Amino acid synthesis
Of the 20 amino acids needed for protein synthesis, humans are only able to make 11. These are termed the non-essential amino acids and are made by either de novo pathways or by conversion from existing amino acids. These pathways involve, in the first instance, the addition of an amino group to a carbon skeleton via a transaminase reaction. The other nine amino acids are termed essential amino acids and have to be obtained from the diet. In a well-nourished adult approximately 50 g of dietary protein is required daily.
The amino acid pool and protein turnover
A major source of amino acids is from the hydrolysis of dietary protein as well as from the breakdown of cellular protein. These sources, in addition to synthesised amino acids, give rise to a pool of free amino acids within the body (Figure 13.1). Cellular protein is continuously undergoing synthesis and breakdown in a process known as protein turnover, which allows damaged or abnormal proteins to be removed from the body. This is a highly regulated process and approximately 400 g of cellular protein is degraded and re-synthesised each day. In a well-nourished adult the rate of protein synthesis is equal to the rate of protein degradation.
Formation of nitrogen-containing molecules
Amino acids are precursors to a number of important nitrogen-containing molecules. These include porphyrins (e.g. heme groups), purines and pyrimidines (e.g. nucleotides for DNA synthesis; Chapter 15), neurotransmitters (e.g. dopamine, noradrenaline) and creatine (found in muscle as the high-energy molecule creatine phosphate) (Figure 13.1).
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