Ok, so you understand the catalytic triad of serine proteases. This mechanism shown is for amide hydrolysis. The mechanism for ester hydrolysis is exactly the same, except it is an oxygen instead of a nitrogen. I don't know the structure of the alcalase enzyme but I am certain this is what is involved. In your mechanism the SER-HIS are shown as free amino acids. In alcalase they are residues. Not sure becuase I don't know your exact procedure but sodium bicarbonate can be a buffer or initiation base to ensure the solution is basic.