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In [[biochemistry]], a '''ligase''' is an [[enzyme]] that can [[catalyze]] the joining of two large molecules by forming a new [[chemical bond]], usually with accompanying [[hydrolysis]] of a small pendant chemical group on one of the larger molecules or the enzyme catalyzing the linking together of two compounds, e.g., enzymes that catalyze joining of C-O, C-S, C-N, etc. In general, a ligase catalyzes the following reaction:
:Ab + C → A–C + b
or sometimes
:Ab + cD → A–D + b + c + d + e + f
where the lowercase letters can signify the small, dependent groups. Ligase can join two complementary fragments of nucleic acid and repair single stranded breaks that arise in double stranded DNA during replication.
The common names of ligases often include the word "ligase", such as [[DNA ligase]], an enzyme commonly used in molecular biology [[laboratory|laboratories]] to join together [[DNA]] fragments. Other common names for ligases include the word "synthetase", because they are used to [[biosynthesis|synthesize]] new molecules.
Biochemical nomenclature has sometimes distinguished synthetases from [[synthase]]s and sometimes treated the words as [[synonym]]s. Under one definition, synthases ''do not'' use energy from [[nucleoside triphosphate]]s (such as ATP, GTP, CTP, TTP, and UTP), whereas synthetases ''do'' use nucleoside triphosphates. It is also said that a synthase is a [[lyase]] (a lyase is an enzyme that catalyzes the breaking of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure) and does not require any energy, whereas a synthetase is a ligase (a ligase is an enzyme that binds two chemicals or compounds) and thus requires energy. However, the [[Joint Commission on Biochemical Nomenclature]] (JCBN) dictates that "synthase" can be used with any enzyme that catalyses synthesis (whether or not it uses nucleoside triphosphates), whereas "synthetase" is to be used synonymously.<ref>{{cite web|url= |title=Synthases and ligases | |date= |accessdate=July 28, 2013}}</ref>
Ligases are classified as '''EC 6''' in the [[Enzyme Commission number|EC number]] classification of enzymes. Ligases can be further classified into six subclasses:
*[[:Category:EC 6.1|EC 6.1]] includes ligases used to form carbon-oxygen bonds
*[[:Category:EC 6.2|EC 6.2]] includes ligases used to form carbon-sulfur bonds
*[[:Category:EC 6.3|EC 6.3]] includes ligases used to form carbon-nitrogen bonds (including [[argininosuccinate synthetase]])
*[[:Category:EC 6.4|EC 6.4]] includes ligases used to form carbon-carbon bonds
*[[:Category:EC 6.5|EC 6.5]] includes ligases used to form [[phosphoric ester]] bonds
*[[:Category:EC 6.6|EC 6.6]] includes ligases used to form nitrogen-metal bonds, as in the [[chelatase]]s
==Membrane-associated ligases==
Some ligases associate with [[biological membranes]] as [[peripheral membrane protein]]s or anchored through a single [[transmembrane helix]]<ref>[ Superfamilies of single-pass transmembrane ligases] in [[Membranome database]]</ref>, for example certain [[ubiquitin ligase]] related proteins.
==Etymology and pronunciation==
The word ''ligase'' uses [[classical compound|combining forms]] of ''[[wikt:lig-#Prefix|lig-]]'' (from the [[Latin language|Latin]] verb ''ligāre'', "to bind" or "to tie together") + ''[[wikt:-ase#Suffix|-ase]]'' (denoting an enzyme), yielding "binding enzyme".
==See also==
*[[DNA ligase]]
* [ EC 6 Introduction] from the Department of Chemistry at [[Queen Mary, University of London]]
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