THE SUBSTRATE-BINDING SITE IN CU NITRITE REDUCTASE AND ITS SIMILARITYTO ZN CARBONIC-ANHYDRASE

Here we investigate the structure of the two types of copper site in n itrite reductase from Alcaligenes xylosoxidans, the molecular organisa tion of the enzyme when the type-2 copper is absent, and its mode of s ubstrate binding. X-ray absorption studies provide evidence for a four th ligand at the type-2 Cu, that substrate binds to this site and indi cates that this binding does not change the type-1 Cu centre. The subs trate replaces a putative water ligand and is accommodated by a length ening of the Cu-histidine bond by approximately 0.08 Angstrom. Modelli ng suggests a similarity between this unusual type-2 Cu site and the Z n site in carbonic anhydrase and that nitrite is anchored by hydrogen bonds to an unligated histidine present in the type-2 Cu cavity.