Free energy hydrolysis, coenzyme

Would you expect the free energy of hydrolysis of aceto-acetyl-coenzyme A (see diagram) to be greater than, equal to, or less than that of acetyl-coenzyme A Provide a chemical rationale for your answer. 

Selected entries from Methods in Enzymology [vol, page(s)] Assay, 1, 611 3, 935-938 63, 33 separation by HPLC, 72, 45 extraction from tissues, 13, 439 formation of, 1, 486, 518, 585 5, 466 free energy of hydrolysis, 1, 694 substrate for the following enzymes [acetyl-coenzyme A acyl carrier protein transacylase, 14, 50 acetyl-coenzyme A carboxylase, 14, 3, 9 acetyl-coenzyme A synthetase, 13, 375 N-acetyltransferase, 17B, 805 aminoacetone... 

Thioesters, in which a sulfur atom replaces the usual oxygen in the ester bond, also have large, negative, standard free energies of hydrolysis. Acetyl-coenzyme A, or acetyl-CoA (Fig. 13-6), is one of many thioesters important in metabolism. The acyl group in... 

RGURE 13-6 Hydrolysis of acetyl-coenzyme A Acetyl-CoA is a thioester with a large, negative, standard free energy of hydrolysis Thioesters contain a sulfur atom in the position occupied by an oxygen atom in oxygen esters. The complete structure of coenzyme A (CoA, or CoASH) is shown in Rgure 8-41. 

On hydrolysis, acetyl coenzyme A makes available a large amount of free energy which can be coupled to other reactions (11.21). Such a transfer occurs in the citric acid cycle (Section 12.5) where oxaloacetic acid is converted into citric acid (11.22). 

Burton K (1955) The free energy change associated with the hydrolysis of the thiol ester bond of acetyl coenzyme A. Biochem J 59 44 6... 

There are two forms of the hydrolytic enzyme, one of which is specific for short chain esters like retinyl acetate, even though this ester does not occur naturally The other has maximum activity with retinyl palmitate as substrate but also hydrolyses other long chain esters. As in the hydrolysis of cholesteryl esters, the enzyme is not just a non-specific esterase, but has quite definite specificity for retinyl esters. In vitamin A deficiency, the activity of the enzyme increases one hundred fold. The esterification enzyme resembles the low energy cholesteryl esterase in that neither ATP nor coenzyme A appear to take part in the reaction nor are free fatty acids or acyl-CoA thiolesters incorporated into retinyl esters. One of the major problems in this area of research is to identify the acyl donor, which may be, as in plasma cholesteryl ester biosynthesis, a phospholipid. 

---