Thiol esters formation

After E2/ubiquitin thiol ester formation, the ubiquitin must be transferred to the substrate, which is sometimes another ubiquitin. An E3 is usually required for this reaction in vitro, and is always required in vivo. There are three known types of E3s the RING domain, HECT domain, and U-box (UED2 homology) families. RING and U-box E3s act as bridging factors for E2s and substrates, but HEGT E3s use a different mechanism, adding an extra step to the pathway (Section 5.6.3.3). 

Methods available before 1971 for the preparation of thiol esters are briefly summarized in a review article.4 Since then, several newer techniques have been developed, to meet a certain set of criteria required for recent synthetic operations. This development may be summarized as follows. Whenever an acid chloride is available, the reaction of the T1(I) salt of a thiolate of virtually any kind, including alkane-, benzene-, 2-benzothiazoline-, and 2-pyridinethiol, proceeds efficiently and near-quan-titatively. However, if selective thiol ester formation in the presence of hydroxy or other functional groups in the same molecule is required, three main procedures are available. First, reaction of an acid (1), with... 

Other methods that can be used to prepare thiol esters from carboxylic acids include the use of aryl thiocyanates,12 thiopyridyl chloroformate,13 2-fIuoro- V-methylpyridinium tosylate,14 1-hydroxybenzotriazole, 5 and boron thiolate.16 Direct conversion of 0-esters to 5-esters can also be effected via aluminum and boron reagents.17 However, the applicability of these 1217 and other more recent methods18 to the selective thiol ester formation discussed above has not been clearly defined. 

IV. Thiol-ester Formation in a-Keto Acid Oxidation. 196... 

E. Racker It is quite jslear that DPN is not involved directly in the glyoxylase reaction. I tried to point out in the talk that this is of particular interest. Glyoxylase may represent a primitive mechanism of thiol-ester formation which does not involve nucleotide participation and it would therefore be very valuable to know more about the detailed mechanisms. On the other hand, it is quite true that once lactoyl glutathione is formed, secondary nucleotide-linked reactions may take place. 

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