Serine deaminase

L-Serine dehydratase [EC 4.2.1.13], also known as serine deaminase and L-hydroxyaminoacid dehydratase, catalyzes the pyridoxal-phosphate-dependent hydrolysis of L-serine to produce pyruvate, ammonia, and water. In a number of organisms, this reaction is also catalyzed by threonine dehydratase. 

SERINE DEHYDRATASE SERINE DEHYDROGENASE SERINE HYDROXYMETHYLTRANSFERASE SERINE PALMITOYLTRANSFERASE Serine deaminase... 

Both the l- and D-serine deaminase catalyze the elimination of the amino functionality of both l- and D-serine, but the mechanism proceeds via the initial elimination of water and these enzymes are thus classified as hydrolyases (l- and D-serine dehydratases E.C. 4.2.1.13 and E.C. 4.2.1.14, respectively)[27, 28. The aminoacrylate generated is unstable and subsequent elimination of the amine results in the formation of pyruvate. Similarly, threonine deaminase is in effect a dehydratase that converts L-threonine into 2-oxobuturate, water and ammonia (E.C. 4.2.1.16) (Scheme 12.6-1). 

Enzymes which show the repression effect include enzymes of glycerol regulation, tryptophanase, D-serine deaminase, histidase, the o/operon, and the lac operon. As far as is known, all the enzymes classified as inducible show the effect. 

While this work was proceeding, Yanofsky and Reissig independently found the L-serine deaminase of Neurospora This enzyme is activated by pyridoxal phosphate, and is not stimulated by ATP, biotin, or glutathione. The Neurospora enzyme is somewhat more active with threonine than with serine, and acts only on the L-isomers. 

During this period I was frustrated by my inability to obtain tryptophan synthetase in pure form from Neurospora. Therefore, I searched for a more suitable enzyme for studies on protein structure alterations resulting from mutations. I characterized d- and L-serine deaminases from Neurospora - with this objective in mind but was unable to develop a convenient isolation procedure for mutants lacking these enzymes and so stopped these investigations. 

Serine is deaminated and dehydrated to pyruvate by serine deaminase (sometimes called serine dehydratase). 

More important evidence, however, is the widespread distribution and considerable activity of serine deaminase in different organisms. The anaerobic deamination of DL-serine by cell-free liver extracts has been ob-... 

D-Serine deaminase from both E. coli (64) and Neurospora (67) was found to require pyridoxal phosphate as a coenzyme. This enzyme was observed to be quite specific for o-serine and did not attack D-threonine. 

Pyridoxal phosphate was demonstrated to be required as a coenzyme by dissociating it from the apoenzyme. This causes a loss of enzyme activity which can be restored by recombining the apoenzyme with pyridoxal phosphate. The optimum activity is at pH 8.3 and the enzyme requires free sulfhydryl groups for activity. The serine deaminase activity of the enz3rme is strongly inhibited by homocysteine and by cystathionine. No synthetic product appears to be formed with cysteine. The Km of the enzyme for serine is 8.1 X 10 Af. Knowledge of the mechanisms of the reactions catalyzed by pyridoxal phosphate-dependent enzymes has been advanced by model experiments with pyridoxal, metal ions, and amino acids. This subject has been reviewed recently by Snell (61). 

The unsaturated intermediate formed in this manner may dissociate off a-aminoacrylic acid, which immediately reacts spontaneously with water to form pyruvic acid and ammonia. This is the essence of the media-nism of the serine deaminase reaction. It is shown structurally in F. 3. The same mechanism applies to the deamination of other /8-hydroxyaniino acids, such as threonine, discussed in the next section. 

Sayre and Greenberg 69) separated L-threonine deaminase of sheep liver from L-serine deaminase and concentrated it about 600-fold. Even higher specific activities have been achieved by Nishimura and Greenberg 70). This enzyme also is pyridoxal phosphate-dependent. It retains considerable activity against L-serine although it is not known for certain whether this represents an intrinsic property of the enz3une or is due to a contamination. The Michaelis constant fof L-threonine was estimated to be 2.9 X 10- M. 

Pardee and co-workers (225,225a,227,227a) have investigated the activity of 9 bacterial enzymes after infection of E. coli with various T phages, i.e., apyrase, ribonuclease, deoxyribonuclease, alkaline and acid proteases, pyruvic oxidase, formic dehydrogenase, serine deaminase, and catalase. The difficulties of assay of labile enzymes in the broken cell systems are indicated by the early report of an increase in apyrase activity (225) which later was attributed to a technical defect in the... 

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