Question

Q: Explain why Co(II) can be substituted for Zn(II) in many hydrolase enzymes to give an active enzyme, whereas Fe(II) and Ni(II) are not effective substitutes.

Answer 1

I know that Zn can activate water (H2O-> OH-) for substrate attack and it can polarize P-O and C-O bonds to increase substrate acidity. Zn has a range or coordination numbers (4-6) and being an intermediate soft/hard acid it can bind both soft and hard bases, it is kinetically labile due to zero LSFE (because d10)..

Points to my answer so far: In terms of size Co(II) and Zn(II) are approx. the same, the other differ by at about 10 pm.

I can't really think of other reasons that don't cancel each other out when comparing, for example: reduction potentials don't support the idea of substitution (Co(II) and Ni(II) are about the same).. Fe(II) and Co(II) are both borderline hard/soft acids.

btw Fe(II) d6 , Co(II) d7, Ni(II) d8