Lyase (source code)
= Lyase
{title2=EC 4}
A <lyase> is an <enzyme> classified by cleavage through means other than <hydrolysis> or <oxidation>. Such reactions commonly eliminate a group to create a <double bond> or a ring; the reverse direction adds a group across the corresponding structural feature. Lyases form EC class 4, with subclasses organized partly by the bonds affected.<Enzyme Nomenclature by the IUBMB>[EC 4 Introduction, definition and naming; EC 4 subclass table]{ref}
The involvement of <water> alone does not make a reaction hydrolysis. <Fumarate hydratase>, also called fumarase, interconverts <fumarate> plus water with the (S) form of <malate>. In the water-removing direction, it generates the double bond of fumarate. In the reverse direction, water adds across that bond.<Fumarate hydratase in IUBMB Enzyme Nomenclature>[EC 4.2.1.2, Reaction and accepted/other names]{ref} The classification concerns the transformation, not merely whether water appears somewhere in its equation.
An invented bookkeeping example begins with $5\,\mu\mathrm{mol}$ of (S)-malate. If $3\,\mu\mathrm{mol}$ undergoes the water-eliminating conversion, $2\,\mu\mathrm{mol}$ malate remains and $3\,\mu\mathrm{mol}$ fumarate is formed. No carbon atom is removed from the four-carbon skeleton in this reaction. This is a stipulated extent, not a prediction of the position of <chemical equilibrium> or the reaction speed.
The distinction can also arise in a multistep reaction. <Ribonuclease T1> first produces a cyclic phosphate end by a phosphorus–oxygen lyase step and subsequently hydrolyses that cyclic product. Its classification emphasizes the primary cleavage reaction, even though the overall process also includes hydrolysis.<Ribonuclease T1 in IUBMB Enzyme Nomenclature>[EC 4.6.1.24, Reaction 1a–1b and classification history]{ref}
Thus a <lyase> classification gives useful chemical information but does not uniquely specify a protein's structure, substrate range or physiological role. The actual reaction and its experimentally observed direction remain essential to interpreting the enzyme's name.
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