Human Aspartyl (Asparaginyl) Hydroxylase. A Multifaceted Enzyme with Broad Intra- and Extracellular Activity

Histidine-rich calcium-binding protein: a molecular integrator of cardiac excitation-contraction coupling

During mammalian cardiomyocyte excitation-contraction coupling, Ca2+ influx through voltage-gated Ca2+ channels triggers Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptor channels. This Ca2+-induced Ca2+ release mechanism controls cardiomyocyte contraction and is exquisitely regulated by SR Ca2+ levels. The histidine-rich calcium-binding protein (HRC) and its aspartic acid-rich paralogue aspolin are high-capacity, low-affinity Ca2+-binding proteins. Aspolin also acts as a trimethylamine N-oxide demethylase. At low intraluminal Ca2+ concentrations, HRC binds to the SR Ca2+-ATPase 2, inhibiting its Ca2+-pumping activity. At high intraluminal Ca2+ levels, HRC interacts with triadin to reduce Ca2+ release through ryanodine receptor channels. This Review analyses the evolution of these Ca2+-regulatory proteins, to gain insights into their roles. It reveals that HRC homologues are present in chordates, annelid worms, molluscs, corals and sea anemones. In contrast, triadin appears to be a chordate innovation. Furthermore, HRC is evolving more rapidly than other cardiac excitation-contraction coupling proteins. This positive selection (or relaxed negative selection) occurs along most of the mammalian HRC protein sequence, with the exception being the C-terminal cysteine-rich region, which is undergoing negative selection. The histidine-rich region of HRC might be involved in pH sensing, as an adaptation to air-breathing, endothermic and terrestrial life. In addition, a cysteine-rich pattern within HRC and aspolin is also found in a wide range of iron-sulfur cluster proteins, suggesting roles in redox reactions and metal binding. The polyaspartic regions of aspolins are likely to underlie their trimethylamine N-oxide demethylase activity, which might be mimicked by the acidic regions of HRCs. These potential roles of HRCs and aspolins await experimental verification.

Effects of Clinical Mutations in the Second Coordination Sphere and Remote Regions on the Catalytic Mechanism of Non-Heme Fe(II)/2-Oxoglutarate-Dependent Aspartyl Hydroxylase AspH

Aspartyl/asparaginyl hydroxylase (AspH) catalyzes the post-translational hydroxylations of vital human proteins, playing an essential role in maintaining their biological functions. Single-point mutations in the Second Coordination Sphere (SCS) and long-range (LR) residues of AspH have been linked to pathological conditions such as the ophthalmologic condition Traboulsi syndrome and chronic kidney disease (CKD). Although the clinical impacts of these mutations are established, there is a critical knowledge gap regarding their specific atomistic effects on the catalytic mechanism of AspH. In this study, we report integrated computational investigations on the potential mechanistic implications of four mutant forms of human AspH with clinical importance: R735W, R735Q, R688Q, and G434V. All the mutant forms exhibited altered binding interactions with the co-substrate 2-oxoglutarate (2OG) and the main substrate in the ferric-superoxo and ferryl complexes, which are critical for catalysis, compared to the wild-type (WT). Importantly, the mutations strongly influence the energetics of the frontier molecular orbitals (FMOs) and, thereby, the activation energies for the hydrogen atom transfer (HAT) step compared to the WT AspH. Insights from our study can contribute to enzyme engineering and the development of selective modulators for WT and mutants of AspH, ultimately aiding in treating cancers, Traboulsi syndrome and, CKD.

Orthogonal Translation for Site-Specific Installation of Post-translational Modifications

Post-translational modifications (PTMs) endow proteins with new properties to respond to environmental changes or growth needs. With the development of advanced proteomics techniques, hundreds of distinct types of PTMs have been observed in a wide range of proteins from bacteria, archaea, and eukarya. To identify the roles of these PTMs, scientists have applied various approaches. However, high dynamics, low stoichiometry, and crosstalk between PTMs make it almost impossible to obtain homogeneously modified proteins for characterization of the site-specific effect of individual PTM on target proteins. To solve this problem, the genetic code expansion (GCE) strategy has been introduced into the field of PTM studies. Instead of modifying proteins after translation, GCE incorporates modified amino acids into proteins during translation, thus generating site-specifically modified proteins at target positions. In this review, we summarize the development of GCE systems for orthogonal translation for site-specific installation of PTMs.

