[kelvone]

I was hoping someone could help me out.  The amino acid selenocysteine, found in the active site of various enzymes, is identical to cysteine except the sulfur atom is replaced by a selenium atom.  In terms of increasing the catalytic potential of an enzyme, selenocysteine is said to have increased nucleophillicity and increased electrophillicity relative to its sulfur containing counterpart (cysteine).  This seems like an oxymoron to me.  I'm not a chemist and it's been many years since I've taken any chemistry courses, and I was hoping someone could give me a concise explanation for how an element can be both a better nucleophile and electrophile.

Thank you