Anion- and pH-linked conformational transition in horseradish peroxidase

In a previous study we have shown that bringing horseradish peroxidase to p H 3.0 induces a spectroscopic transition (G. Smulevich et al., Biochemistry 36 (1997) 640). We have extended the investigation on this pH-Iinked confo rmational change to different experimental conditions, such as (i) in phosp hate alone, (ii) in HCl alone and (iii) in phosphate + NaCl. The data obtai ned allow a number of conclusions to be drawn, namely: (a) the exposure to ph 3.0 under all conditions brings about an alteration of the distal portio n of the heme pocket, leading to the rapid formation of a hexa-coordinated species; (b) only in the presence of phosphate is the hexa-coordination fol lowed by a slow cleavage (or severe weakening) of the proximal Fe-His bond, and (c) the rate of this second process is speeded up in the presence of C l- ions. Such observations underline the presence of a communication pathwa y between the two opposite sides of the heme pocket, such that any alterati on of the structural arrangement on one side of the heme cavity is transmit ted to the other, inducing a corresponding conformational change. (C) 2000 Elsevier Science Inc. All rights reserved.