Unusual catalytic strategy by non-heme Fe(ii)/2-oxoglutarate-dependent aspartyl hydroxylase AspH

Biocatalytic C-H oxidation reactions are of important synthetic utility, provide a sustainable route for selective synthesis of important organic molecules, and are an integral part of fundamental cell processes. The multidomain non-heme Fe(ii)/2-oxoglutarate (2OG) dependent oxygenase AspH catalyzes stereoselective (3R)-hydroxylation of aspartyl- and asparaginyl-residues. Unusually, compared to other 2OG hydroxylases, crystallography has shown that AspH lacks the carboxylate residue of the characteristic two-His-one-Asp/Glu Fe-binding triad. Instead, AspH has a water molecule that coordinates Fe(ii) in the coordination position usually occupied by the Asp/Glu carboxylate. Molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) studies reveal that the iron coordinating water is stabilized by hydrogen bonding with a second coordination sphere (SCS) carboxylate residue Asp721, an arrangement that helps maintain the six coordinated Fe(ii) distorted octahedral coordination geometry and enable catalysis. AspH catalysis follows a dioxygen activation-hydrogen atom transfer (HAT)-rebound hydroxylation mechanism, unusually exhibiting higher activation energy for rebound hydroxylation than for HAT, indicating that the rebound step may be rate-limiting. The HAT step, along with substrate positioning modulated by the non-covalent interactions with SCS residues (Arg688, Arg686, Lys666, Asp721, and Gln664), are essential in determining stereoselectivity, which likely proceeds with retention of configuration. The tetratricopeptide repeat (TPR) domain of AspH influences substrate binding and manifests dynamic motions during catalysis, an observation of interest with respect to other 2OG oxygenases with TPR domains. The results provide unique insights into how non-heme Fe(ii) oxygenases can effectively catalyze stereoselective hydroxylation using only two enzyme-derived Fe-ligating residues, potentially guiding enzyme engineering for stereoselective biocatalysis, thus advancing the development of non-heme Fe(ii) based biomimetic C-H oxidation catalysts, and supporting the proposal that the 2OG oxygenase superfamily may be larger than once perceived.

5-Substituted Pyridine-2,4-dicarboxylate Derivatives Have Potential for Selective Inhibition of Human Jumonji-C Domain-Containing Protein 5

Jumonji-C domain-containing protein 5 (JMJD5) is a 2-oxoglutarate (2OG)-dependent oxygenase that plays important roles in development, circadian rhythm, and cancer through unclear mechanisms. JMJD5 has been reported to have activity as a histone protease, as an Nε-methyl lysine demethylase, and as an arginine residue hydroxylase. Small-molecule JMJD5-selective inhibitors will be useful for investigating its (patho)physiological roles. Following the observation that the broad-spectrum 2OG oxygenase inhibitor pyridine-2,4-dicarboxylic acid (2,4-PDCA) is a 2OG-competing JMJD5 inhibitor, we report that 5-aminoalkyl-substituted 2,4-PDCA derivatives are potent JMJD5 inhibitors manifesting selectivity for JMJD5 over other human 2OG oxygenases. Crystallographic analyses with five inhibitors imply induced fit binding and reveal that the 2,4-PDCA C5 substituent orients into the JMJD5 substrate-binding pocket. Cellular studies indicate that the lead compounds display similar phenotypes as reported for clinically observed JMJD5 variants, which have a reduced catalytic activity compared to wild-type JMJD5.

