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building blocks for protein synthesis
precursors of nucleotides and heme
source of energy
neurotransmitters
precursors of neurotransmitters and hormones
We have seen before that, during digestion in the gut, proteins are broken down to their constituent amino acids. Proteins contain twenty standard amino acids, which are incorporated into them during translation. Proteins may also contain several non-standard amino acids; these are mostly formed by post-translational modification, and they are much less abundant than the standard amino acids. In this chapter, we will focus on the standard amino acids, although some non-standard ones will appear in the urea cycle.
Animals and humans obtain many important metabolites from their food and have a fairly lazy synthetic metabolism. While many other organisms, even simple ones such as E. coli, can make all their amino acids from scratch, we possess synthetic pathways for only 11 out of the 20 standard amino acids. The remaining ones must be obtained from the diet and accordingly are referred to as the essential amino acids.75 A certain amount of dietary protein is therefore strictly necessary, and the lack of food protein is a very common form of malnutrition in impoverished countries. Nevertheless, during day-to-day protein turnover, most of the amino acids used in protein synthesis are obtained not from food but rather through endogenous protein breakdown; food protein only replaces the fraction of amino acids diverted toward other destinations.