Glycosyl transfer mechanism

The lack of glycosyl transfer reaction is the class of pectinolytic hydrolases is in agreement with the observed inversion of the anomeric configuration of the newly formed reducing ends of the products. All three polygalacturonases studied here utilize the single displacement mechanism of hydrolysis. 

Pont Lezica355 speculated that the role of dolichyl phosphates, a-satu-rated polyisoprenyl phosphates of chain length between 80 and 105 carbon atoms, in glycosyl transfer during biosynthetic mechanisms is a general feature of eukaryotic cells, whereas, in prokaryotic cells, the lipids fulfilling this function are a-saturated polyisoprenyl phosphates of shorter chain-length, such as undecaprenyl phosphate (which contains 55 carbon atoms). 

M. L. Sinnott, Catalytic mechanisms of enzymic glycosyl transfer, Chem. Rev. 90 1171 (1990). G. W. J. Fleet, Homochiral compounds from sugars, Chem. Br. 25 287 (1989). 

Hen egg white lysozyme is a small protein of Mr 14 500 and 129 amino acid residues. This enzyme was introduced in Chapter 1, where it was pointed out that examination of the crystal structure of the enzyme stimulated most of the solution studies. Hen egg white lysozyme has the distinction of being the first enzyme to have had its structure solved by x-ray crystallography.207 It is an atypical member of the hexosaminidase class of glycosyl transfer enzymes. It catalyzes the hydrolysis of substrates with retention of stereochemistry. T4 lysozyme was for many years thought to have the same fold and mechanism of lysozyme, despite there being no sequence homology. But it has now been found that the T4 enzyme has inversion of configuration and so operates by a different mechanism.208,209 A mechanism proposed for the enzymatic reaction was based on the structure of the... 

Sinnott, M.L. Catalitic Mechanisms of Enzymic Glycosyl Transfer. 1990 [95]... 

Comprehensive Biological Catalysis—a Mechanistic Reference Volume has recently been published. The fiiU contents list (approximate number of references in parentheses) is as follows S-adenosylmethionine-dependent methyltransferases (110) prenyl transfer and the enzymes of terpenoid and steroid biosynthesis (330) glycosyl transfer (800) mechanism of folate-requiring enzymes in one-carbon metabohsm (260) hydride and alkyl group shifts in the reactions of aldehydes and ketones (150) phosphoenolpyruvate as an electrophile carboxyvinyl transfer reactions (140) physical organic chemistry of acyl transfer reactions (220) catalytic mechanisms of the aspartic proteinases (90) the serine proteinases (135) cysteine proteinases (350) zinc proteinases (200) esterases and lipases (160) reactions of carbon at the carbon dioxide level of oxidation (390) transfer of the POj group (230) phosphate diesterases and triesterases (160) ribozymes (70) catalysis of tRNA aminoacylation by class I and class II aminoacyl-tRNA synthetases (220) thio-disulfide exchange of divalent sulfirr (150) and sulfotransferases (50). 

Zechel DL, Withers SG (1999) In Poulter CD (ed), Mechanisms of glycosyl transfer, Comprehensive Natural Products Chemistry, chapter 5, Elsevier, New York, p 279... 

Scheme 5.4 Rapid equilibrium random mechanism for a two-substrate enzyme, illustrated for a glycosyl transfer.

Slow proton transfer makes possible the occurrence of iso mechanisms -mechanisms in which the form of the enzyme released after catalysis is different to that at the start of the cycle. A candidate would be any inverting glycosidase, which is released with the acid group deprotonated and the basic group protonated [Figure 1(b)], although no example in the glycosyl transfer area has yet been demonstrated (the best example is proline racemase, " in which two cysteines act, one as an acid and the other as a base). 

---