Thioester Bond Energy

Coenzyme A forms high-energy thioester bonds with acyl groups. The energy-rich nature of thioesters, as compared with ordinary esters, is related primarily to resonance stabilization (Figure 14.9). Most esters have two resonance forms (Figure 14.9). Stabilization involves -electron overlap, giving partial double-bond character to the C-O link.

In thioesters, the larger atomic size of S (as compared with O) reduces the -electron overlap between C and S, so that the C=S structure does not contribute significantly to resonance stabilization. As a result, the thioester is destabilized relative to an ester, thus releasing more energy on hydrolysis.

The lack of double-bond character in the C-S bond of acyl-CoAs makes this bond weaker than the corresponding C-O bond in ordinary esters, thus making the thioalkoxide ion, a good leaving group in nucleophilic displacement reactions. Consequently, the acyl group is readily transferred to other metabolites, as occurs in the first reaction of the citric acid cycle (Figure 14.3).

See also: Coenzyme A