Thiolester conjugation

Structure of a conjugating enzyme-ubiquitin thiolester intermediate reveals a novel role for the ubiquitin tail. Structure 2001, 9, 897-904. 

The reagents for the next step are both disguised and are both unmasked by NaOH. thiolester is hydrolysed to the free thiolate and the nitrocompound eliminates acetate in an E reaction. An excellent conjugate addition follows both nucleophile (thiolate) and Michael acce (nitroalkene) are ideal. 

The acyl-CoA conjugates thus formed are seldom excreted, but they can be isolated and characterized relatively easily in in vitro studies. They may also be hydrolyzed back to the parent acid by thiolester hydrolases (EC... 

In contrast to aliphatic esters or lactones which only show end absorption around 230 nm, thiolesters exhibit a strong absorption band with X 235 nm, corresponding to a n-n transition [16]. Conjugation of the C(0)S group with an additional C=C double bond shifts the maximum to 265-270 nm as illustrated for 2(3//)-thiophenones [17] or 2(5/I)-thiophenones [18] (Scheme 2). These latter compounds exhibit an additional n-ci absorption band at X 320 nm. 

Another reaction that goes better with thiol esters than with ordinary esters is enolization. This is an equilibrium reaction and the enol has lost the conjugation present in the ester. Again, a thiol ester has less to lose so is more enolized, and it is the enolization of thiolesters of coenzyme A that we are now going to discuss. 

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