An investigation of the ligand-binding properties of XXXX @entity3542 reductase.

@entity796 @entity3793 @entity3542 @entity1853 @entity3789 @entity8793
The low-temperature e.p.r. and m.c.d. (magnetic-circular-dichroism) spectra of @entity796 @entity3542 reductase, together with those of its partially and fully @entity3789 -bound derivatives, were investigated. The m.c.d. spectra in the range 600-2000 nm indicate that the native axial ligands to haem c are @entity3793 and @entity1853 , and furthermore that it is the @entity1853 ligand that must be displaced before @entity3789 binding at this haem. The m.c.d. spectra in the range 1000-2000 nm contain no charge-transfer bands arising from low-spin ferric haem d1, a @entity8793 . New optical transitions in the region 700-850 nm were found for the @entity3789 adduct of haem d1. The g-values of haem d1 in the native enzyme are 2.51, 2.43 and 1.71, suggesting co-ordination by two @entity3793 ligands in the oxidized state. There is clear evidence in the e.p.r. data of an interaction between the c and d1 haem groups. This is not apparent in the optical spectra. The results are interpreted in terms of haem groups that are remote from each other, their interaction being mediated through protein conformational changes. The possible implications of this in relation to reduction processes catalysed by the enzyme are considered.
