Structural and functional characterization of the aminoacetone utilization microcompartment from Mycobacterium smegmatis MC2 155
Bacterial microcompartments are proteinacious complexes made by bacteria, which metabolize volatile or cytotoxic chemicals, by sequestering a series of reactions within a selectively permeable shell. This thesis focused on characterizing the catalytic functions and thus the metabolism of an aminoacetone utilization microcompartment (AAUM) found in Rhodococcus and Mycobacterium species, using the prototypical operon found in Mycobacterium smegmatis MC2 155 as a subject for investigation. Of the four enzymes associated with the AAUM, catalytic functions and structures were determined for two of the enzymes from M. smegmatis and for a homolog of a third enzyme. The first enzyme characterized was a stereospecific alcohol dehydrogenase catalyzing the reduction of 1-amino-2-propanone forming S-(+)-1-amino-2-propanol. The second enzyme was characterized as a 1-amino-2-propanol O-kinase with a preference for the S-isomer, able to also phosphorylate the R-isomer and amino-alcohols of varying lengths. An ortholog of the AAUM associated class-III aminotransferase from Mesorhizobium loti was characterized as a phosphopropanolamine phospholyase, forming ammonia, inorganic phosphate, and propionaldehyde. The remaining enzyme from the AAUM is annotated as a coenzyme A acylating aldehyde dehydrogenase, proposed to acylate coenzyme A with propionaldehyde produced by the phospholyase enzyme resulting in propionyl-CoA production. From the determined enzyme functions, I propose a metabolic pathway for the AAUM, converting aminoacetone to propionyl-CoA for use in central metabolism. The structures of the four shell proteins from the AAUM were determined by X-ray crystallography; proposed to form an icosahedral shell, with hexagonal oligomers forming the facets capped at the apices by pentagonal oligomers. The hexagonal hexameric bacterial microcompartment shell protein (BMC-H) formed a negatively charged pore, proposed to function in transport of a positively charged molecule across the shell. Both hexagonal trimeric bacterial microcompartment shell proteins (BMC-Ts) formed stacked trimeric rings with loops forming a tight interface at the center of each ring, proposed to undergo conformational shifts to open a large diameter pore for intermittent large molecule exchange. The pentagonal pentameric bacterial microcompartment shell protein (BMC-P) formed a roughly pyramidal oligomer proposed to fulfill an apex capping function. Using current modeling techniques, we were able to propose an architecture for the microcompartment shell.