Toward Structure Determination of Methyltetrahydrofolate: Corrinoid/iron Sulfur Protein Methyltransferase From Clostridium Thermoaceticum. Tzanko I. Doukov¹, Stephen W. Ragsdale², and John J. Stezowski¹, Department of ¹Chemistry, ²Biochemistry, University of Nebraska-Lincoln, Lincoln, NE 68588-0304

Methyltetrahydrofolate: corrinoid/iron-sulfur protein methyltransferase (MeTr) transfers the N⁵-methyl group of (6S)-methyltetrahydrofolate to the cobalt(I) of a corrinoid/iron sulfur protein in the reductive acetyl-CoA pathway for CO₂ fixation in Clostridium thermoaceticum (Ragsdale, 1991).

MeTr and its selenomethionine substitute has been crystallized and characterized by X-ray diffraction measurements (Doukov et al., 1995). Crystals of the enzyme displayed orthorhombic symmetry and belonged to space group P2₁2₁2₁. Unit cell dimensions are: a=64.6 (, b=89.6 (, c=114.9 ( and there are eight monomeric molecules per unit cell (two per asymmetric unit). Longest axis shrunk ~ 4% upon cryocooling. The crystals scattered to approximately 2.5 ( resolution. Four datasets at wavelengths around the Se absorption edge were collected at beam line 1-5 (MAD) at SSRL. These were reduced with the HKL package: DENZO and SCALEPACK. Despite the good quality of the data (~90% I>3( and 98+% overall completeness; R_sym ~ 6-8 %), problems still exist in localizing 12 Se sites per monomer. We will report the latest developments during the meeting.