Respiratory substrates Learning objectives Define the term respiratory

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 2870 k. J mol-1 • It takes 30. 6 KJ 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 Co. A – this requires hydrolysis of ATP to AMP • This breakdown occurs via the ß-oxidation pathway. Reduced NAD and reduced FAD are formed. • Co. A is releases newly formed acetate as it enters the Krebs cycle. 3 NAD, 1 FAD and 1 ATP (substrate level phosphorylation) are produced in one turn of the cycle • Large amounts of reduced acetate is reoxidised in the electron

Energy values per gram of different respiratory substrates Respiratory substrate Mean energy value/ k. J g-1 Carbohydrate 15. 8 Lipid 39. 4 Protein 17. 0