Following a gene duplication event, random mutation in one copy may cause inactivation or rarely, may confer new properties to the gene product. A fourth mechanism is multifunctionality, i.e., the use of an existing protein to play a new role while it continues to carry out its original function. Energy physiology in fed and fasting humans Before modern times, periods of low food availability occurred frequently. The development of a system for storing energy and for releasing it in a controlled fashion would have major benefit for survival. In humans, this role is assumed by lipid energy metabolism. If food supply is interrupted for more than a few hours, endogenous fatty acids progressively become the most important source of body energy. . The term “energy homeostasis” refers to the provision of sufficient energy to tissues under all nutritional states, varying from overabundance of available energy after eating to a complete lack of
exogenous energy during fasting. Energy homeostasis is achieved by efficiently integrating the metabolism of fatty acids, carbohydrates and proteins (Cahill, 2006). It is conceptually useful to consider energy production from fatty acids, carbohydrates and proteins separately, but they are interrelated, and none is completely extinguished when another is activated. Lipid energy metabolism in humans is highly regulated and is distributed among several organs (Fig. 2b). In the hours following a meal, some plasma fatty acids are derived from the action of lipoprotein lipase, an enzyme found on the inner surface of blood vessels. It cleaves fatty acids from triglycerides within the lipoproteins that circulate in the blood after eating (Figs. 2b and 3). The resulting nonesterified (‘free’) fatty acids are then taken up by many tissues for oxidation in their mitochondria. In white adipose tissue, fatty acids are used to synthesize triglycerides. During prolonged fasting, fatty acid degradation is active (Fig. 3). Glucose is in short supply. This reduces insulin secretion, which in turn activates the degradation of triglycerides to fatty acids (lipolysis). During fasting, fatty acids that are liberated by lipolysis enter the circulation. They are taken up by cells and degraded to provide energy. In liver mitochondria, an additional process is fueled by fatty acids: ketone body synthesis (ketogenesis). Ketone bodies diffuse from the liver for use by other tissues. From estimates of the
Please cite this article in press as: Wang, S.P., et al., Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example, Journal of Human Evolution (2014), http://dx.doi.org/10.1016/j.jhevol.2014.06.013
4
S.P. Wang et al. / Journal of Human Evolution xxx (2014) 1e9
TG synthesis
Lipolysis
Lipid Droplet TG Storage
TG
TG DGAT CGI-58
PLIN-1
DG HSL
PL PLIN-1
MGAT
GPAT
ATGL
ATGL
FA-CoA
FA-CoA
PLIN-1
MG MGL
FA-CoA FA
CoA
HSL
PLIN-1 PLIN-1
MGL (+ HSL?) FA
Glucose
Glucose
FA
LPL
TG
Glycerol FA
Figure 3. Cytoplasmic triglyceride metabolism in adipocytes. This can be divided into three phases, shown from left to right. First, TG synthesis in the endoplasmic reticulum begins with glycerol-3-phosphate, a byproduct of glycolysis, and with FAs derived from plasma lipoproteins (LPL is shown) or from endogenous synthesis (not shown). These FAs are esterified to CoA. Specific enzymes add the first, second and third FA molecules as shown. Phospholipids (PL), used for signaling purposes and for membrane structure (not shown), share the first part of this synthetic pathway. The synthesis of TGs can be viewed as a branch pathway of phospholipid metabolism. Second, TGs are transported to and stored in the lipid droplet. The lipid droplet is coated by a phospholipid monolayer in which surface proteins are embedded. In mature adipocytes, the main lipid droplet surface protein is perilipin (PLIN-1); for simplicity it is the only one of this class shown. During fasting or adrenergic stress, lipolysis is activated and TGs are sequentially cleaved, first by adipose triglyceride lipase (ATGL) in the presence of its coactivator, CGI-58, then by hormone-sensitive lipase (HSL) and finally by monoglyceride lipase (MGL). The end products are FAs and glycerol, both of which are exported and circulate to other tissues. Specific cell membrane transporters are shown by different geometrical shapes.
