Kinetic Investigations of the Role of Factor Inhibiting Hypoxia-inducible Factor (FIH) as an Oxygen Sensor.

The hypoxia-inducible factor (HIF) hydroxylases regulate hypoxia sensing in animals. In humans, they comprise three prolyl hydroxylases (PHD1-3 or EGLN1-3) and factor inhibiting HIF (FIH). FIH is an asparaginyl hydroxylase catalyzing post-translational modification of HIF-α, resulting in reduction of HIF-mediated transcription. Like the PHDs, FIH is proposed to have a hypoxia-sensing role in cells, enabling responses to changes in cellular O2 availability. PHD2, the most important human PHD isoform, is proposed to be biochemically/kinetically suited as a hypoxia sensor due to its relatively high sensitivity to changes in O2 concentration and slow reaction with O2. To ascertain whether these parameters are conserved among the HIF hydroxylases, we compared the reactions of FIH and PHD2 with O2. Consistent with previous reports, we found lower Km(app)(O2) values for FIH than for PHD2 with all HIF-derived substrates. Under pre-steady-state conditions, the O2-initiated FIH reaction is significantly faster than that of PHD2. We then investigated the kinetics with respect to O2 of the FIH reaction with ankyrin repeat domain (ARD) substrates. FIH has lower Km(app)(O2) values for the tested ARDs than HIF-α substrates, and pre-steady-state O2-initiated reactions were faster with ARDs than with HIF-α substrates. The results correlate with cellular studies showing that FIH is active at lower O2 concentrations than the PHDs and suggest that competition between HIF-α and ARDs for FIH is likely to be biologically relevant, particularly in hypoxic conditions. The overall results are consistent with the proposal that the kinetic properties of individual oxygenases reflect their biological capacity to act as hypoxia sensors.

Selection-based discovery of macrocyclic peptides for the next generation therapeutics.

Naturally occurring macrocyclic peptides represent a unique class of compounds that exhibit various biological activities ranging from antibiotics to immunosuppressant. Although the discovery of such macrocyclic peptides had relied on their isolation from living organisms, recent advances in ribosomal peptide synthesis and in display techniques made it possible to use artificially generated macrocyclic peptide libraries for selection of ligands for biologically relevant proteins. In this review, we discuss the technologies and their applications for the discovery of peptide ligands.

Human oxygen sensing may have origins in prokaryotic elongation factor Tu prolyl-hydroxylation.

The roles of 2-oxoglutarate (2OG)-dependent prolyl-hydroxylases in eukaryotes include collagen stabilization, hypoxia sensing, and translational regulation. The hypoxia-inducible factor (HIF) sensing system is conserved in animals, but not in other organisms. However, bioinformatics imply that 2OG-dependent prolyl-hydroxylases (PHDs) homologous to those acting as sensing components for the HIF system in animals occur in prokaryotes. We report cellular, biochemical, and crystallographic analyses revealing that Pseudomonas prolyl-hydroxylase domain containing protein (PPHD) contain a 2OG oxygenase related in structure and function to the animal PHDs. A Pseudomonas aeruginosa PPHD knockout mutant displays impaired growth in the presence of iron chelators and increased production of the virulence factor pyocyanin. We identify elongation factor Tu (EF-Tu) as a PPHD substrate, which undergoes prolyl-4-hydroxylation on its switch I loop. A crystal structure of PPHD reveals striking similarity to human PHD2 and a Chlamydomonas reinhardtii prolyl-4-hydroxylase. A crystal structure of PPHD complexed with intact EF-Tu reveals that major conformational changes occur in both PPHD and EF-Tu, including a >20-Å movement of the EF-Tu switch I loop. Comparison of the PPHD structures with those of HIF and collagen PHDs reveals conservation in substrate recognition despite diverse biological roles and origins. The observed changes will be useful in designing new types of 2OG oxygenase inhibitors based on various conformational states, rather than active site iron chelators, which make up most reported 2OG oxygenase inhibitors. Structurally informed phylogenetic analyses suggest that the role of prolyl-hydroxylation in human hypoxia sensing has ancient origins.

Investigating the contribution of the active site environment to the slow reaction of hypoxia-inducible factor prolyl hydroxylase domain 2 with oxygen.

