Proteins are respired only when carbohydrate and lipid stores are low, for example during prolonged starvation. They are first hydrolysed by protease enzymes into amino acids.Amino acids cannot be respired directly, because they contain nitrogen in their amine group (NH2). In mammals, excess amino acids are deaminated in the liver: the amine group is removed and forms ammonia. Ammonia is very toxic, so it is converted into urea in the ornithine cycle and excreted by the kidneys.The part of the amino acid that remains is a keto acid (an organic acid). Depending on the amino acid, the keto acid is converted into:• pyruvate, which enters the link reaction,
• acetyl CoA, which enters the Krebs cycle, or
• an intermediate of the Krebs cycle itself, such as oxaloacetate, which enters the cycle directly.The keto acid is then oxidised in the same way as other substrates. Hydrogen atoms are carried by reduced NAD and reduced FAD to the electron transport chain, and ATP is made by oxidative phosphorylation.Proteins release about 17 kJ g−1, slightly more than carbohydrates, because some amino acids have more hydrogen atoms available. However, respiring protein means breaking down muscle and enzymes, so it is a last resort.Worked example: alanine is deaminated to form pyruvate, a 3-carbon compound, which then passes through the link reaction to form acetyl CoA and carbon dioxide.
• acetyl CoA, which enters the Krebs cycle, or
• an intermediate of the Krebs cycle itself, such as oxaloacetate, which enters the cycle directly.The keto acid is then oxidised in the same way as other substrates. Hydrogen atoms are carried by reduced NAD and reduced FAD to the electron transport chain, and ATP is made by oxidative phosphorylation.Proteins release about 17 kJ g−1, slightly more than carbohydrates, because some amino acids have more hydrogen atoms available. However, respiring protein means breaking down muscle and enzymes, so it is a last resort.Worked example: alanine is deaminated to form pyruvate, a 3-carbon compound, which then passes through the link reaction to form acetyl CoA and carbon dioxide.