Deep Mutational Scanning Reveals the Active-Site Sequence Requirements for the Colistin Antibiotic Resistance Enzyme MCR-1

Polymyxin antibiotics are used as last-line antibiotics in treating infections caused by multidrug-resistant pathogens. However, widespread use of polymyxins has led to the emergence of resistance.

Reconstructing the Evolutionary History of Nitrogenases: Evidence for Ancestral Molybdenum-Cofactor Utilization

ABSTRACTThe nitrogenase metalloenzyme family, essential for supplying fixed nitrogen to the biosphere, is one of life’s key biogeochemical innovations. The three isozymes of nitrogenase differ in their metal dependence, each binding either a FeMo-, FeV-, or FeFe-cofactor where the reduction of dinitrogen takes place. The history of nitrogenase metal dependence has been of particular interest due to the possible implication that ancient marine metal availabilities have significantly constrained nitrogenase evolution over geologic time. Here, we reconstructed the evolutionary history of nitrogenases, and combined phylogenetic reconstruction, ancestral sequence inference, and structural homology modeling to evaluate the potential metal dependence of ancient nitrogenases. We find that active-site sequence features can reliably distinguish extant Mo-nitrogenases from V- and Fe-nitrogenases, and that inferred ancestral sequences at the deepest nodes of the phylogeny suggest these ancient proteins most resemble modern Mo-nitrogenases. Taxa representing early-branching nitrogenase lineages lack one or more biosynthetic nifE and nifN genes that both contribute to the assembly of the FeMo-cofactor in studied organisms, suggesting that early Mo-nitrogenases may have utilized an alternate and/or simplified pathway for cofactor biosynthesis. Our results underscore the profound impacts that protein-level innovations likely had on shaping global biogeochemical cycles throughout the Precambrian, in contrast to organism-level innovations that characterize the Phanerozoic Eon.

Structure-based discovery of cyclin-dependent protein kinase inhibitors

The cell fate-determining roles played by members of the cyclin-dependent protein kinase (CDK) family explain why their dysregulation can promote proliferative diseases, and identify them as potential targets for drug discovery in oncology and beyond. After many years of research, the first efficacious CDK inhibitors have now been registered for clinical use in a defined segment of breast cancer. Research is underway to identify inhibitors with appropriate CDK-inhibitory profiles to recapitulate this success in other disease settings. Here, we review the structural data that illustrate the interactions and properties that confer upon inhibitors affinity and/or selectivity toward different CDK family members. We conclude that where CDK inhibitors display selectivity, that selectivity derives from exploiting active site sequence peculiarities and/or from the capacity of the target CDK(s) to access conformations compatible with optimizing inhibitor–target interactions.

2-Carboxy-D-arabinitol 1-phosphate (CA1P) phosphatase: evidence for a wider role in plant Rubisco regulation

The genes for CA1Pase (2-carboxy-D-arabinitol-1-bisphosphate phosphatase) from French bean, wheat, Arabidopsis and tobacco were identified and cloned. The deduced protein sequence included an N-terminal motif identical with the PGM (phosphoglycerate mutase) active site sequence [LIVM]-x-R-H-G-[EQ]-x-x-[WN]. The corresponding gene from wheat coded for an enzyme with the properties published for CA1Pase. The expressed protein lacked PGM activity but rapidly dephosphorylated 2,3-DPG (2,3-diphosphoglycerate) to 2-phosphoglycerate. DTT (dithiothreitol) activation and GSSG inactivation of this enzyme was pH-sensitive, the greatest difference being apparent at pH 8. The presence of the expressed protein during in vitro measurement of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) activity prevented a progressive decline in Rubisco turnover. This was due to the removal of an inhibitory bisphosphate that was present in the RuBP (ribulose-1,5-bisphosphate) preparation, and was found to be PDBP (D-glycero-2,3-pentodiulose-1,5-bisphosphate). The substrate specificity of the expressed protein indicates a role for CA1Pase in the removal of ‘misfire’ products of Rubisco.

The Monothiol Single-Domain Glutaredoxin Is Conserved in the Highly Reduced Mitochondria of Giardia intestinalis

ABSTRACT The highly reduced mitochondria (mitosomes) of Giardia intestinalis are recently discovered organelles for which, it was suggested, iron-sulfur cluster assembly was their only conserved function. However, only an incomplete set of the components required for FeS cluster biogenesis was localized to the mitosomes. Via proteomic analysis of a mitosome-rich cellular fraction together with immunofluorescence microscopy, we identified a novel mitosomal protein homologous to monothiol glutaredoxins containing a CGFS motif at the active site. Sequence analysis revealed the presence of long nonconserved N-terminal extension of 77 amino acids, which was absent in the mature protein. Expression of the complete and N-terminally truncated forms of the glutaredoxin indicated that the extension is involved in glutaredoxin import into mitosomes. However, the mechanism of preprotein processing is unclear, as the mitosomal processing peptidase is unable to cleave this type of extension. The recombinant mature protein was shown to form a homodimeric structure, which binds a labile FeS cluster. The cluster is stabilized by glutathione and dithiothreitol. Phylogenetic analysis showed that giardial glutaredoxin is related to the mitochondrial monothiol glutaredoxins involved in FeS cluster assembly. The identification of a mitochondrial-type monothiol glutaredoxin in the mitosomes of G. intestinalis thus completes the mitosomal FeS cluster biosynthetic pathway and provides further evidence for the mitochondrial origin of these organelles.

Characterization of the phenylurea hydrolases A and B: founding members of a novel amidohydrolase subgroup

Mycobacterium brisbanense strain JK1, a bacterium capable of degrading the herbicide diuron, was isolated from herbicide-exposed soil. A gene/enzyme system with diuron hydrolase activity was isolated from this strain and named PUH (phenylurea hydrolase) B (puhB/PuhB) because of its close similarity to the previously characterized PUH A (puhA/PuhA). Both PUHs were heterologously expressed, purified and characterized. The PUHs were found to oligomerize as hexamers in solution, with each monomer containing a mononuclear Zn2+ active site. Sequence analysis showed that these enzymes belong to the metal-dependent amidohydrolase superfamily, although they contain a hitherto unreported Asn-X-His metal-binding motif and appear to form a novel sub-group within this superfamily. The effects of temperature and solvent on the enzymes were characterized. Determination of the kinetic parameters of the PUHs was used alongside Brønsted plots to develop a plausible catalytic mechanism, which is similar to that used by urease. In addition to the primary PUH activity, both enzymes are catalytically promiscuous, efficiently hydrolysing esters, carbamates and phosphotriesters. In fact, an analogue of diuron, in which the C–N bond was replaced by a C–O bond, was found to be turned over as efficiently as diuron, suggesting that the substrate specificity is predominantly determined by steric factors. The discovery of PuhA and PuhB on separate continents, and the absence of any other close homologues in the available sequence databases, poses a challenging question regarding the evolutionary origins of these enzymes.