An Envelope Protein of a Prokaryotic Organelle Function towards Conserving Catalytic Activity of the Native Enzyme at Higher Temperatures

A classic example of an all-protein natural nano-bioreactor, the bacterial microcompartments are a special kind of prokaryotic organelles that confine enzymes within a small volume enveloped by an outer layer of shell proteins. This arrangement provides conditional metabolic aid to the bacteria. The outer shell allows selective diffusion of small molecules and sequesters toxic metabolites. In this work we use 1,2-propanediol utilization microcompartment as a model to study the effect of molecular confinement on the stability and catalytic activity of native enzymes in microcompartment. We observe a 50% decrease in the activity of free enzyme PduCDE at 45{degrees}C, while PduMCP retains its optimum activity till 50{degrees}C followed by more than 40% reduced activity at 55{degrees}C. PduBB, the major component of the outer shell contributes to the increased catalytic activity of PduCDE. PduBB also prevents the unfolding and aggregation of PduCDE under thermal stress. Using a combination of experimental and theoretical studies we probe the interactions of the shell proteins PduBB, N-terminal truncated PduB and single mutant PduBM38L with PduCDE. We observe that all the three variants of PduB* shell proteins interact with the enzyme in vitro, but only PduBB influences its activity and stability, underscoring the significance of the unique combination of PduB and PduB in PduMCP assembly.