Esterases free energy

Since the hydrolysis of most substrates is a highly exergonic reaction, the position at equilibrium is greatly in favor of the products of the split. Furthermore, the presence of a great excess of water, common in most biological systems, favors the hydrolytic reaction as opposed to the synthetic reaction. Some esterases, however, catalyze reactions with a free energy close to zero, and so the equilibrium mixture will contain appreciable quantities of reactant. If, on the other hand, the elements of phosphoric acid, rather than water, are inserted, then the energy released upon... 

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. 

---