Histone monoaminylation is catalyzed by
transglutaminase 2 (TGM2) in a calcium-dependent manner, and relies upon the intracellular bioavailability of monoamine substrates. Generally, protein monoaminylation occurs in the
cytoplasm; however, histone monoaminylation only occurs within the
nucleus. The active site itself is composed of a well conserved
catalytic triad (Cys277–His335–Asp358) situated within a substrate binding channel, which is bordered by two conserved residues (Trp241 and Trp332) that facilitate catalysis through stabilization of the transition state. Once intracellular Ca2+ binds to
TGM2 and exposes the substrate binding channel, the glutamine residue of the substrate protein (i.e.,
histone H3) is free to enter the enzyme active site. As a
transamidation reaction, the mechanism for histone monoaminylation can be summarized in two parts: an initial thioester formation, followed by isopeptide bond formation.
Fig. 1 Mechanism for Histone Monoaminylation Monoaminylation is a two step, Ca2+-dependent reaction in which
TGM2 catalyzes the covalent attachment of a monoamine (i.e.,
dopamine,
serotonin,
histamine) onto the
glutamine residue of
histone proteins.
(A) The
catalytic cysteine residue (Cys277) of TGM2 facilitates an initial acyl transfer reaction, which is ultimately followed by
isopeptide bond formation
(B). When intracellular Ca2+ and monoamine concentrations are sufficient, TGM2-catalyzed monoaminylation of
histone H3 can occur. First, the
catalytic cysteine residue (Cys277) within the TGM2 active site nucleophilically attacks the 𝛾-carboxamido group of the glutamine residue in an acyl transfer reaction (
Fig. 1A), forming a thioester intermediate and releasing one molecule of ammonia (NH3) as a result. Next, the deprotonated primary amine of the monoamine substrate nucleophilically attacks the 𝛾-thioester group of the intermediate, forming a stable isopeptide bond and ultimately releasing the enzyme (
Fig. 1B). == Function ==