The Great Metabolic Race

Organisms are constantly undergoing various chemical reactions and pathways that enable for them to maintain life. These pathways are part of metabolism, involving catabolism (break down of organic nutrients for extraction of useful) and anabolism (energy dependent conversion of small precursor molecules in complex molecules); some of which are energy coupled to provide energy efficiency. This intermediate coupling is due to the “energy currency” within the body, known as Adenosine Triphosphate (ATP). These metabolic pathways are reliant on certain energies of reactions; according to Gibbs’ free energy (ΔG), referring to the change in usable energy available for a reaction. Many of these cellular pathways involve ΔG, where if ΔG < 0 it is an exergonic reaction, and if ΔG > 0 then it is an endergonic reaction. We are able to observe these various metabolic pathways within the body of an athlete as they perform a 1 hour race. At the very start of the race, muscles in the body are used profusely and the athlete will undergo anaerobic pathways of energy.

Therefore at the beginning of the race, in order to attain a large amount of ATP or energy in the most efficient way, the body perform lipid catabolism via the beta-oxidation pathway. Fats are used as the body does not require energy to be released rapidly until the point when the athlete achieves a relatively stable “metabolic pace.” The production of ATP is quicker from glucose than from fatty acids though, from anaerobic respiration. By 5 minutes once the athlete gets settled into a steady pace, aerobic pathways begin to take over. This aerobic pathway involves the catabolism of glucose, as a fuel. Reliance on glucose is due to the ability to produce a lot of energy in a relatively short time span. The glucose stores in the body of the athlete are located mainly in the liver and muscles; the liver which acts to regulate blood glucose, and muscles for the excess work that is performed which requires a lot of energy. Glucose is stored in the form of the polysaccharide, glycogen, which is a highly branched structure of glucose.

Branching parts are hydrolyzed via glycogenolysis. The glucose that reaches cells for initiation of cellular respiration undergo a series of processes that allow for the production of ATP. The first process of glucose catabolism is the glycolysis phase, involving a preparatory phase and a pay-off phase. The preparatory phase begins with a phosphorylation of a single glucose molecule via hexokinase to form glucose-6-phophate. This initial reaction is both exergonic with a negative ΔG and requires the conversion of ATP into ADP. After this energy coupled reaction that pulls the catabolic process forward, glucose-6-phosphate becomes isomerized by phosphohexokinase isomerase; giving fructose-6-phosphate. This second reaction is endergonic with a positive ΔG, meaning it favors a backward reaction, but the exergonic properties of the following phosphorylation reaction continues to pull this reaction forward and subsequent reactions as well. Phosphofrutokinase-1 (PFK-1) is implemented with the conversion of ATP to ADP, in order to result in Fructose 1,6-biphosphate.

The subsequent step involves aldose enzyme which cleaves the Fructose 1,6-biphosphate into two, glyceraldehyde 3-phosphate, which is the end product of the preparatory phase of glycolysis; and Dihydroxyacetone phosphate, which is isomerized via triose phosphate isomerase to also form glyceraldehyde 3-phosphate. The overall preparatory phase has given the athlete a net loss of 2ATP for one glucose molecule. However pay-off phase begins with the oxidation and phosphorylation of 2 glyceraldehyde 3-phosphate and 2 inorganic phosphate via the enzyme glyceraldehyde 3-phosphate dehydrogenase to form 2 1,3-Biphosphoglycerate; oxidizing NAD+ to form the electron carrier and proton NADH and H+, required for the latter process of the electron transport chain. Substrate level phosphorylation then occurs via phosphoglycerate kinase that produces 2ATP molecules and the two 3-phophoglycerate; under the influence of a strong exergonic reaction.

The phosphate is then transferred and the result is dehydrated to form 2 molecules of phosphoenolpyruvate (PEP) and water. These endergonic reactions are pushed forward by the final extremely exergonic substrate level phosphorylation. As a result of glycolysis the net ATP formed is 2. The fate of the 2 pyruvates at the end of the process is to be transported into the mitochondrion. During the translocation, CO2 is released to convert and oxidize pyruvate to acetate while reducing NAD+ to NADH. Coenzyme A is attached to the acetyl group, forming acetyl CoA’s, which is the unstable compound ready to undergo the Citric Acid Cycle (CAC). CAC involves the addition of two carbon fragments of acetyl CoA to oxaloacetate, forming citrate via an exergonic reaction. This is converted to its isomer, isocitrate, which subsequently loses a CO2 resulting in α-ketoglutarate; reducing NAD+ to NADH and H+.

