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Imidazole glycerol phosphate synthase (ImGPS) is an allosteric bienzyme complex in which substrate binding to the synthase subunit HisF stimulates the glutaminase subunit HisH. To control this stimulation with light, we have incorporated the photo-responsive unnatural amino acids phenylalanine-4'-azobenzene (AzoF), o-nitropiperonyl-O-tyrosine (NPY), and methyl-o-nitropiperonyllysine (mNPK) at strategic positions of HisF. The light-mediated isomerization of AzoF at position 55 (fS55AzoF(E) fS55AzoF(Z)) resulted in a reversible 10-fold regulation of HisH activity. The light-mediated decaging of NPY at position 39 (fY39NPY -> fY39) and of mNPK at position 99 (fK99mNPK -> fK99) led to a 4- to 6-fold increase of HisH activity. Molecular dynamics simulations explained how the unnatural amino acids interfere with the allosteric machinery of ImGPS and revealed additional aspects of HisH stimulation in wild-type ImGPS. Our findings show that unnatural amino acids can be used as a powerful tool for the spatio-temporal control of a central metabolic enzyme complex by light.

Formylglycinamide ribonucleotide (FGAR) amidotransferase (FGAR-AT) takes part in purine biosynthesis and is a multidomain enzyme with multiple spatially separated active sites. FGAR-AT contains a glutaminase domain that is responsible for the generation of ammonia from glutamine. Ammonia is then transferred via a channel to a second active site located in the synthetase domain and utilized to convert FGAR to formylglycinamidine ribonucleotide (FGAM) in an adenosine triphosphate (ATP) dependent reaction. In some ammonia-channelling enzymes ligand binding triggers interdomain signalling between the two diverse active centres and also assists in formation of the ammonia channel. Previously, the structure of FGAR-AT from Salmonella typhimurium containing a glutamyl thioester intermediate covalently bound in the glutaminase active site was determined. In this work, the roles played by various ligands of FGAR-AT in inducing catalytic coupling are investigated. Structures of FGAR-AT from S. typhimurium were determined in two different states: the unliganded form and the binary complex with an ATP analogue in the presence of the glutamyl thioester intermediate. The structures were compared in order to decipher the roles of these two states in interdomain communication. Using a process of elimination, the results indicated that binding of FGAR is most likely to be the major mechanism by which catalytic coupling occurs. This is because conformational changes do not occur either upon formation of the glutamyl thioester intermediate or upon subsequent ATP complexation. A model of the FGAR-bound form of the enzyme suggested that the loop in the synthetase domain may be responsible for initiating catalytic coupling via its inter­action with the N-terminal domain.

Formylglycinamide ribonucleotide amidotransferase (FGAR-AT) is a 140 kDa bi-functional enzyme involved in a coupled reaction, where the glutaminase active site produces ammonia that is subsequently utilized to convert FGAR to its corresponding amidine in an ATP assisted fashion. The structure of FGAR-AT has been previously determined in an inactive state and the mechanism of activation remains largely unknown. In the current study, hydrophobic cavities were used as markers to identify regions involved in domain movements that facilitate catalytic coupling and subsequent activation of the enzyme. Three internal hydrophobic cavities were located by xenon trapping experiments on FGAR-AT crystals and further, these cavities were perturbed via site-directed mutagenesis. Biophysical characterization of the mutants demonstrated that two of these three voids are crucial for stability and function of the protein, although being ∼20 Å from the active centers. Interestingly, correlation analysis corroborated the experimental findings, and revealed that amino acids lining the functionally important cavities form correlated sets (co-evolving residues) that connect these regions to the amidotransferase active center. It was further proposed that the first cavity is transient and allows for breathing motion to occur and thereby serves as an allosteric hotspot. In contrast, the third cavity which lacks correlated residues was found to be highly plastic and accommodated steric congestion by local adjustment of the structure without affecting either stability or activity.