Explain how proteins can be used as a respiratory substrate.

When supplies of carbohydrate and lipid are inadequate, the body can use protein as a respiratory substrate- in times of prolonged starvation, tissue protein from muscles can even be broekn down for this purpose. The proteins are hydrolysed into their constituent amino acids, which are then deaminated in the liver. The amino groups are converted into urea and excreted in the urine, and the remaining amino acid residues are converted into acetyl CoA or pyruvate by enzymes- both of which are intermediates of the Krebs Cycle. The Krebs cycle then continues the process of respiration.