Alleviated Inhibition of Single Enzyme in Confined and Crowded Environment.

ABSTRACT

Most proteins perform functions in intracellular milieu. The crowding, compartmentalized cytosol environment affects the protein structure, folding, conformational stability, substrate diffusion, and substrate-enzyme binding. Moreover, enzymes are available at single or very low copy numbers in a cell, and thus the conformation fluctuations of a single enzyme in a crowding environment could also greatly influence its kinetics. However, the crowding effect is poorly understood in the kinetical aspect of enzymatic reactions. In the present study, individual horseradish peroxidase (HRP) is encapsulated in a liposome containing crowding reagents as mimics of viscous cytosol. The confined crowding environment possesses a profound influence on both the catalytic activity and the product inhibition of enzymes. By analyzing the correlation between product generation and product inhibition, we find that the allosteric noncompetitive inhibition of HRP is alleviated in the crowded and confined milieu. Small-angle X-ray scattering experiments provide straightforward proofs of structural changes of enzymes in crowding environments, which are responsible for the reduced enzyme activity and increased enzyme-substrate affinity. We expect that this work may deepen the understanding of correlations between enzymatic conformations and activity performance in real cellular environments.