Which enzyme directly adds a phosphate group to a substrate by breaking bonds within the substrate?

Which enzyme directly adds a phosphate group to a substrate by breaking bonds within the substrate?

• A. Kinase
• B. Phosphatase
• C. Phosphorylase
• D. Ligase

Answer: (C)

Phosphorylase stands out because it uses inorganic phosphate to attach a phosphate group directly to a substrate while cleaving a bond within that substrate. In glycogen breakdown, glycogen phosphorylase adds Pi to the glucose units, breaking the glycosidic bond and producing glucose-1-phosphate. This mechanism is distinct from kinases, which transfer a phosphate from ATP to a substrate without necessarily cleaving the substrate’s own bonds, and from phosphatases, which remove phosphate groups. Ligases form new bonds between molecules and don’t primarily act by phosphorylating substrates via internal bond cleavage.

Phosphorylase stands out because it uses inorganic phosphate to attach a phosphate group directly to a substrate while cleaving a bond within that substrate. In glycogen breakdown, glycogen phosphorylase adds Pi to the glucose units, breaking the glycosidic bond and producing glucose-1-phosphate. This mechanism is distinct from kinases, which transfer a phosphate from ATP to a substrate without necessarily cleaving the substrate’s own bonds, and from phosphatases, which remove phosphate groups. Ligases form new bonds between molecules and don’t primarily act by phosphorylating substrates via internal bond cleavage.