The essential amino acids are those that cannot be synthesized in sufficient amounts in the body and so must be supplied in the diet in sufficient amounts to meet the body's needs. Therefore, discussion of amino acid synthesis applies only to the nonessential amino acids. Nonessential amino acids fall into two groups on the basis of their synthesis: (a) amino acids that are synthesized by transferring a nitrogen to a carbon skeleton precursor that has come from the TCA cycle or from glycolysis of glucose and (b) amino acids synthesized specifically from other amino acids. Because this latter group of amino acids depends upon the availability of other specific amino acids, they are particularly vulnerable to becoming essential if the dietary supply of a precursor amino acid becomes limiting. In contrast, the former group is rarely rate limited in synthesis because of the ample precursor availability of carbon skeletons from the TCA cycle and from the labile amino-N pool of transaminating amino acids.
The pathways of nonessential amino acid synthesis are shown in Figure...2,.4. As with amino acid degradation, glutamate is central to the synthesis of several amino acids by providing the N. Glutamate, alanine, and aspartate may share amino-N transaminating back and forth among them ( Fig 2.2). As Figure..2,.4 is drawn, glutamate derives its N from ammonia with a-ketoglutarate, and that glutamate goes on to promote the synthesis of other amino acids. Under most circumstances, the transaminating amino acids shown in Figure.2.2 supply more than adequate amino N to glutamate. The transaminating amino acids provide a buffer pool of N that can absorb an increase in N from increased degradation or supply N when there is a drain. From this pool, glutamate provides material to maintain synthesis of ornithine and proline, the latter particularly important in synthesis of collagen and related proteins.
Figure 2.4. Pathways of synthesis of nonessential amino acids. Glutamate is produced from ammonia and a-ketoglutarate. That glutamate becomes the N source added to carbon precursors (pyruvate, oxaloacetate, glycolysis products of glucose, and glycerol) to form most of the other nonessential amino acids. Cysteine and tyrosine are different in that they require essential amino acid input for their production.
Serine may be produced from hydroxypyruvate derived either from glycolysis of glucose or from glycerol. Serine may then be used to produce glycine through a process that transfers a methylene group to tetrahydrofolate. This pathway could (and probably should) have been listed in T.aMe 2.:.6. as a degradative pathway for serine. However, it is not usually considered an important means of degrading serine but as a source of glycine and one-carbon-unit generation ( 21, 22). On the other hand, the pathway backward from glycine to serine is also quite active in humans. When [ 15N]glycine is given orally, the primary transfer of 15N is to serine (23). Therefore, there is significant reverse synthesis of serine from glycine. The other major place where 15N appeared was in glutamate and glutamine, indicating that the ammonia released by glycine oxidation is immediately picked up and incorporated into glutamate and the transaminating-N pool.
All of the amino acids shown in Figure..2,.4 have active routes of synthesis in the body (17), in contrast to the essential amino acids for which no routes of synthesis exist in humans. This statement should be a simple definition of "essential" versus "nonessential." However, in nutrition, we define a "nonessential" amino acid as an amino acid that is dispensable from the diet (7). This definition is different from defining the presence or absence of enzymatic pathways for an amino acid's synthesis. For example, two of the nonessential amino acids depend upon degradation of essential amino acids for their production: cysteine and tyrosine. Although serine provides the carbon skeleton and amino group of cysteine, methionine provides the sulfur through condensation of homocysteine and serine to form cystathionine (20). The above discussion explains why neither the carbon skeleton nor amino group of serine are likely to be in short supply, but provision of sulfur from methionine may become limiting. Therefore, cysteine synthesis depends heavily upon the availability of the essential amino acid methionine. The same is true for tyrosine. Tyrosine is produced by hydroxylation of phenylalanine, which is also the degradative pathway of phenylalanine. The availability of tyrosine is strictly dependent upon the availability of phenylalanine and the liver's ability to perform the hydroxylation.
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