12 Amino acid metabolism
12.1 Overview of amino acid degradation
12.3 The urea cycle
12.4 Degradative pathways of individual amino acids
12.5 Enzyme defects in amino acid metabolism
We have seen before that during digestion in the intestines proteins are broken down to their constituent amino acids. Proteins contain 20 standard amino acids, which are incorporated into them during translation, and several non-standard ones that are mostly formed by post-translational modification. These are much less abundant than the standard amino acids and will not be considered here. Blah.
- As building blocks for our own protein synthesis. Animals, including humans, are essentially parasites and have a lazy synthetic metabolism. Accordingly, we possess synthetic pathways for only 10 out of the 20 standard amino acids. The residual 10 amino acids have to be obtained from the diet and are called the essential amino acids.
- As a source of energy. Depending on the composition of the diet, this role may be very significant. Carbohydrates occur nearly exclusively in plant-derived foodstuffs. Therefore, on a meat-only diet, amino acids become the major source of glucose.
- As building blocks for other things such as nucleotides and heme.
We will here focus energy metabolism, i.e. on the degradation of amino acids. Synthesis will be neglected, save for some trivial pathways that involve no more than transamination (see below). We will only state that nine out of twenty amino acids cannot be synthesized in mammalian cells and therefore have to be obtained with the diet. These essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophane, and valine. Arginine is apparently not synthesized in sufficient amount and often listed as a tenth essential amino acid.