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Difference from Transferases[edit]

The reaction presented:

Ab + C → A–C + b

Seems not much different from the one corresponding to transferase

A–X + B → A + B–X

especially if rename the participants:

B–A + C → B + C–A

The distinction between the two classes likely should be highlighted, given the similarity in the presented mechanisms. I can somewhat get the impression that the difference between the two classes is the size/importance of the transferred group, but from the current phrasing I wouldn't be sure of it. Nor would I know at what size group would a ligase become a transferase, nor if the group size is the only difference between the two groups of enzymes. -- (talk) 19:41, 22 November 2010 (UTC)

According to IUBMB Commission on enzyme nomenclature, EC 6 Ligase constitutes the following:

"EC6 Ligases: Ligases are enzymes that catalyse the joining of two molecules with concomitant hydrolysis of the diphosphate bond in ATP or a similar triphosphate. 'Ligase' is commonly used for the common name, but, in a few cases, 'synthase' or 'carboxylase' is used. 'Synthetase' may be used in place of 'synthase' for enzymes in this class."

Synthases that do not require ATP hydrolysis catalyse the reverse of the lyase reaction, and belong in EC 4 and are not ligases. At one time this was clearly distinguished by the difference in name 'synthase' and 'synthetase', but too many exceptions have crept in to the literature.

"EC 4. Lyases

Lyases are enzymes cleaving C-C, C-O, C-N and other bonds by other means than by hydrolysis or oxidation. They differ from other enzymes in that two substrates are involved in one reaction direction, but only one in the other direction. When acting on the single substrate, a molecule is eliminated and this generates either a new double bond or a new ring. The systematic name is formed according to 'substrate group-lyase'. In common names, expressions like decarboxylase, aldolase, etc. are used. 'Dehydratase' is used for those enzymes eliminating water. In cases where the reverse reaction is the more important, or the only one to be demonstrated, 'synthase' may be used in the name."

In general, ligase reactions are reversed hydrolysis reactions. Since hydrolysis is almost invariably thermodynamically favourable, an energy source is needed for the reversal. The reaction represented below is trying to be too generic to the point of causing confusion.

Ab + C → A–C + b

A better version would be:

A-OH + B-H + ATP → A-B + ADP + Pi or A-OH + B-H + ATP → A-B + AMP + PPi

The H2O formed in the ligase reaction is consumed by the hydrolysis of ATP. (talk) 19:55, 17 February 2011 (UTC)