maximal rate of ketogenesis (Flatt, 1972; Garber et al., 1974; Reichard et al., 1974), it has been calculated that ketones can potentially furnish nearly half of basal energy requirements in humans during prolonged fasting or starvation (Mitchell and Fukao, 2001). Plasma ketone body concentrations in adult humans range from
Contents lists available at ScienceDirect
Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol
Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example Shu Pei Wang a, Hao Yang a, b, Jiang Wei Wu a, Nicolas Gauthier a, Toshiyuki Fukao c, Grant A. Mitchell a, * ^te Sainte-Catherine, Montreal H3T 1C5, QC, Division of Medical Genetics, Department of Pediatrics, Universit e de Montr eal and CHU Sainte-Justine, 3175 Co Canada b Laboratory of Animal Fat Deposition and Muscle Development, College of Animal Science and Technology, Northwest A&F University, Yangling 712100, Shaanxi, China c Department of Pediatrics, Gifu University School of Medicine, Gifu 500, Japan a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2013 Accepted 18 June 2014 Available online xxx
Genes and the environment both influence the metabolic processes that determine fitness. To illustrate the importance of metabolism for human brain evolution and health, we use the example of lipid energy metabolism, i.e. the use of fat (lipid) to produce energy and the advantages that this metabolic pathway provides for the brain during environmental energy shortage. We briefly describe some features of metabolism in ancestral organisms, which provided a molecular toolkit for later development. In modern humans, lipid energy metabolism is a regulated multi-organ pathway that links triglycerides in fat tissue to the mitochondria of many tissues including the brain. Three important control points are each suppressed by insulin. (1) Lipid reserves in adipose tissue are released by lipolysis during fasting and stress, producing fatty acids (FAs) which circulate in the blood and are taken up by cells. (2) FA oxidation. Mitochondrial entry is controlled by carnitine palmitoyl transferase 1 (CPT1). Inside the mitochondria, FAs undergo beta oxidation and energy production in the Krebs cycle and respiratory chain. (3) In liver mitochondria, the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) pathway produces ketone bodies for the brain and other organs. Unlike most tissues, the brain does not capture and metabolize circulating FAs for energy production. However, the brain can use ketone bodies for energy. We discuss two examples of genetic metabolic traits that may be advantageous under most conditions but deleterious in others. (1) A CPT1A variant prevalent in Inuit people may allow increased FA oxidation under nonfasting conditions but also predispose to hypoglycemic episodes. (2) The thrifty genotype theory, which holds that energy expenditure is efficient so as to maximize energy stores, predicts that these adaptations may enhance survival in periods of famine but predispose to obesity in modern dietary environments. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Triglycerides Fatty acids Ketone bodies Brain evolution Thrifty genotype Inuit Carnitine palmitoyl transferase
Introduction Biochemistry provides a link between two pillars of current studies of evolution, the genome and studies of the environmental conditions under which humans and their ancestors lived and evolved. We illustrate this using the model of lipid energy metabolism, the process by which energy is derived from fat stores.
Abbreviations: AcAc, acetoacetate; CoA, coenzyme A; CPT, carnitine palmitoyl transferase; HMG, 3-hydroxy-3-methylglutaric acid; mHS, mitochondrial HMG-CoA synthase; SCOT, succinyl-CoA: 3-oxoacid transferase; TG, triglycerides. * Corresponding author. E-mail address: [email protected] (G.A. Mitchell).
Energy metabolism is important for human brain evolution: the adult human brain weighs only 2% of body mass but accounts for 20% of whole body resting energy expenditure; in human newborns, the corresponding figures are ~11% of body weight and >50% of energy consumption (Kennedy and Sokoloff, 1957; Sokoloff, 1989). Energy use is thought to be an important determinant of brain size (Laughlin and Sejnowski, 2003). Mammals have developed complex strategies for storing and releasing energy. After fasting longer than a few hours, mammals derive energy principally from stores of fat. Lipid energy metabolism involves white adipose tissue, which stores and releases fatty acids, and fatty acid metabolizing tissues, which use fatty acids or their byproducts as fuel.
