3.4.23.1 glycoprotein - 3.4.23.1 phosphoprotein contains 0.37 molecules phosphorous per molecule 3.4.23.1 proteolytic modification zymogen: pepsinogen 3.4.23.1 proteolytic modification activation of pepsinogen to pepsin occurs between pH 1.0 and pH 4.0 for wild-type, between pH 1.0 and pH 6.0 for N-glycosylated mutants 3.4.23.1 side-chain modification glycoprotein