Studies on the structure, mechanism and protein engineering of Bacillus subtilis pimeloyl-CoA synthetase (PCAS)
Biotin is an essential vitamin in plants and mammals functioning as the carbon dioxide carrier within central lipid metabolism. Biotin is composed of a fused bicylic ring system and a five carbon, carboxylic acid chain. Biotin biosynthesis in bacteria is catalysed by a series of enzymes that use fatty acid, amino acid and sulfur-containing substrates. In Bacillus subtilis, pimeloyl-CoA synthetase (PCAS, EC 22.214.171.124, UNIPROT code: P53559, 29.6 kDa) is the first enzyme in the biotin biosynthetic pathway and acts as a highly specific substrate selection gate ensuring the integrity of the carbon chain in biotin synthesis. PCAS catalyses the synthesis of the key acyl-thioester, pimeloyl-CoA in two steps; the first involves activation of pimelic acid (C7 dicarboxylic acid) using ATP to give an acyl-adenylate, enzyme-bound intermediate and pyrophosphate (PPi), and in the second step, this pimeloyl-adenylate reacts with coenzyme A (CoASH) to form the pimeloyl-CoA thioester. This thesis describes the results of biochemical, structural and mechanistic studies of B. subtilis PCAS. Recombinant PCAS was prepared by expressing the B. subtilis BioW gene in E. coli in various hexa-histidine affinity-tagged forms and the enzyme purified in high purity and yield. Enzyme activity and kinetic constants were measured using reverse-phase HPLC and enzyme coupled spectroscopic assays. These revealed the enzyme to have a strict carboxylic acid specificity. In collaboration with colleagues at the University of St. Andrews various commercial and in-house screens were used to obtain diffraction-quality crystals suitable for X-ray crystallography. This also included the generation of seleno-methionine (SeMet) labelled PCAS, as well as heavy-metal derivatives. Structures of B. subtilis PCAS in complex with the substrate pimelic acid and the pimeloyl-adenylate intermediate and product PPi were determined at 2.04 Å and 2.34 Å resolution respectively. The B. subtilis PCAS displays a novel 3D fold and defines a new class (Class IV) in the ANL superfamily of adenylate forming enzymes. The enzyme is a homodimer composed of two domains, a short N-terminus and a large C-terminal domain and the ligand-bound structures revealed the residues potentially involved in substrate specificity and enzyme catalysis. The enzyme uses an internal ruler composed of a number of conserved arginine residues (Arg213, Arg227 and Arg170) to select the correct dicarboxylic acid substrate. The X-ray structures guided the production of a number of site directed mutants to identify residues involved in the catalytic mechanism and stabilising the acyl-adenylate intermediate. This also allowed rational engineering of the PCAS active site to generate mutants with altered substrate specificity. Mutant PCAS Y211F was shown to synthesise both heptanoyl (C7) and octanoyl (C8) mono carboxylic acid-CoA and C8 dicarboxylic-CoA thioester products, highlighting the synthetic potential of PCAS. The PCAS pimeloyl-CoA product is the substrate for the next enzyme in the biotin pathway, a pyridoxal 5‟phosphate (PLP)-dependent 8-amino 7-oxononanoate synthase (AONS). AONS catalyses the condensation of pimeloyl-CoA with L-alanine to give AON which is converted to biotin by the action of three other enzymes. We used genome mining to identify a putative ~66 kDa, bi-functional PCAS/AONS enzyme with an N-terminal PCAS domain fused to C-terminal AONS domain in the organism Corynebacterium amycolatum. A recombinant C. amycolatum PCAS/AONS fusion protein was expressed and purified from E. coli and initial studies suggest that it forms a functional, fused, dimeric enzyme.