Serine dehydratase, function

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

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). 

Other enzymes in the aconitase family include isopropylmalate isomerase and homoaconitase enzymes functioning in the chain elongation pathways to leucine and lysine, both of which are pictured in Fig. 17-18.90 There are also iron-sulfur dehydratases, some of which may function by a mechanism similar to that of aconitase. Among these are the two fumarate hydratases, fumarases A and B, which are formed in place of fumarase C by cells of E. coli growing anaerobically.9192 Also related may be bacterial L-serine and L-threonine dehydratases. These function without the coenzyme pyridoxal phosphate (Chapter 14) but contain iron-sulfur centers.93-95 A lactyl-CoA... 

The activities involved in yeast fatty acid biosynthesis are covalently linked as separate domains of two multifunctional polypeptides, a and p, encoded by the fas2 and fasl genes, respectively (Fig. 2) [57,58]. The functionalities associated with the 220 kDa a subunit include -ketoacyl synthase activity, -ketoacyl reductase activity, and an AGP domain which bears a phosphopantetheinylated serine. The 208 kDa -subunit has acetyl and malonyl CoA transacylase, palmi-toyl transferase, -hydroxyacyl-enzyme dehydratase, and enoyl acyl-enzyme reductase activities. The two subunits can be readily dissociated, and the individual activities maybe measured [57]. 

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). 

For the time being, a number of perplexing questions are still unsolved. For example - why are exclusively B-elimina-ting lyases, such as serine (threonine) dehydratase, alliin lyase etc., incapable to effect a replacement step (Michael addition) — what is the difference in dynamic topography (stereochemistry) between this enzyme subtype and the ambi-functional Q-lyases, displaying the same behaviour towards L-cycloserine or aminothiol inhibitors,etc. Further investl-ons are needed to shed light on these and other obscure aspects of the problem. 

---