Acid anhydrides, group transfer potential

Acyl residues are usually activated by transfer to coenzyme A (2). In coenzyme A (see p. 12), pantetheine is linked to 3 -phos-pho-ADP by a phosphoric acid anhydride bond. Pantetheine consists of three components connected by amide bonds—pantoic acid, alanine, and cysteamine. The latter two components are biogenic amines formed by the decarboxylation of aspartate and cysteine, respectively. The compound formed from pantoic acid and p-alanine (pantothenic acid) has vitamin-like characteristics for humans (see p. 368). Reactions between the thiol group of the cysteamine residue and carboxylic acids give rise to thioesters, such as acetyl CoA. This reaction is strongly endergonic, and it is therefore coupled to exergonic processes. Thioesters represent the activated form of carboxylic adds, because acyl residues of this type have a high chemical potential and are easily transferred to other molecules. This property is often exploited in metabolism. 

In the case of methanol addition, it was attempted to determine the amount of methoxyl end-groups in the polymer. It was found that one mole of methanol per mole of initiator was incorporated into the polymer and no more. It could not be ascertained from the data whether the methoxyl groups were ends of the polymer chain, or whether a reaction product of SnCl4 with one mole of methanol was quantitatively absorbed on the polymer chain. Acetate end-groups could not be introduced by using acetic anhydride as a potential chain transfer E ent. Formic acid in small amounts, unlike water and methanol, sdso did not cause any decrease in DP in larger than 1 1 amounts (with respect to initiator). 

Bisphosphoglycerate is an acyl phosphate, which is a mixed anhydride of phosphoric acid and a carboxylic acid. Such compounds have a high phosphoryl-transfer potential one of its phosphoryl groups is transferred to ADP in the next step in glycolysis. 

A perusal of current biochemistry texts suggests that the use of the squiggle has largely died out, but the concept of a group potential and the importance of these phosphate anhydride bonds in energy storage and transfer is universally accepted. Lipmann shared the Nobel Prize in Physiology/Medicine in 1953 with Hans Krebs of the Krebs (citric acid) cycle. His Nobel citation read in part,... 

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