15N and 13C NMR studies of ligands bound to the 280,000-dalton protein porphobilinogen synthase elucidate the structures of enzyme-bound product and a Schiff base intermediate

Porphobilinogen synthase (PBGS) catalyzes the asymmetric condensation of two molecules of 5-aminolevulinic acid (ALA). Despite the 280,000-dalton size of PBGS, much can be learned about the reaction mechanism through 13C and 15N NMR. To our knowledge, these studies represent the largest protein complex for which individual nuclei have been characterized by 13C or 15N NMR. Here we extend our 13C NMR studies to PBGS complexes with 3,3-2H2,3-13C]ALA and report 15N NMR studies of 15N]ALA bound to PBGS. As in our previous 13C NMR studies, observation of enzyme-bound 15N-labeled species was facilitated by deuteration at nitrogens that are attached to slowly exchanging hydrogens. For holo-PBGS at neutral pH, the NMR spectra reflect the structure of the enzyme-bound product porphobilinogen (PBG), whose chemical shifts are uniformly consistent with deprotonation of the amino group whose solution pKa is 11. Despite this local environment, the protons of the amino group are in rapid exchange with solvent (kexchange greater than 10(2) s-1). For methyl methanethiosulfonate (MMTS) modified PBGS, the NMR spectra reflect the chemistry of an enzyme-bound Schiff base intermediate that is formed between C4 of ALA and an active-site lysine. The 13C chemical shift of 3,3-2H2,3-13C]ALA confirms that the Schiff base is an imine of E stereochemistry. By comparison to model imines formed between 15N]ALA and hydrazine or hydroxylamine, the 15N chemical shift of the enzyme-bound Schiff base suggests that the free amino group is an environment resembling partial deprotonation; again the protons are in rapid exchange with solvent. Deprotonation of the amino group would facilitate formation of a Schiff base between the amino group of the enzyme-bound Schiff base and C4 of the second ALA substrate. This is the first evidence supporting carbon-nitrogen bond formation as the initial site of interaction between the two substrate molecules.