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Crystal structure of reduced thioredoxin reductase from Escherichia coli : Structural flexibility in the isoalloxazine ring of the flavin adenine dinucleotide cofactor
Author(s) -
Len Brett W.,
Williams Charles H.,
Ludwig Martha L.
Publication year - 1999
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1110/ps.8.11.2366
Subject(s) - flavin adenine dinucleotide , cofactor , flavoprotein , thioredoxin reductase , chemistry , flavin group , active site , crystallography , reductase , stereochemistry , ring (chemistry) , protonation , steric effects , thioredoxin , enzyme , biochemistry , organic chemistry , ion
Catalysis by thioredoxin reductase (TrxR) from Escherichia coli requires alternation between two domain arrangements. One of these conformations has been observed by X‐ray crystallography (Waksman G, Krishna TSR, Williams CH Jr, Kuriyan J, 1994, J Mol Biol 236 :800‐816). This form of TrxR, denoted FO, permits the reaction of enzyme‐bound reduced FAD with a redox‐active disulfide on TrxR. As part of an investigation of conformational changes and intermediates in catalysis by TrxR, an X‐ray structure of the FO form of TrxR with both the FAD and active site disulfide reduced has been determined. Reduction after crystallization resulted in significant local conformation changes. The isoalloxazine ring of the FAD cofactor, which is essentially planar in the oxidized enzyme, assumes a 34° “butterfly” bend about the N(5)‐N(10) axis in reduced TrxR. Theoretical calculations reported by others predict ring bending of 15‐28° for reduced isoalloxazines protonated at N(1). The large bending in reduced TrxR is attributed in part to steric interactions between the isoalloxazine ring and the sulfur of Cys138, formed by reduction of the active site disulfide, and is accompanied by changes in the positions and interactions of several of the ribityl side‐chain atoms of FAD. The bending angle in reduced TrxR is larger than that for any flavoprotein in the Protein Data Bank. Distributions of bending angles in published oxidized and reduced flavoenzyme structures are different from those found in studies of free flavins, indicating that the protein environment has a significant effect on bending.