http://dx.doi.org/10.1016/j.jhevol.2014.06.013 0047-2484/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, S.P., et al., Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example, Journal of Human Evolution (2014), http://dx.doi.org/10.1016/j.jhevol.2014.06.013
2
S.P. Wang et al. / Journal of Human Evolution xxx (2014) 1e9
We begin this article with discussion of ancient metabolic pathways. Then we describe the general structure and control of lipid energy metabolism in humans today and how current metabolic pathways may have been pieced together from the prehistoric genomes. Finally, we note some examples of gene-environment combinations of current interest, which may be advantageous or cause disease depending upon environmental conditions, suggesting that the refinement of lipid energy metabolism is an ongoing process. Ancient metabolism: the wellsprings of life Oxygen, lipids and energy metabolism Fascinating hypotheses have been advanced concerning (1) how organic compounds and chemical reactions resembling those used in metabolic pathways of modern humans may have arisen on earth before cellular life emerged (Bada and Lazcano, 2002; MelendezHevia et al., 2008; Fani and Fondi, 2009; Brown, 2012; Cleaves et al., 2012; Danger et al., 2012) and (2) how the first lipid-like molecules may have arisen (Segre et al., 2001). These subjects are beyond the scope of this review. The first cells did not use oxygen, i.e., they were anaerobic. Most anaerobic energy metabolism proceeds by partial degradation of molecules (fermentation), accompanied by the production of small amounts of energy and the release of the partially degraded, energy-rich molecules from the cell. One such metabolic pathway, that is found in nearly all organisms, is the partial degradation of glucose to pyruvate (glycolysis) in the cytoplasm, with two branches leading from pyruvate (Fig. 1). These core reactions permit energy production and provide many of the substrates for synthesis of amino acids and other essential compounds (Bada and Lazcano, 2002; Melendez-Hevia et al., 2008; Cleaves et al., 2012; Danger et al., 2012). In aerobic organisms, oxygen is consumed to produce energy. Oxygen, produced by photosynthesis in plants, began to accumulate in the atmosphere about 500 million years ago (Gould, 1994). Oxidative metabolism offered a new, much more efficient way than simple glycolysis to obtain energy from food. Fatty acids are degraded by oxidation. The oxidation of fatty acids can be viewed in three steps, beta oxidation, the Krebs cycle and the respiratory chain. Fatty acids are long molecules. Each cycle of beta oxidation shortens the fatty acid and produces a molecule of acetyl-coenzyme A (CoA), plus energy-rich reduced nucleotides. The shortened fatty acyl-CoA that is produced can undergo further beta oxidation. Eventually most fatty acids are completely converted to acetyl-CoA. In the Krebs cycle, acetyl-CoA enters carbon dioxide is released (Fig. 2a). In contrast to glycolysis, the Krebs cycle is located in the
Glycerol-P Glycine Ribose
ATP
Glutamate Porphyrins AcCoA
Glucose
Glutamine
FAs
Serine
Glycolysis 1
Pyruvate
succinyl-CoA citrate 2-KG [H] [H] [H] [H]
[H]
oxaloacetate
( )
succinate
2
Figure 1. The core pathways of metabolism. The earliest cells are thought to have possessed a central core of anaerobic metabolism from which limited amounts of energy were derived and that furnished the precursors for synthesizing essential cell components such as ribonucleic acids, amino acids, FAs, porphyrins and reducing equivalents (here indicated as [H]). The Krebs cycle can be created by the addition of a single reaction (succinyl coenzyme A synthetase, shown between parentheses on the far right of the figure). Dashed lines indicate segments composed of more than one enzymatic reaction. Based on (Melendez-Hevia et al., 2008). Numbers in circles: 1 ¼ glycolysis; 2 ¼ Krebs cycle-related reactions.
mitochondrion. The Krebs cycle could have arisen by the acquisition of a single enzyme, by which the end compounds of the two branches of the pathways of metabolism from pyruvate were connected (e.g., Melendez-Hevia et al., 2008), thus creating a cycle (Fig. 1). In the Krebs cycle, nucleotides such as nicotinamide adenine dinucleotide (NAD) undergo reduction and their reduced forms (e.g., NADH) are rich in energy. The third step is terminal oxidation by the electron transport chain (also known as the respiratory chain). The electron transport chain is a marvel of complexity (DiMauro and Schon, 2003; Vafai and Mootha, 2012). Situated in the inner membrane of mitochondria, it receives high energy hydrogen atoms from NADH and from reduced flavin nucleotides. These high energy compounds are produced mainly by the Krebs cycle and by beta oxidation of fatty acids. In the course of oxidative metabolism, the hydrogen atom is split into its component proton and electron. The chemical energy derived from the sequential reactions along the electron transport chain is used to expel protons from the matrix space inside the mitochondrion. Three of the enzyme complexes of the electron transport chain, complexes I, III and IV, each expel protons from the matrix. This creates a proton (pH) gradient between the mitochondrial matrix and the extramitochondrial space, with