# Allosteric activation unveils protein-mass modulation of ATP phosphoribosyltransferase product release

## TL;DR

This study shows how changing the mass of a protein affects its catalytic activity when regulated by another protein, revealing insights into enzyme dynamics.

## Contribution

The study reveals a mass-dependent effect on product release in an allosterically regulated enzyme, linking protein mass to thermal motions.

## Key findings

- Isotope-labeled HisGS shows mass-dependent decreases in catalytic rate when activated by HisZ at low temperatures.
- The effect is tied to product release, not the chemical step, and is abolished by disrupting enzyme-product interactions.
- Allosteric modulation influences thermal motions beyond the chemical step in the catalytic cycle.

## Abstract

Heavy-isotope substitution into enzymes slows down bond vibrations and may alter transition-state barrier crossing probability if this is coupled to fast protein motions. ATP phosphoribosyltransferase from Acinetobacter baumannii is a multi-protein complex where the regulatory protein HisZ allosterically enhances catalysis by the catalytic protein HisGS. This is accompanied by a shift in rate-limiting step from chemistry to product release. Here we report that isotope-labelling of HisGS has no effect on the nonactivated reaction, which involves negative activation heat capacity, while HisZ-activated HisGS catalytic rate decreases in a strictly mass-dependent fashion across five different HisGS masses, at low temperatures. Surprisingly, the effect is not linked to the chemical step, but to fast motions governing product release in the activated enzyme. Disruption of a specific enzyme-product interaction abolishes the isotope effects. Results highlight how altered protein mass perturbs allosterically modulated thermal motions relevant to the catalytic cycle beyond the chemical step.

ATP phosphoribosyltransferase is a multi-protein complex where the catalytic protein HisGS is allosterically regulated by the regulatory protein HisZ; however, the protein dynamics of HisGS in enzyme catalysis remain underexplored. Here, the authors investigate the catalytic effect of isotope-labeled HisGS, revealing that the catalytic rate of HisZ-activated HisGS decreases in a mass-dependent fashion at low temperatures, which correlates to product release.