The reactions of glutaminase and glutamine synthetase.

Methionine is synthesized from oxaloacetate. Succinyl-CoA participates and cysteine donates a sulfur group to the molecule. Oxaloacetate is first converted into homoserine as described above in the threonine biosynthetic pathway. Homoserine then has a sulfur attached to the end in two steps and finally methionine is formed by the addition of a methyl group.

The following diagram summarizes the regulation of bacterial glutaminesynthetase:

You have previously studied the oxidative deamination of glutamate byglutamate dehydrogenase, in which NH3 and ketoglutarateare produced. The -ketoglutarate produced is thenavailable for accepting amino groups in other transamination reactions, but theaccumulation of ammonia as the other product of this reaction is a problembecause, in high concentrations, it is toxic. To keep the level of NH3in a controlled range, a rising level of -ketoglutarateactivates glutamine synthetase, increasing the production of glutamine, whichdonates its amino group in various other reactions.


Figure 1 - The synthesis of glutamate.

Glutamate synthetase reaction: starting compounds are glutamate and NH4+.

FIGURE 15–10 Biosynthesis of δ-aminolevulinate. (a) In mammalsand other higher eukaryotes, δ-aminolevulinate is synthesized fromglycine and succinyl-CoA. The atoms furnished by glycine are shownin red. (b) In bacteria and plants, the precursor of δ-aminolevulinateis glutamate. (c) Biosynthesis of heme from δ-aminolevulinate.


Protein Lounge: Biosynthesis of Glycine and Serine

Glutamate can be formed inside cells by a short pathway of two enzyme catalysed steps that well illustrates the features of a typical biosynthetic pathway.

Ornithine biosynthesis, glutamate => ornithine

Although the pathway seems to start with its product glutamate , the second step generates glutamate molecules. Thus the net effect is conversion of an organic acid precursor with assimilation of nitrogen into an amino acid monomer, reduction of the precursor, wastage of ATP, and creation of a monomer that can used in protein synthesis.

humancyc Pathway glutamate biosynthesis/degradation

The first step in glutamate pathway, enzyme catalysed synthesis of glutamine, is actually a also used for glutamine biosynthesis. Glutamine, like glutamate, is an amino acid monomer used for building proteins.

Glutamate is the most abundant amino acid in the brain.


FIGURE 15–3
(a) Biosynthesis of serine from 3-phosphoglycerate and of glycine from serine inall organisms.
(b) Biosynthesis of cysteine from serine in bacteria and plants.
(c) The origin of reduced sulfur
(d) Biosynthesis of cysteine from homocysteine and serine in mammals. Thehomocysteine is formed from methionine.

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Important and widely known products of biosynthesis include , , and , but all components of living beings depend on this process. Biosynthesis is harvested by the biotechnology industry to manufacture flavoring compounds such as mono sodium glutamate and lysine, vitamins, and therapeutic proteins such as and .