a lower concentration of protons (higher pH) inside the mitochondrial matrix than outside. Complex V of the electron transport chain harnesses the proton gradient, and couples the reentry of protons into the mitochondrial matrix to the synthesis of adenosine triphosphate (ATP). ATP is the main fuel molecule of the cell. There is great interest in understanding how this elaborate system evolved from simpler elements (Albert et al., 2002; Muller and Gruber, 2003; Vafai and Mootha, 2012) but we will not pursue the question further in this review. Mitochondria: the cellular furnace Mitochondria are thought to be descendants of an aerobic monocellular organism that was engulfed by another cell but that survived. This relationship between two organisms, or endosymbiosis (Margulis and Chapman, 1998; Dyall et al., 2004), would have provided the host cell with the major advantage of deriving over ten-fold more energy from available fuels than from glycolysis alone. Arguably, endosymbiosis was one of the most important events in eukaryotic evolution. Mitochondria retain many general features of prokaryotes, which differ from those of the rest of the cell (Gray, 2012; Vafai and Mootha, 2012). This includes the presence in mitochondria of multiple copies of a small circular chromosome that resembles a bacterial plasmid, of mitochondria-specific systems of DNA replication and RNA synthesis, of a genetic code that differs from that of nuclear DNA, and of distinct mitochondrial protein translation machinery. Also of note, mitochondria are bounded by two membranes, an outer membrane in which the lipid content is similar to that of other eukaryotic cell membranes, and an inner membrane with a distinct lipid composition and a specific set of mitochondrial transporters that ensure efficient movement of proteins and metabolites. Thus, the earliest cells evolved a system for extracting a high fraction of the chemical energy of foods for the synthesis of the basic chemicals of life (Figs. 1 and 2a). This system provided the framework on which energy metabolism in multicellular organisms was constructed. The mechanisms of metabolic evolution Metabolic evolution has occurred by several mechanisms, leading to the metabolic state of today's higher organisms including humans. Lipid energy metabolism provides examples of at least
Please cite this article in press as: Wang, S.P., et al., Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example, Journal of Human Evolution (2014), http://dx.doi.org/10.1016/j.jhevol.2014.06.013
S.P. Wang et al. / Journal of Human Evolution xxx (2014) 1e9
a (Glucose
1
Pyr)
AcCoA
2
AcCoA
[H]
Energy
(Leucine) AcCoA
CoA
HMG-CoA
5 CoA
3
3
7?
AcAc 8
(Isoprenoids)
FA-CoA
4
AcAcCoA
AcAc
6 Succ
Succ-CoA
b
Figure 2. Lipid energy metabolism (a) in early prokaryotic cells and (b) in humans. Glycolysis, the Krebs cycle, the respiratory chain and FA beta oxidation are shown. (a) Hypothetical early energy metabolism. SCOT may have been present, and would have provided a means to cleave excess acetoacetyl-CoA or a similar acetyl-CoA, allowing it to exit from the cell. If the cell was not partitioned, as is the case in most prokaryotic cells, it is possible that HMG-CoA synthase may potentially have allowed for cholesterol synthesis and for ketogenesis. In this scenario, ketogenesis can be seen as a fermentation by which excess short chain products of FA degradation are released in order to free coenzyme A for other tasks. Elements not involved in lipid energy metabolism are shown in parentheses. Numbers in circles: 1 ¼ glycolysis; 2 ¼ Krebs cycle; 3 ¼ respiratory chain; 4 ¼ beta oxidation of fatty acids; 5 ¼ short chain 3-ketothiolase; 6 ¼ SCOT; 7 ¼ HMG-CoA synthase; 8 ¼ HMG-CoA lyase. (b) Human lipid energy metabolism. Human lipid energy metabolism is distributed among several specialized organs. TGs are the source of fuel. They are synthesized and stored in adipose tissue (Ad). Other sources of TGs are the gut, in which absorbed FAs are synthesized to TGs and exported as chylomicrons (CHYL) and the liver, in which cytoplasmic FA-CoAs are used to synthesize TGs that are exported as very low density lipoprotein (VLDL). In the liver and most tissues other than the brain, FAs are internalized and degraded as follows: activation by esterification with CoA, import to mitochondria via the carnitine shuttle, and oxidation to acetyl-CoA by beta oxidation (b-Ox). A branch of this pathway, that in adult mammals is essentially confined to the liver, is the HMG-CoA pathway of ketogenesis. In the liver, SCOT activity is suppressed and ketogenesis proceeds via mitochondrial HMG-CoA synthase. The following reaction, HMG-CoA lyase, is irreversible, conferring directionality to ketogenesis in liver. HMG-CoA lyase is multifunctional, serving also in leucine catabolism. Finally, extrahepatic organs, including the brain, can take up ketone bodies and use them for energy or synthesis. Numbers in circles: 1e8 as in Fig. 2aeb; 9 ¼ 3-hydroxybutyrate dehydrogenase; 10 ¼ the carnitine shuttle (CPT1 ¼ black square; carnitine/acylcarnitine transporter ¼ gray circle; CPT2 ¼ gray square); 11 ¼ cell membrane FA entry and exit; 12 ¼ adipocyte lipolysis; 13 ¼ intravascular hydrolysis of TGs in lipoproteins by lipoprotein lipase; 14 ¼ cytoplasmic TG synthesis.
