- act as biological catalysts, increasing the rate of reaction (of chemical reactions), but not being affected by the reaction

- the catalytic ability of an enzyme is related to the structure of the particular protein, and so can only catalyse a limited number of reactions

- active site – the specific cleft/indentation on an enzyme’s area where the substrate binds is 2-4% of the enzyme, involving 3-10 residues

- Process

o old chemical bonds must break

o new bonds must form

o transition state – where neither original or final molecules are present

o energy is required to break the old bonds (activation energy)

- The enzyme’s impact

o creates a modified transition state, with a much lower activation energy

o this allows the reaction to proceed more rapidly

o overall reaction in terms of free energy change does not change

o will not affect the equilibrium position

- Binding between the enzyme and the substrate is a result of non-covalent interactions (ionic, hydrogen, hydrophobic)

o perfect fit (shape and charge)

o may still fit (partial shape and good charge)

o does not fit

- Catalysis

o acid-base reactions

o stabilisation of charged intermediates

o highly reactive nucleophilic groups

- Active Site

o binding of substrate

o catalysis

o binding of the substrate produces precise orientation of the substrate with respect to the functional groups of the enzyme involved in catalysis

o addition/removal of H+

o promotion of charged transition state

- Prosthetic Groups

o constituents that provide reactive groups at the active site

- Specificity

o enzymes only oxidise a specific class of reactions (oxidation etc)

o enzymes can accept a broad range of closely related substrates, or accept only one substrate

- Enzyme kinetics

o Michaelis- Menten Plot = V verses [S]

o Lineweaver Burke Plot = 1/V verse 1/[S]

- enzyme inhibitors

o two classes

§ irreversible inhibition

§ reversible inhibition

· Competitive inhibition – inhibitor binds to the active site, lines pass through the same point on the 1/V axis

· Uncompetitive inhibition – inhibitor binds to a separate site, only on the ES complex, lines are parallel

· Mixed (non-competitive) inhibition – binds anywhere, and my not affect the binding of the substrate, lines intersect at a point not on the 1/V axis

- allosteric enzymes

o quaternary structure

o anomalous MM kinetics

o substrate binding causes further change to facilitate the binding of more substrates (sigmoidal kinetics)

- zymogens

o synthesised as catalytically inactive pre-cursor proteins

o activated by cleaving a limited number of specific peptide bonds

o digestive enzymes and blood coagulation enzymes

- isosomes

o multiple forms of the same enzyme occurring within the same species

o different forms in different tissues, or occurring in different compartments of the cell

Copyright Monash University

At the start of the race, 65% percent of the required energy is met by catabolism of fats and 35% from carbohydrates. Anerobic pathways provide the initial energy to begin running, sourcing glucose (to produce pyruvate) from the breakdown of glycogen. Pyruvate undergoes fermentation to produce lactate and ATP. This method of producing energy is very inefficient compared to the energy produced under aerobic conditions. One mol of glucose produces a net gain of 2 mol of lactate and 2 mol of ATP. There is no net gain of NADH, because it is oxidised directly back to NAD+ to allow glycolysis to continue.

After 5 Minutes

After 5 minutes of running, 85% of the energy required is provided by carbohydrates, which are broken down in the liver and skeletal muscle, via aerobic pathways. Free glucose derived from glycogen mobilisation is exported from the liver. Glucose is degraded to pyruvate by glycolysis by a ten reaction pathway.

The first five reactions of glycolysis (preparatory phase) consume ATP. During the preparatory stage, the molecule of glucose is phosphorylated, and converted into two molecules of glyceraldehyde-3-phosphate. The phosphate groups are obtained from 2 ATP and attached in place of the OH groups, producing glyceraldehyde-3-phosphate, and leaving 2 ADP molecules behind (stages 1 and 3). Although the conversion of glucose to glucose-6-phosphate is endergonic (requires energy) the overall process of glycolysis still proceeds. This is because the overall reaction is exogonic, and the later stages have excess energy to help the first reactions proceed.

The last five reactions (pay off phase) produce ATP. In the payoff phase, the glyceraldehyde-3-phosphate is converted to pyruvate. This releases four ATP molecules, and produces two NADH molecules (stages 7 and 10). A molecule of Pi replaces the H group, releasing electrons to form NADH and 1,3-biphosphoglycerate. The phosphate group is then removed by ADP to form ATP and 3-phosphoglycerate. Substrate-level phosphorylation is the transfer of a high energy phosphate group from 1,3-biphosphoylate/phosphoenolpyruvate to ADP to produce ATP and a molecule of 3-phosphoglycerate/pyruvate.

In aerobic conditions, pyruvate is further oxidised into acetyl-CoA.

The citric acid cycle converts acetyl-CoA into CO₂, transferring more electrons to NADH and FADH₂. For every molecule of acetyl-CoA degraded, 10 ATP are produced.

In the electron transfer chain, electrons flow from NADH and FADH₂, ultimately finishing at oxygen, which is then reduced to form water. The energy from this flow of electrons drives the synthesis of ATP.

The total amount of energy produced is 5.3 ATP per carbohydrate carbon atom.

After 45 Minutes

After 45 minutes of running, most of the energy need is being met by the catabolism of fats. Fats are stored in adipose tissue as triacylglycerols and must be mobilised before use.

Low glucose levels in the blood cause the release of hormones which activate the enzyme triacylglycerol lipase. This stimulates the breakdown of stored fatty acids into fatty acids and glycerol – the fatty acids are transported by serum albumin to the muscles and the glycerol is recycled back to the liver.

Once in the muscles, the B-oxidation pathway degrades the fatty acids into acetyl-CoA and reduced co-enzymes, via the cyclic repetition of four reactions in the mitochondria. Each cycle consists of a dehydrogenation (oxidation) which produces FADH₂ from FAD, a hydrolysis and a second dehydrogenation which produces NADH from NAD+. The final reaction, catalysed by thiolase, releases acetyl-CoA and reduces the length of the fatty acid by two carbons (thiolytic cleavage). The acetyl-CoA is then subjected to the citric acid cycle.

The glycerol backbone of the triaglycerol is converted to glycerol-3-phosphate by ATP to ADP, then dihydroxyacetone phosphate by NAD+ to NADH and H+. It is then further catabolised via the glycolysis pathway.

One fatty acid carbon atom produces 6.6 ATP.