An analysis of the substrate-induced rate effect in the phosphoglucomutase system

Area of Science:

• Biochemistry
• Enzyme kinetics
• Protein-ligand interactions

Background:

• Rabbit muscle phosphoglucomutase facilitates the transfer of a phosphate (PO3) group.
• The enzyme exhibits a significant rate difference between water and its normal substrate, glucose 1-phosphate.

Purpose of the Study:

• To investigate the 'substrate-induced rate effect' by dissecting the contributions of different substrate moieties.
• To quantify the impact of various factors on the PO3 transfer rate from phosphoglucomutase.

Main Methods:

• Studied the rate of PO3 transfer to water and various alcoholic acceptors under different conditions.
• Investigated the effect of binding phosphate-containing molecules (ethyl phosphate, inorganic phosphite) and substrate fragments (xylose) to the enzyme.
• Utilized intramolecular transfer by creating linked acceptor-PO3 group molecules.

Main Results:

• Binding of a second PO3 group increased the rate of PO3 transfer to water by up to 1000-fold.
• Alcoholic acceptors, particularly in the presence of bound phosphite, further enhanced transfer efficiency.
• Binding of the xylosyl moiety of the substrate also increased PO3 transfer rates.

Conclusions:

• The large rate difference between water and glucose 1-phosphate reactions is attributable to four main factors: PO4 binding, alcohol vs. water acceptance, nucleophile binding, and bridging effects.
• These factors collectively explain the substrate-induced rate enhancement observed in phosphoglucomutase catalysis.