four such mechanisms. First, specific steps in energy metabolism became concentrated in specialized tissues, such as liver and adipose tissue. Second, hormonal and neurological control of cell functions developed, which alter gene expression and the concentrations of signaling metabolites. A third mechanism is gene duplication. One copy of a gene suffices to carry out the original function. Following a gene duplication event, random mutation in one copy may cause inactivation or rarely, may confer new properties to the gene product. A fourth mechanism is multifunctionality, i.e., the use of an existing protein to play a new role while it continues to carry out its original function. Energy physiology in fed and fasting humans Before modern times, periods of low food availability occurred frequently. The development of a system for storing energy and for releasing it in a controlled fashion would have major benefit for survival. In humans, this role is assumed by lipid energy metabolism. If food supply is interrupted for more than a few hours, endogenous fatty acids progressively become the most important source of body energy. . The term “energy homeostasis” refers to the provision of sufficient energy to tissues under all nutritional states, varying from overabundance of available energy after eating to a complete lack of
exogenous energy during fasting. Energy homeostasis is achieved by efficiently integrating the metabolism of fatty acids, carbohydrates and proteins (Cahill, 2006). It is conceptually useful to consider energy production from fatty acids, carbohydrates and proteins separately, but they are interrelated, and none is completely extinguished when another is activated. Lipid energy metabolism in humans is highly regulated and is distributed among several organs (Fig. 2b). In the hours following a meal, some plasma fatty acids are derived from the action of lipoprotein lipase, an enzyme found on the inner surface of blood vessels. It cleaves fatty acids from triglycerides within the lipoproteins that circulate in the blood after eating (Figs. 2b and 3). The resulting nonesterified (‘free’) fatty acids are then taken up by many tissues for oxidation in their mitochondria. In white adipose tissue, fatty acids are used to synthesize triglycerides. During prolonged fasting, fatty acid degradation is active (Fig. 3). Glucose is in short supply. This reduces insulin secretion, which in turn activates the degradation of triglycerides to fatty acids (lipolysis). During fasting, fatty acids that are liberated by lipolysis enter the circulation. They are taken up by cells and degraded to provide energy. In liver mitochondria, an additional process is fueled by fatty acids: ketone body synthesis (ketogenesis). Ketone bodies diffuse from the liver for use by other tissues. From estimates of the
Please cite this article in press as: Wang, S.P., et al., Metabolism as a tool for understanding human brain evolution: Lipid energy metabolism as an example, Journal of Human Evolution (2014), http://dx.doi.org/10.1016/j.jhevol.2014.06.013
4
S.P. Wang et al. / Journal of Human Evolution xxx (2014) 1e9
TG synthesis
Lipolysis
Lipid Droplet TG Storage
TG
TG DGAT CGI-58
PLIN-1
DG HSL
PL PLIN-1
MGAT
GPAT
ATGL
ATGL
FA-CoA
FA-CoA
PLIN-1
MG MGL
FA-CoA FA
CoA
HSL
PLIN-1 PLIN-1
MGL (+ HSL?) FA
Glucose
Glucose
FA
LPL
TG
Glycerol FA
Figure 3. Cytoplasmic triglyceride metabolism in adipocytes. This can be divided into three phases, shown from left to right. First, TG synthesis in the endoplasmic reticulum begins with glycerol-3-phosphate, a byproduct of glycolysis, and with FAs derived from plasma lipoproteins (LPL is shown) or from endogenous synthesis (not shown). These FAs are esterified to CoA. Specific enzymes add the first, second and third FA molecules as shown. Phospholipids (PL), used for signaling purposes and for membrane structure (not shown), share the first part of this synthetic pathway. The synthesis of TGs can be viewed as a branch pathway of phospholipid metabolism. Second, TGs are transported to and stored in the lipid droplet. The lipid droplet is coated by a phospholipid monolayer in which surface proteins are embedded. In mature adipocytes, the main lipid droplet surface protein is perilipin (PLIN-1); for simplicity it is the only one of this class shown. During fasting or adrenergic stress, lipolysis is activated and TGs are sequentially cleaved, first by adipose triglyceride lipase (ATGL) in the presence of its coactivator, CGI-58, then by hormone-sensitive lipase (HSL) and finally by monoglyceride lipase (MGL). The end products are FAs and glycerol, both of which are exported and circulate to other tissues. Specific cell membrane transporters are shown by different geometrical shapes.
maximal rate of ketogenesis (Flatt, 1972; Garber et al., 1974; Reichard et al., 1974), it has been calculated that ketones can potentially furnish nearly half of basal energy requirements in humans during prolonged fasting or starvation (Mitchell and Fukao, 2001). Plasma ketone body concentrations in adult humans range from
