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Energy and Metabolism

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Abstract

Although firmly grounded in metabolic biochemistry, the study of energy metabolism has gone well beyond this discipline and become integrative and comparative as well as ecological and evolutionary in scope. At the cellular level, ATP is hydrolyzed by energy‐expending processes and resynthesized by pathways in bioenergetics. A significant development in the study of bioenergetics is the realization that fluxes through pathways as well as metabolic rates in cells, tissues, organs, and whole organisms are “system properties.” Therefore, studies of energy metabolism have become, increasingly, experiments in systems biology. A significant challenge continues to be the integration of phenomena over multiple levels of organization. Body mass and temperature are said to account for most of the variation in metabolic rates found in nature. A mechanistic foundation for the understanding of these patterns is outlined. It is emphasized that evolution, leading to adaptation to diverse lifestyles and environments, has resulted in a tremendous amount of deviation from popularly accepted scaling “rules.” This is especially so in the deep sea which constitutes most of the biosphere. © 2012 American Physiological Society. Compr Physiol 2:2527‐2540, 2012.

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Figure 1. Figure 1.

The transport of fuels and O2 to cells at various rates of energy expenditure [from Weibel , with permission]. The illustration emphasizes how, at basal metabolic rate (BMR), ATP hydrolysis by processes such as biosynthesis and ion transport mainly determine the rate of whole‐body energy metabolism. Physical activity results in increasing contribution to ATP hydrolysis by muscle work such that, at maximum metabolic rate (MMR or o2max), 90% or more of whole‐body metabolic rate is due to the respiration of muscle mitochondria. It is proposed that, at BMR, energy expenditure dominates the control of whole‐body o2 while, at MMR, processes involved in the delivery of fuels and/or O2 to cells contribute to the control of o2max.

Figure 2. Figure 2.

Flux control coefficients of ATP turnover (solid line), substrate oxidation (thick dashed line), and proton leak (thin dashed line). Results from top‐down control analysis show how contributions to control change as the system approaches 100% of state 3 respiration rate. Adapted, with permission, from Suarez and Darveau ; redrawn, with permission, from Brand et al. .

Figure 3. Figure 3.

Pathways of glucose (left) and fatty acid oxidation (right) in vertebrate muscles, highly simplified, and redrawn, with permission, from Suarez et al. . Readers are encouraged to consult a biochemistry textbook for details. Diagram shows the role played by the malate‐aspartate shuttle in maintaining high cytoplasmic [NAD+]/[NADH] during the oxidation of glucose. Abbreviations: HK, hexokinase; G6P, glucose 6‐phosphate; GAP, glyceraldehyde 3‐phosphate; 1,3 DPG, 1,3‐diphosphoglycerate; NAD+, nicotinamide adenine dinucleotide (oxidized); NADH, nicotinamide adenine dinucleotide (reduced); Oxa, oxaloacetate; Mal, malate; 2‐KGA, 2‐ketoglutarate; Glu, glutamate; Asp, aspartate; CPT, carnitine palmitoyltransferase; CT, carnitine acyltranslocase.

Figure 4. Figure 4.

Carbohydrate oxidation in bee flight muscles, also highly simplified, from Suarez et al. . Readers are encouraged to consult a biochemistry textbook for details. Diagram highlights the roles played by fat body, hemolymph, cytoplasmic, and mitochondrial reactions and shows the main routes of carbon flow, the role played by the glycerol 3‐phosphate (G3P) shuttle in maintaining high cytoplasmic [NAD+]/[NADH] during the oxidation of glucose, and anaplerotic (pyruvate carboxylase and proline oxidation) reactions. Abbreviations: Pi, inorganic phosphate; G6P, glucose 6‐phosphate; F6P, fructose 6‐phosphate; FDP, fructose 1,6‐diphosphate; DHAP, dihydroxyacetone phosphate; GAP, glyceraldehyde 3‐phosphate; pyr, pyruvate; G3P, glycerol 3‐phosphate; H, reducing equivalent; OXA, oxaloacetate; CIT, citrate; αKG, alpha‐ketoglutarate; glut, glutamate; PROL, proline; CYTO, cytoplasmic compartment; MITO, mitochondrial matrix.

Figure 5. Figure 5.

