Why does the amino acid serine form a catalytic triad while

Why does the amino acid serine form a catalytic triad while the amino acid cysteine form a catalytic dyad?

Serine versus Cysteine proteases Asp O.. O Ser N His Cys His Catalytic Triad Catalytic Dyad

Solution

Serine is also known as 3C proteases as they are formed by a combination of 3amino acids(histidine, asparticacid ,serine).These three amino acids form a triad where the serine is active and it acts a nucelophile during the proces of catalysis by donating and electron to the carbonul carbon present in the peptide bond.

On the other hand cysteine produced a dyad which comprises of cys and his(cysteine and histidine)where the Cys sulfhydryl acts as a nucleophile to attack on the carbonyl group of the peptide bond to be hydrolized.