Yeast and some other organisms produce proteins that closely resemble O6-alkylguanine-DNA alkyltransferase but lack a Cys at

Question:

Yeast and some other organisms produce proteins that closely resemble O6-alkylguanine-DNA alkyltransferase but lack a Cys at the active site.
(a) Explain why these proteins cannot remove an alkyl group attached to guanine.
(b) These proteins do protect cells against DNA alkylation damage. In which type of repair process are these proteins most likely to participate?