Serine deamination

The early work on serine deamination revealed two distinct enzymes, one active on the L and the other on the D-form in E. coli (53, 64) and Neurospora (57, 68),... 

Many of the amino acids originally tested by Krebs were racemic mixtures. When naturally occurring L-amino acids became available the oxidase was found to be sterically restricted to the unnatural, D series. [D-serine occurs in worms free and as D-phosphoryl lombricine (Ennor, 1959)]. It could not therefore be the enzyme used in the liver to release NH3 in amino acid metabolism. D-amino acid oxidase was shown by Warburg and Christian (1938) to be a flavoprotein with FAD as its prosthetic group. A few years later Green found an L-amino acid oxidase in liver. It was however limited in its specificity for amino acid substrates and not very active—characteristics which again precluded its central role in deamination. 

Glutamate dehydrogenation is involved in deamination of most of the amino acids. The first two reactions are not involved in the overall deamination system they are included here for completeness and because they are of some general interest. The complete biochemical description of these reactions is given in Appendix 8.4. For a few amino acids, e.g. threonine and serine, other specific reactions are responsible for deamination. 

Eliminating deamination takes place in the degradation of histidine and serine. H2O is first eliminated here, yielding an unsaturated intermediate. In the case of serine, this intermediate is first rearranged into an imine (not shown), which is hydrolyzed in the second step into NH3 and pyruvate, with H2O being taken up. H2O does not therefore appear in the reaction equation. 

Deamination appears to proceed equally rapidly both in the presence and absence of oxygen. Stein and Weiss (S18) examined this phenomenon and deduced that deamination can be accompanied by both oxidation and reduction. In the absence of oxygen the H atom, rather than the HO radical is the effective agent. This mechanism is satisfactory for the simple amino acids (glycine, alanine, and serine) studied by these workers. With the more complex amino acids yields of ammonia become less and side chains may be preferentially broken. 

An alternative pathway by which some acetogenic bacteria form acetate is via reversal of the glycine decarboxylase reaction of Fig. 15-20. Methylene-THF is formed by reduction of C02, and together with NH3 and C02 a lipoamide group of the enzyme and PLP forms glycine. The latter reacts with a second methylene-THF to form serine, which can be deaminated to pyruvate and assimilated. Methanogens may use similar pathways but ones that involve methanopterin (Fig. 15-17).191... 

Now let us consider the further conversion of PEP and of the triose phosphates to glucose 1-phosphate, the key intermediate in biosynthesis of other sugars and polysaccharides. The conversion of PEP to glucose 1-P represents a reversal of part of the glycolysis sequence. It is convenient to discuss this along with gluconeogenesis, the reversal of the complete glycolysis sequence from lactic acid. This is an essential part of the Cori cycle (Section F) in our own bodies, and the same process may be used to convert pyruvate derived from deamination of alanine or serine (Chapter 24) into carbohydrates. 

Glycine is then transported to the mitochondrial matrix where the conversion of two glycines to one serine occurs with the loss of CO2 and NH3 from the pool of fixed molecules. The serine is transported into the peroxisome, where it is deaminated to glycerate. The glycer-ate is transported back to the chloroplast, where it is phosphorylated to 3-phosphoglycerate for the Calvin-Benson cycle. 

Although the nitrogen atoms of most amino acids are transferred to a-ketoglutarate before removal, the a-amino groups of serine and threonine can be directly converted into NH4 +. These direct deaminations are catalyzed by serine dehydratase and threonine dehydratase, in which PLP is the prosthetic group. 

These enzymes are called dehydratases because dehydration precedes deamination. Serine loses a hydrogen ion from its a-carbon atom and a hydroxide ion group from its P-carbon atom to yield aminoacrylate. This unstable compound reacts with H2O to give pyruvate and NH4 +. Thus, the presence of a hydroxyl group attached to the P-carbon atom in each of these amino acids permits the direct deamination. 

Another simple reaction in the degradation of amino acids is the deamination of serine to pyruvate by serine dehydratase (Section 23.3.4). 

In addition to being a precursor of methionine in the activated methyl cycle, homocysteine is an intermediate in the synthesis of cysteine. Serine and homocysteine condense to form cystathionine. This reaction is catalyzed hy cystathionine -synthase. Cystathionine is then deaminated and cleaved to cysteine and a-ketohutyrate hy cystathioninase. Both of these enzymes utilize PLP and are homologous to aspartate aminotransferase. The net reaction is... 

Amino acids such as serine and histidine are deaminated non-oxidatively... 

L-Serine and L-threonine dehydratases dehydrate and subsequently deaminate the amino add to the corresponding a-keto add. These enzymes are known to require pjn-idoxal-S -phosphate as a coenzyme. They can function in a biosynthetic or catabolic marmer (99). Both enzymes can cause problems for the whole-cell-based production of L-serine (100). 

Serine and threonine are deaminated by serine dehydratase, which requires pyridoxal phosphate. Serine is converted to pyruvate, and threonine to a-ketobutyrate NH4+ is released. 

We now turn to studies with the isolated a protein and ySj protein. The a protein promotes the cleavage reaction of Eqn. 5 at about % of the rate exhibited by the complex, for this conversion. On the other hand the pyridoxal-P containing 2 protein catalyses the condensation reaction between indole and L-serine (Eqn. 6). The efficiency of this process is about 5% of that observed with the intact complex. The 2 protein in the absence of indole also catalyses the deamination reaction ... 

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