Anthocyanin acyltransferases: specificities, mechanism, phylogenetics, and applications

Anthocyanins are responsible for the orange to blue coloration of flowers, fruits, and leaves. They are beneficial to human health and widely used as food colorants. Anthocyanin acyltransferases (AATs) are the plant enzymes that catalyze the regiospecific acyl transfer from acyl-CoA to the sugar moiety of anthocyanins. AATs are classified on the basis of their acyl-donor specificity into two categories; i.e. aliphatic and aromatic acyltransferases. However, the acyl-acceptor specificity of AAT differs greatly with the enzyme. Primary structural analyses of several AATs revealed that AATs form a subfamily within the versatile acyltransferase family and share highly conserved sequences such as motif 1 (-His-Xaa3-Asp-) and motif 3 (-Asp-Phe-Gly-Trp-Gly-) with each other. It is proposed that AAT-catalyzed acyl transfer proceeds with a general acid/base mechanism, where the enzyme and both acyl donor and acyl acceptor form a ternary complex before catalysis can occur. The histidine and aspartic acid residues located at motifs 1 and 3, respectively, appear to play very important roles during the proposed general acid/base catalysis. AAT cDNAs have been expressed in heterologous systems, providing a basis for applications of AATs in biotechnology, such as flower color modification and food colorant production by metabolic engineering of anthocyanin biosynthesis in plants.