Hja

Pyruvate

Amino acids

(alanine, leucine,valine)

Lipids (fatty acids)

Amino acids (arginine, glutamate, glutamine, proline)

Lipids (fatty acids)

Amino acids (arginine, glutamate, glutamine, proline)

ACP. The glycerol component of the fat is synthesized from dihydroxyacetone phosphate.

Amino Acid Synthesis

Proteins are composed of various combinations of 20 different amino acids. Amino acids can be grouped into structurally related families that share common pathways of biosynthesis. Some are synthesized from precursor metabolites formed during gly-colysis, while others are derived from compounds of the TCA cycle (see table 6.3).

Branched pathways are used to synthesize some families of amino acids. These are controlled at key points by allosteric enzymes, regulating the flow of certain branches as well as the common initial steps of the pathway. Amino acids that are the end products of various branches serve as feedback inhibitors.

Glutamate

Although all amino acids are necessary for protein synthesis, glutamate is especially important because it is used to form many other amino acids. In addition, its synthesis provides a mechanism for bacteria to incorporate nitrogen into an organic material. Recall from chapter 4 that many bacteria utilize ammonium (NH4+) provided in the medium as their source of nitrogen; it is primarily through the synthesis of glutamate that they do this.

Bacteria that synthesize glutamate use a single-step reaction that incorporates ammonia into the precursor metabolite a-ketoglutarate, produced in the TCA cycle (figure 6.30a). Once glutamate has been produced, its amino group can be transferred to other carbon compounds to produce amino acids such as aspartate (figure 6.30b). This transfer of the amino group, a transamination, regenerates a-ketoglutarate from

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Perspective, Fourth Edition

162 Chapter 6 Metabolism: Fueling Cell Growth

a-ketoglutarate (a)

Figure 6.30 Glutamate (a) Glutamate is synthesized in a single-step reaction that incorporates ammonia into the precursor metabolite a-ketoglutarate. (b)The amino group of glutamate can be transferred to other carbon compounds in order to produce other amino acids. For example, transferring it to oxaloacetate produces aspartate.

Glutamate

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