The structure of horseradish peroxidase C characterized as a molten globule state after Ca(2+) depletion

The structure and activity of native horseradish peroxidase C (HRP) is stabilized by two bound Ca(2+) ions. Earlier studies suggested a critical role of one of the bound Ca(2+) ions but with conflicting conclusions concerning their respective importance. In this work we compare the native and totally Ca(2+)-depleted forms of the enzyme using pH-, pressure-, viscosity- and temperature-dependent UV absorption, CD, H/D exchange-FTIR spectroscopy and by binding the substrate benzohydroxamic acid (BHA). We report that Ca(2+)-depletion does not change the alpha helical content of the protein, but strongly modifies the tertiary structure and dynamics to yield a homogeneously loosened molten globule-like structure. We relate observed tertiary changes in the heme pocket to changes in the dipole orientation and coordination of a distal water molecule. Deprotonation of distal His42, linked to Asp43, itself coordinated to the distal Ca(2+), perturbs a H-bonding network connecting this Ca(2+) to the heme crevice that involves the distal water. The measured effects of Ca(2)(+) depletion can be interpreted as supporting a structural role for the distal Ca(2+) and for its enhanced significance in finetuning the protein structure to optimize enzyme activity.