Phosphorylation of protein occurs on many residues, but mainly serine (Ser; 90-95%), threonine (Thr; 5-10%) and tyrosine (Tyr; <10%) are phosphorylated. Phosphorylation does not always lead to increase in enzyme activity, and in some cases may actually lead to decrease in activity (e.g. glycogen synthase). Sometimes positive and negative regulatory phosphorylation sites are present in the same enzyme, so that same enzyme may need two kinases and two phosphatases.
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