Respiratory substrates Learning objectives Define the term respiratory substrate Explain the difference in relative energy values of carbohydrate, lipid and protein respiratory substrate

Carbohydrate Theoretical maximum yield for glucose is 2870kJ mol-1 It takes 30.6KJ to produce 1 mol ATP, therefore theoretic maximum yield of 1 mol of glucose is nearly 94 mol ATP The actual yield is more like 30 mol ATP, which is an efficiency of 32% The remaining energy is lost as heat – helps to maintain a suitable temperature for enzymes to run efficiently.

Protein Excess amino acids are deaminated in the liver and excreted in urine. The rest of the molecule is changed to glycogen or fat for respiration later During periods of starvation/fasting, muscle protein can be hydrolysed to amino acids, which can be respired. Some is converted to pyruvate or acetate, which can then enter the Krebs cycle. Some can enter the Krebs cycle directly. Number of H atoms per mole accepted by NAD and subsequently used in oxidative phosphorylation is slightly more than glucose. This means that proteins release slightly more energy than equivalent masses of carbohydrate.

Lipids Fatty acids are long chain hydrocarbons – this means lots of hydrogen that can be oxidised and then used in oxidative phosphorylation. Fatty acids combine with CoA – this requires hydrolysis of ATP to AMP This breakdown occurs via the ß-oxidation pathway. Reduced NAD and reduced FAD are formed. CoA is releases newly formed acetate as it enters the Krebs cycle. 3NAD, 1FAD and 1ATP (substrate level phosphorylation) are produced in one turn of the cycle Large amounts of reduced acetate is reoxidised in the electron transport chain, producing lots of ATP at chemiosmosis.

Energy values per gram of different respiratory substrates Mean energy value/ kJ g-1 Carbohydrate 15.8 Lipid 39.4 Protein 17.0