The prolyl hydroxylase domain proteins (PHDs) catalyse the post-translational hydroxylation of the hypoxia-inducible factor (HIF), a modification that regulates the hypoxic response in humans. The PHDs are Fe(II)/2-oxoglutarate (2OG) oxygenases; their catalysis is proposed to provide a link between cellular HIF levels and changes in O2 availability. Transient kinetic studies have shown that purified PHD2 reacts slowly with O2 compared with some other studied 2OG oxygenases, a property which may be related to its hypoxia-sensing role. PHD2 forms a stable complex with Fe(II) and 2OG; crystallographic and kinetic analyses indicate that an Fe(II)-co-ordinated water molecule, which must be displaced before O2 binding, is relatively stable in the active site of PHD2. We used active site substitutions to investigate whether these properties are related to the slow reaction of PHD2 with O2. While disruption of 2OG binding in a R383K variant did not accelerate O2 activation, we found that substitution of the Fe(II)-binding aspartate for a glutamate residue (D315E) manifested significantly reduced Fe(II) binding, yet maintained catalytic activity with a 5-fold faster reaction with O2. The results inform on how the precise active site environment of oxygenases can affect rates of O2 activation and provide insights into limiting steps in PHD catalysis.

Comparison of the substrate selectivity and biochemical properties of human and bacterial γ-butyrobetaine hydroxylase.

2-Oxoglutarate and iron dependent oxygenases have potential for the stereoselective hydroxylation of amino acids and related compounds. The biochemical and kinetic properties of recombinant γ-butyrobetaine hydroxylase from human and Pseudomonas sp. AK1 were compared. The results reveal differences between the two BBOXs, including in their stimulation by ascorbate. Despite their closely related sequences, the two enzymes also display different substrate selectivities, including for the production of (di)hydroxylated betaines, implying use of engineered BBOXs for biocatalytic purposes may be productive.

Non-enzymatic chemistry enables 2-hydroxyglutarate-mediated activation of 2-oxoglutarate oxygenases.

Accumulation of (R)-2-hydroxyglutarate in cells results from mutations to isocitrate dehydrogenase that correlate with cancer. A recent study reports that (R)-, but not (S)-2-hydroxyglutarate, acts as a co-substrate for the hypoxia-inducible factor prolyl hydroxylases via enzyme-catalysed oxidation to 2-oxoglutarate. Here we investigate the mechanism of 2-hydroxyglutarate-enabled activation of 2-oxoglutarate oxygenases, including prolyl hydroxylase domain 2, the most important human prolyl hydroxylase isoform. We observe that 2-hydroxyglutarate-enabled catalysis by prolyl hydroxylase domain 2 is not enantiomer-specific and is stimulated by ferrous/ferric ion and reducing agents including L-ascorbate. The results reveal that 2-hydroxyglutarate is oxidized to 2-oxoglutarate non-enzymatically, likely via iron-mediated Fenton-chemistry, at levels supporting in vitro catalysis by 2-oxoglutarate oxygenases. Succinic semialdehyde and succinate are also identified as products of 2-hydroxyglutarate oxidation. Overall, the results rationalize the reported effects of 2-hydroxyglutarate on catalysis by prolyl hydroxylases in vitro and suggest that non-enzymatic 2-hydroxyglutarate oxidation may be of biological interest.

Oxygenase-catalyzed ribosome hydroxylation occurs in prokaryotes and humans.

The finding that oxygenase-catalyzed protein hydroxylation regulates animal transcription raises questions as to whether the translation machinery and prokaryotic proteins are analogously modified. Escherichia coli ycfD is a growth-regulating 2-oxoglutarate oxygenase catalyzing arginyl hydroxylation of the ribosomal protein Rpl16. Human ycfD homologs, Myc-induced nuclear antigen (MINA53) and NO66, are also linked to growth and catalyze histidyl hydroxylation of Rpl27a and Rpl8, respectively. This work reveals new therapeutic possibilities via oxygenase inhibition and by targeting modified over unmodified ribosomes.

The 2-oxoglutarate-dependent oxygenase JMJD6 catalyses oxidation of lysine residues to give 5S-hydroxylysine residues.

Amino acid analyses reveal that JMJD6-catalysed hydroxylation of RNA-splicing regulatory protein fragments occurs to give hydroxylysine products with 5S stereochemistry. This contrasts with collagen lysyl hydroxylases, which give 5R-hydroxylated products. The work suggests that more than one subfamily of lysyl hydroxylases has evolved and illustrates the importance of stereochemical assignments in proteomic analyses.