A remote surface loop modulates core structure and cold activity in phosphopantetheine adenylyltransferase

TL;DR

A unique loop in a cold-adapted enzyme helps it function at low temperatures by keeping its structure flexible and guiding substrates to the active site.

Contribution

The study identifies a novel allosteric mechanism in cold-adapted PPAT involving a surface loop that modulates core structure and electrostatics.

Findings

The SCRLS loop insertion in MpaPPAT prevents structural rigidity and maintains core flexibility at low temperatures.

Deletion of the SCRLS loop causes a shift in electrostatic potential, impairing substrate binding in cold conditions.

The loop's presence ensures a positively charged central pore, aiding substrate entry into the active site.

Abstract

Phosphopantetheine adenylyltransferase (PPAT), a key enzyme in the universal Coenzyme A biosynthetic pathway, is essential for cellular metabolism. However, the adaptive mechanisms of PPAT in psychrophilic (cold-adapted) organisms remain poorly understood. Here, we characterize PPAT from the psychrophilic methanotroph Methylocapsa palsarum (MpaPPAT). Sequence analysis identified a unique five-amino-acid insertion (SCRLS) within a surface-exposed loop, a feature conserved among psychrophilic homologues. To investigate its function, we determined the crystal structures of wild-type (WT) MpaPPAT and a loop-deletion mutant (MpaPPAT(Δ67–71)) and performed comparative biochemical analyses. Structurally, MpaPPAT forms a dimer-of-trimers hexamer. Biochemically, WT MpaPPAT maintains high catalytic activity at low temperatures (10–20 °C), whereas the MpaPPAT(Δ67–71) mutant exhibits impaired cold activity. The mutant structure reveals that the deletion of the distant surface loop induces a long-range allosteric change, resulting in a dual impairment: 1) a stabilization and rigidification (“clamping”) of the central α-helix 4 (H4) at the hexameric core interface, and 2) a dramatic shift in the central pore’s electrostatic potential from positive (WT) to negative (mutant). Our findings reveal that the SCRLS insertion is a critical allosteric modulator that provides a sophisticated dual mechanism for enzymatic cold adaptation. It maintains the conformational flexibility of the hexameric core, preventing the “clamping” effect, and simultaneously ensures a positively charged central channel to electrostatically steer negatively charged substrates (ATP and phosphopantetheine) into the active site, thereby overcoming the kinetic challenges of a low-temperature environment.

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