General Base-General Acid Catalysis in Human Histone Deacetylase 8.

Histone deacetylases (HDACs) regulate cellular processes such as differentiation and apoptosis and are targeted by anticancer therapeutics in development and in the Clinic. HDAC8 is a metal-dependent class I HDAC and is proposed to use a general acid base catalytic pair in the mechanism of amide bond hydrolysis. Here, we report site-directed mutagenesis and enzymological measurements to elucidate the catalytic mechanism of HDAC8. Specifically, we focus on the catalytic function of Y306 and the histidine-aspartate dyads H142-D176 and H143-D183. Additionally, we report X-ray crystal structures of four representative HDAC8 mutants: D176N, D176N/Y306F, D176A/Y306F, and H142A/Y306F. These structures provide a useful framework for understanding enzymological measurements. The pH dependence of k(cat)/K-M for wild-type Co(II)-HDAC8 is bell-shaped with two pK(a) values of 7.4 and 10.0. The upper pK(a) reflects the ionization of the metal-bound water molecule and shifts to 9.1 in Zn(II)-HDAC8. The H142A mutant has activity 230-fold lower than that of wild-type HDAC8, but the pK(al) value is not altered. Y306F HDAC8 is 150-fold less active than the wild-type enzyme; crystal structures show that Y306 hydrogen bonds with the zinc-bound substrate carbonyl, poised for transition state stabilization. The H143A and H142A/H143A mutants exhibit activity that is >80000-fold lower than that of wild type HDAC8; the buried D176N and D176A mutants have significant catalytic effects, with more subtle effects caused by D183N and D183A. These enzymological and structural studies strongly suggest that H143 functions as a single general base general acid catalyst, while H142 remains positively charged and serves as an electrostatic catalyst for transition state stabilization.