Water-assisted dual mode cofactor recognition by HhaI DNA methyltransferase

Swaminathan, Chittoor P and Sankpal, Umesh T and Rao, Desirazu N and Surolia, Avadhesha (2002) Water-assisted dual mode cofactor recognition by HhaI DNA methyltransferase. In: Journal of Biological Chemistry, 277 (6). pp. 4042-4049.

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Abstract

Energetically competent binary recognition of the cofactorS-adenosyl-l-methionine (AdoMet) and the product S-adenosyl-l-homocysteine (AdoHcy) by the DNA (cytosine C-5) methyltransferase (M.HhaI) is demonstrated herein. Titration calorimetry reveals a dual mode, involving a primary dominant exothermic reaction followed by a weaker endothermic one, for the recognition of AdoMet and AdoHcy by M.HhaI. Conservation of the bimodal recognition in W41I and W41Y mutants of M.HhaI excludes the cation−π interaction between the methylsulfonium group of AdoMet and the π face of the Trp41 indole ring from a role in its origin. Small magnitude of temperature-independent heat capacity changes upon AdoMet or AdoHcy binding by M.HhaI preclude appreciable conformational alterations in the reacting species. Coupled osmotic-calorimetric analyses of AdoMet and AdoHcy binding by M.HhaI indicate that a net uptake of nearly eight and 10 water molecules, respectively, assists their primary recognition. A change in water activity at constant temperature and pH is sufficient to engender and conserve enthalpy−entropy compensation, consistent with a true osmotic effect. The results implicate solvent reorganization in providing the major contribution to the origin of this isoequilibrium phenomenon in AdoMet and AdoHcy recognition by M.HhaI. The observations provide unequivocal evidence for the binding of AdoMet as well as AdoHcy to M.HhaI in solution state. Isotope partitioning analysis and preincubation studies favor a random mechanism for M.HhaI-catalyzed reaction. Taken together, the results clearly resolve the issue of cofactor recognition by free M.HhaI, specifically in the absence of DNA, leading to the formation of an energetically and catalytically competent binary complex.

Item Type:	Journal Article
Publication:	Journal of Biological Chemistry
Publisher:	The American Society for Biochemistry and Molecular Biology
Additional Information:	Copyright of this article belongs to The American Society for Biochemistry and Molecular Biology.
Keywords:	Enthalpy-Entropy Compensation;Adenosyl-L-Methionine;Dihydrofolate-Reductase;Kinetic Mechanism;Crystal-Structure;Escherichia-Coli;Osmotic-Stress;Bound Water;Target Base;Binding
Department/Centre:	Division of Biological Sciences > Biochemistry
Date Deposited:	19 Nov 2009 09:25
Last Modified:	19 Sep 2010 05:00
URI:	http://eprints.iisc.ac.in/id/eprint/18240

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