Correlation of substrate-stabilization patterns with proposed mechanisms for three nucleoside phosphorylases

Area of Science:

• Biochemistry
• Enzymology
• Molecular Biology

Background:

• Uridine phosphorylase (UP), thymidine phosphorylase (TP), and purine-nucleoside phosphorylase (PNP) are key enzymes in nucleotide metabolism.
• Previous kinetic analyses suggested sequential reaction mechanisms for these phosphorylases, with varying substrate addition orders.
• Understanding enzyme stabilization by substrates can provide insights into their catalytic mechanisms and conformational states.

Purpose of the Study:

• To investigate the substrate-stabilization effects on three bacterial phosphorylases: uridine phosphorylase, thymidine phosphorylase, and purine-nucleoside phosphorylase from Escherichia coli.
• To correlate substrate-stabilization patterns with previously proposed sequential reaction mechanisms for these enzymes.
• To determine the relative effectiveness of different substrate types (nucleosides, aglycones, phosphate, pentose-1-phosphate esters) in stabilizing these enzymes.

Main Methods:

• Heat-inactivation experiments were employed to assess the stabilization of uridine phosphorylase, thymidine phosphorylase, and purine-nucleoside phosphorylase by various substrates.
• Enzymes were incubated with different substrate classes, including nucleosides, aglycones, phosphate, and pentose-1-phosphate esters, followed by thermal challenge.
• The degree of enzyme activity remaining after heat treatment was measured to quantify stabilization.

Main Results:

• Nucleoside substrates stabilized uridine phosphorylase and purine-nucleoside phosphorylase, but not thymidine phosphorylase.
• Aglycone substrates demonstrated a stabilizing effect exclusively on uridine phosphorylase.
• Phosphate and pentose-1-phosphate ester substrates stabilized all three enzymes, with pentose-1-phosphate esters being more effective than phosphate.

Conclusions:

• The observed substrate-stabilization patterns are consistent with the proposed sequential reaction mechanisms for uridine phosphorylase, thymidine phosphorylase, and purine-nucleoside phosphorylase.
• Different substrate classes bind to distinct sites on the enzymes, influencing their thermal stability in a manner reflective of their catalytic roles.
• The findings support the hypothesis that substrate binding order and interactions are critical determinants of enzyme stability and mechanism.