The following steps involve multi-enzyme processes, converting another NAD+ to NADH and H+, and removing a CO2 as well as attaching back the CoA to form succinyl CoA via an exergonic reaction. Substrate level phosphorylation occurs next and we see the high energy bonds of succinyl CoA become broken; the phosphate being transferred from GDP (guanosine diphosphate) to form GTP (guanosine triphosphate) and succinate and effectively the donated phosphate group to ADP to form ATP. Succinate is then oxidized to fumarate, catalyzed by the enzyme dehydrogenase; effectively reducing FAD to form FADH2. Fumarate is exergonically added to water to form malate; reducing NAD+ to NADH, and is oxidized to regenerate oxaloacetate in order to begin the cycle again. This last reaction is endergonic but the reaction is pulled forward due to the net exergonic nature of the CAC. Effectively two turns of the CAC occur and 2 ATP result through substrate-level phosphorylation. The main ATP output however occurs via the electron carriers of FADH2 and NADH via biological oxidation processes in the Electron Transport Chain (ETC). The ETC in the inner mitochondrial membrane involves embedded protein complexes.

The first process is the transfer of electrons from NADH and H+ to the NADH dehydrogenase Complex I of the ETC. Effectively 2 electrons are transferred and 4H+ ions are pumped across the membrane. In Complex II, FADH is oxidized to FAD, donating 2 electrons to the succinate dehydrogenase complex where 4H+ are pumped across the membrane. The electrons that get transferred to Complexes I and II, eventually flow to ubiquinone structure Q, which carries these electrons to the successive cytochrome Complex III in the form of QH2. This third complex reoxidises QH2 to Q and this results in the pumping of 4H+. Complex IV involves the formation of water via oxygen so effectively 4H+ gets pumped across the membrane. This accumulation of H+ generates a proton gradient, storing potential energy for use in phosphorylation of ADP. The final complex involves ATP synthase, which uses the proton gradient in order to catalyze the phosphorylation of ADP to ATP.

Using the electrons transferred from the electron carriers ATP synthase can cause a “rotation” of its structure and hence the catalysis of ADP to ATP as protons flow back across the membrane via this protein pump. This chemiosmosis model is the final step in the cellular respiration pathway for glucose catabolism. Effectively the net ATP attained by the entire process is 32 per glucose molecule. By 45 minutes in the race, glycogen stores have decreased and blood sugar levels will have dropped allowing for fatty acids to be mobilized to produce ATP via the formation of acetyl CoA and also to give the body time to increase blood sugar levels via gluconeogenesis. Lipids are stored in the form of triglycerides in adipose tissue. Mobilization of triglycerides occurs via the enzyme triacylglycerol lipase; breaking the fat down into fatty acids, and consequently transportation into the blood stream via serum albumin to areas like skeletal muscles, heart and renal cortex. Fatty acids dissociate from albumin and re-enter the cells.

The fate of glycerol components of lipid is to enter the glycolytic pathway; to be phosphorylated, oxidized and isomerized via various enzymes in order to form glyceraldehyde-3-phosphate, which effectively performs normal glycolysis pay-off phase and hence normal cellular respiration. However, fatty acids must be activated via the fatty acyl-CoA synthetase, forming fatty acyl CoA via an endergonic reaction. Binding with carnitine, the fatty acyl-CoA can be transported across the inter-membrane space of the mitochondria to undergo beta-oxidation to form acetyl CoA. Once across the membrane, fatty acyl-CoA dissociates with the carnitine, forming simple fatty acyl-CoA and CoA-SH. The first step the beta-oxidation phase is the conversion of fatty acyl-CoA to a trans-form via acyl-CoA dehydrogenase and the oxidation of FAD to FADH. The second step is the hydration of the product via the enzyme enoyl-CoA hydratase and the addition of water to form trans-hydroxyacyl-CoA.

Following this step is the oxidation of NAD+ to NADH and H+ via the enzyme hydroxyacyl-CoA dehydrogenase, producing β-Ketoacyl-CoA. The final step is cleavage of the product back to fatty acyl-CoA, via the enzyme thiolase which integrates CoA-SH (the reduced CoA) and ultimately forms Acetyl-CoA. This cycle can repeat several times until the fatty acid is completely broken down and for each cycle the products include a molecule of Acetyl CoA, 1 FADH2 and 1 NADH; these are the key components of cellular respiration that will eventually take part in the synthesis of ATP via the CAC and the ETC. During the 1 hour race, the catabolic processes of lipids and carbohydrates occur at different times, and more prominently at certain moments. Before the race starts fats are the primary source of energy, and then carbohydrates start to kick in and provide the large amount of energy needed to satiate the muscles’ need for quick energy. Then as carbohydrate stores run low, the athlete experiences the mobilization of fatty acids from adipose tissue as the prime source for ATP synthesis, in order for the athlete to exert the same amount on his muscles for a prolonged period of time.