[mrlucky0]

The problem:

The R-group of Cysteine  (Cys) is often found bound to various Lewis acids (such as Zn2+) in the active
site of enzymes. A certain Cys in the active site of an enzyme was determined to have a pKa
of 6.6. What is the percent ionization of this particular Cys residue at physiological pH?
Draw the two forms of the Cys residue.

I can't figure out what the two forms of the Cys residue are. Since the pH > pKa, I'm guessing that one form of the amino acid must be positively charged and the other form should be neutral.

With the Cys sulfur having 2 lone pairs, how does Zn2+ bond with Cys? If I make a double bond, the sulfur is still neutral. My other guess is that deprotonation of the R group occurs. Any ideas?

[optimusprime]

Why would a zinc cation be found associated with a cationic residue?

Thiols are fairly acidic, so you'll be looking at a deprotonated anion.

From that it's simple Henderson-Hasselbach.

7.4 = 6.6 + log([anion]/[neutral])

[Yggdrasil]

In many proteins (so called zinc finger proteins), Zn²⁺ ions are tetrahedrally coordinated by two histidines and two cysteines, so Zn binding to one cysteine will not fully coordinate the Zn.