Lysine or histidine? examining the role of the proximal ligand at the active heme center of cytochrome c nitrite reductase

Abstract The identity of the proximal ligand of its catalytic heme 1 center - lysine or histidine - may directly affect how efficiently pentaheme cytochrome c nitrite reductase catalyzes the six-electron reduction of nitrite (NO 2 − ) to ammonium (NH 4 + ) and may also alter substrate specificity. This reaction is part of the pathway of dissimilatory nitrate reduction to ammonium, which competes with denitrification and avoids the formation of unreactive dinitrogen (N 2 ). In this study, we compare the biochemical properties and structures of cytochrome c nitrite reductase from Campylobacter jejuni with those of the enzyme from Wolinella succinogenes and examine its Lys134His variant. Notably, both the Campylobacter nitrite reductase and the Lys134His variant possess a catalytic heme 1 center coordinated by histidine as the proximal ligand, whereas the native Wolinella enzyme is coordinated by lysine. Despite this difference, both enzymes catalyze the reduction of NO 2 − to NH 4 + with high specific activities. To enable biochemical and structural characterization of these enzymes, we developed optimized protocols for protein expression and purification. The data we present allow, for the first time, a direct analysis of the influence of histidine or lysine as the proximal nitrogen ligand at the catalytic heme 1 center of pentaheme cytochrome c nitrite reductase within otherwise similar protein backgrounds.