EC 6. 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.
EC 6.1 Forming Carbon-Oxygen Bonds
To this subclass belong the enzymes acylating a transfer RNA with the corresponding amino acid (amino-acid-tRNA ligases) (EC 6.1.1).
EC 6.2 Forming Carbon-Sulfur Bonds
To this subclass belong the enzymes synthesizing acyl-CoA derivatives.
EC 6.3 Forming Carbon-Nitrogen Bonds
To this subclass belong the amide synthases (EC 6.3.1), the peptide synthases (EC 6.3.2), enzymes forming heterocyclic rings (EC 6.3.3), enzymes using glutamine as amido-N-donor (EC 6.3.5) and a few others (EC 6.3.4).
EC 6.4 Forming Carbon-Carbon Bonds
These are the carboxylating enzymes, mostly biotinyl-proteins.
EC 6.5 Forming Phosphoric Ester Bonds
These include the enzymes restoring broken phosphodiester bonds in the nucleic acids (often called repair enzymes).
EC 6.6 Forming Nitrogen�Metal Bonds
This covers metal chelation of a tetrapyrrole ring system.
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