Metabolic rate ( o2) as a function of body mass in rodents, denoted by various symbols (left to right: hazel mouse, edible dormouse, ground squirrel, European hamster, European hedgehog, and alpine marmot). Solid line shows that basal metabolic rate (BMR) scales allometrically during euthermia while dashed line shows how, during hibernation, metabolic rate scales isometrically. Euthermic and hibernating metabolic rates differ due to changes in the rates of energy expenditure in internal organs. Adapted, with permission, from Suarez and Darveau ; redrawn, with permission, from Singer et al. .



Figure 1.

The transport of fuels and O2 to cells at various rates of energy expenditure [from Weibel , with permission]. The illustration emphasizes how, at basal metabolic rate (BMR), ATP hydrolysis by processes such as biosynthesis and ion transport mainly determine the rate of whole‐body energy metabolism. Physical activity results in increasing contribution to ATP hydrolysis by muscle work such that, at maximum metabolic rate (MMR or o2max), 90% or more of whole‐body metabolic rate is due to the respiration of muscle mitochondria. It is proposed that, at BMR, energy expenditure dominates the control of whole‐body o2 while, at MMR, processes involved in the delivery of fuels and/or O2 to cells contribute to the control of o2max.



Figure 2.

Flux control coefficients of ATP turnover (solid line), substrate oxidation (thick dashed line), and proton leak (thin dashed line). Results from top‐down control analysis show how contributions to control change as the system approaches 100% of state 3 respiration rate. Adapted, with permission, from Suarez and Darveau ; redrawn, with permission, from Brand et al. .



Figure 3.

Pathways of glucose (left) and fatty acid oxidation (right) in vertebrate muscles, highly simplified, and redrawn, with permission, from Suarez et al. . Readers are encouraged to consult a biochemistry textbook for details. Diagram shows the role played by the malate‐aspartate shuttle in maintaining high cytoplasmic [NAD+]/[NADH] during the oxidation of glucose. Abbreviations: HK, hexokinase; G6P, glucose 6‐phosphate; GAP, glyceraldehyde 3‐phosphate; 1,3 DPG, 1,3‐diphosphoglycerate; NAD+, nicotinamide adenine dinucleotide (oxidized); NADH, nicotinamide adenine dinucleotide (reduced); Oxa, oxaloacetate; Mal, malate; 2‐KGA, 2‐ketoglutarate; Glu, glutamate; Asp, aspartate; CPT, carnitine palmitoyltransferase; CT, carnitine acyltranslocase.



Figure 4.

Carbohydrate oxidation in bee flight muscles, also highly simplified, from Suarez et al. . Readers are encouraged to consult a biochemistry textbook for details. Diagram highlights the roles played by fat body, hemolymph, cytoplasmic, and mitochondrial reactions and shows the main routes of carbon flow, the role played by the glycerol 3‐phosphate (G3P) shuttle in maintaining high cytoplasmic [NAD+]/[NADH] during the oxidation of glucose, and anaplerotic (pyruvate carboxylase and proline oxidation) reactions. Abbreviations: Pi, inorganic phosphate; G6P, glucose 6‐phosphate; F6P, fructose 6‐phosphate; FDP, fructose 1,6‐diphosphate; DHAP, dihydroxyacetone phosphate; GAP, glyceraldehyde 3‐phosphate; pyr, pyruvate; G3P, glycerol 3‐phosphate; H, reducing equivalent; OXA, oxaloacetate; CIT, citrate; αKG, alpha‐ketoglutarate; glut, glutamate; PROL, proline; CYTO, cytoplasmic compartment; MITO, mitochondrial matrix.



Figure 5.

Metabolic rate ( o2) as a function of body mass in rodents, denoted by various symbols (left to right: hazel mouse, edible dormouse, ground squirrel, European hamster, European hedgehog, and alpine marmot). Solid line shows that basal metabolic rate (BMR) scales allometrically during euthermia while dashed line shows how, during hibernation, metabolic rate scales isometrically. Euthermic and hibernating metabolic rates differ due to changes in the rates of energy expenditure in internal organs. Adapted, with permission, from Suarez and Darveau ; redrawn, with permission, from Singer et al. .

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How to Cite

Raul K. Suarez. Energy and Metabolism. Compr Physiol 2012, 2: 2527-2540. doi: 10.1002/cphy.c110009