The key catalytic amino acids of the chymotrypsin active site in the free enzyme are: A. a serine, an aspartic acid with a deprotonated side chain and a histidine with a protonated side chain. B. a serine, an aspartic acid with a deprotonated side chain and a histidine with a deprotonated side chain. C. a serine, an aspartic acid with a protonated side chain and a histidine with a protonated side chain. D. a serine, an aspartic acid with a protonated side chain and a histidine with a deprotonated side chain. E. a serine, an aspartic acid, and a histidine; each of which has a deprotonated side chain.