THE GREAT METABOLIC RACE INTRODUCTION During exercise, carbohydrates and lipids are required in order to provide energy for the working body. The inherent reduced nature of these compounds allows for partial or complete oxidation in extracting energy in the form of adenosine triphosphate (ATP). The varying chemical structure of the macromolecules evokes different processes for their complete utilisation. This paper seeks to expound on the mobilisation of these fuels, the biochemical pathways that are used and the amount of ATP yielded with reference to different points during physical exertion, such as a long distance race.
BEGINNING OF RACE At the inception of the race, effectively 0 minutes, internal energetic laws dictate how products will be utilised and the extent to which these reactions will proceed. According to the free energy of the system, if Gibbs Free Energy (? G) is negative, it will see the formation of products from the reactants stemming from carbohydrates and lipids. The source of energy initially here comes from the stored ATP within the body.
There are three components to this molecule that allow for its easy, fast utilisation according to free energy laws. Firstly, at equilibrium the ATP molecule carries four closely spaced negative charges based on its charged groups. This causes an internal strain from these repulsing forces which are easily released once ATP is hydrolysed. Secondly, the products of Adenosine Diphosphate (ADP) and inorganic phosphate (Pi) are resonating structures that are stable, therefore their creation from ATP is favourable.
Lastly, the H+ ions created as a result of ATP hydrolysis by water (H20) drive the formation of products as their concentration rises shifting the reaction towards the right to minimise the disturbance cause by the protons to re-establish equilibrium according to Le Chatelier’s principle. CARBOHYDRATE UTILISATION Once the ATP stores of the body are depleted, energy is provided through carbohydrates under anaerobic conditions. Approximately 85% of energy is supplied to the body by sugars at about five minutes into exercise.
Firstly however, the mobilisation of glucose from glycogen must take place before it is utilised. Glycogen is a branched molecule that is stored in both the liver and active muscles. It is cleaved to glucose monomers by glycogen phosphorylase act on the non-reducing ends to produce glucose-1-phosphate (G1P), which is then converted to glucose-6-phosphate (G6P) by phosphoglucomutase. This G6P can then enter the glycolysis pathway in the cytosol of cells where it can be converted to pyruvate.
Under anaerobic conditions pyruvate is converted to lactate, a ‘dead-end’ molecule that is not further oxidised. Approximately 2 ATP molecules are yielded by glycolysis. The preparatory phase steps of 1 and 3 consume 2 ATP whilst steps 7 and 10 in the payoff phase yield 4 ATP. Due to this quick mobilisation and utilisation before adequate oxygen can be consumed, carbohydrates provide expeditious energy near the start of the race. CROSS OVER IN FUELS As exercise approaches thirty minutes in duration, under aerobic conditions, 50% of energy comes from lipids and 50% from carbohydrates.
With oxygen present, both sugars and fats can be utilised in the citric acid cycle and eventually oxidative phosphorylation within mitochondria to create ATP. The citric acid cycle (CAC) is the vital pathway by which complete oxidation of both carbohydrate and lipids occur. The initial molecule used here is acetyl-CoA, a three-carbon compound which is created from both pyruvate and glycerol under aerobic conditions. Acetyl-CoA reacts with oxaloacetate to form citric acid, the first product of the cycle.
This pathway sees the regeneration of oxaloacetate and the creation of reduced electron carriers of FADH2 and NADH needed in electron transport. The final reaction of the CAC, the conversion of malate to oxaloacetate is endergonic but is coupled to the highly exergonic first step of the CAC, pulling the reaction forward. Oxidative phosphorylation that takes place in the intermembrane space of the mitochondria sees the formation of ATP as oxygen (O2) acts as an electron acceptor. O2 accepts electrons from FADH2 and NADH and a proton gradient is established by the movement of H+ ions into the matrix.
This ‘proton motive force’ drives ATP synthase to phosphorylate ADP and ultimately create ATP. FAT UTILISATION Fats are utilised under aerobic conditions through a process known as ? -oxidation. They become the primary fuel source at about forty five minutes into exercise. Mobilisation of fats first takes place before they can be fully broken down. Fats are stored in adipose tissue. Here, under hormonal control, a hormone attaches to a receptor which activates adenyl cyclise to create cAMP as a substrate for protein kinases.
These phosphorylate hormone-sensitive lipases which breakdown triacylglycerols (TAGs) into free fatty acids (FAs) and glycerol. Then in a process known as activation, FAs are then combined with Acetyl-CoA to create Fatty Acid-CoA which can then enter the mitochondria via a cartinine transporter where it can undergo ? -oxidation. This occurs by turning Fatty Acid-CoA into Fatty Acid- Cartinine then back again once it is in the inner mitochondrial membrane. This reaction is catalysed by cartinine acyltransferase and is reversible.
Each ? -oxidation cycle consists of four steps of oxidation, hydration, further oxidation and finally thiolysis. After each cycle of four reactions two carbons are cut off from the FA and produces acetyl-CoA which can enter the CAC where it yields 10 ATP per molecule. The glycerol molecule is simply utilised where it is converted to glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase and then to dihydroxyacetone phosphate which can be oxidised in glycolysis to create glyceraldehyde 3-phosphate.
This yields 2 ATP per glycerol molecule. CONCLUSION Complete oxidation of glucose yields 32 ATP whilst the complete oxidation of a six-carbon fatty acid would yield 36 ATP. Due to their different structures, different periods of time and varying pathways are required to utilise these fuels sources there are varying yields in both fuel sources. This results in the utilisation of theses fuels in a way that links all these biochemical pathways together.