The inhibitory effect of human umbilical cord mesenchymal stem cells expressing anti-HAAH scFv-sTRAIL fusion protein on glioma

Glioma is the most common malignant intracranial tumor with low 5-year survival rate. In this study, we constructed a plasmid expressing anti-HAAH single-chain antibody and sTRAIL fusion protein (scFv-sTRAIL), and explored the effects of the double gene modified human umbilical cord mesenchyreal stem cells (hucMSCs) on the growth of glioma in vitro and in vivo. The isolated hucMSCs were identified by detecting the adipogenic differentiation ability and the osteogenic differentiation ability. The phenotypes of hucMSCs were determined by the flow cytometry. The hucMSCs were infected with lentivirus expression scFv-sTRAIL fusion protein. The expression of sTRAIL in hucMSCs were detected by immunofluorescence staining, western blot and ELISA. The tropism of hucMSCs toward U87G cells was assessed by transwell assay. The inhibitory effect of hucMSCs on U87G cells were explored by CCK8 and apoptosis assay. The xenograft tumor was established by subcutaneously injection of U87G cells into the back of mice. The hucMSCs were injected via tail veins. The inhibitory effect of hucMSCs on glioma in vivo was assessed by TUNEL assay. The hucMSCs migrated into the xenograft tumor were revealed by detecting the green fluorescent. The results showed that the scFv-sTRAIL expression did not affect the phenotypes of hucMSCs. The scFv-sTRAIL expression promoted the tropism of hucMSCs toward U87G cells, enhanced the inhibitory effect and tumor killing effect of hucMSCs on U87G cells. The in vivo study showed that hucMSCs expressing scFv-sTRAIL demonstrated significantly higher inhibitory effect and tumor killing effect than hucMSCs expressing sTRAIL. The green fluorescence intensity in the mice injected with hucMSCs expressing scFv-sTRAIL was significantly higher than that injected with hucMSCs expressing sTRAIL. These data suggested that the scFv conferred the targeting effect of hucMSCs tropism towards the xenograft tumor. In conclusion, the hucMSCs expressing scFv-sTRAIL fusion protein gained the capability to target and kill gliomas cells in vitro and in vivo. These findings shed light on a potential therapy for glioma treatment.

Combined proteomic and biochemical analyses redefine the consensus sequence requirement for epidermal growth factor-like domain hydroxylation

Epidermal growth factor-like domains (EGFDs) have important functions in cell-cell signaling. Both secreted and cell surface human EGFDs are subject to extensive modifications, including aspartate and asparagine residue C3-hydroxylations catalyzed by the 2-oxoglutarate oxygenase aspartate/asparagine-β-hydroxylase (AspH). Although genetic studies show AspH is important in human biology, studies on its physiological roles have been limited by incomplete knowledge of its substrates. Here, we redefine the consensus sequence requirements for AspH-catalyzed EGFD hydroxylation based on combined analysis of proteomic mass spectrometric data and mass spectrometry-based assays with isolated AspH and peptide substrates. We provide cellular and biochemical evidence that the preferred site of EGFD hydroxylation is embedded within a disulfide-bridged macrocycle formed of 10 amino acid residues. This definition enabled the identification of previously unassigned hydroxylation sites in three EGFDs of human fibulins as AspH substrates. A non-EGFD containing protein, lymphocyte antigen-6/plasminogen activator urokinase receptor domain containing protein 6B (LYPD6B) was shown to be a substrate for isolated AspH, but we did not observe evidence for LYPD6B hydroxylation in cells. AspH-catalyzed hydroxylation of fibulins is of particular interest given their important roles in extracellular matrix dynamics. In conclusion, these results lead to a revision of the consensus substrate requirements for AspH and expand the range of observed and potential AspH-catalyzed hydroxylation in cells, which will enable future study of the biological roles of AspH.

2-Oxoglutarate derivatives can selectively enhance or inhibit the activity of human oxygenases

2-Oxoglutarate (2OG) oxygenases are validated agrochemical and human drug targets. The potential for modulating their activity with 2OG derivatives has not been explored, possibly due to concerns regarding selectivity. We report proof-of-principle studies demonstrating selective enhancement or inhibition of 2OG oxygenase activity by 2-oxo acids. The human 2OG oxygenases studied, factor inhibiting hypoxia-inducible transcription factor HIF-α (FIH) and aspartate/asparagine-β-hydroxylase (AspH), catalyze C3 hydroxylations of Asp/Asn-residues. Of 35 tested 2OG derivatives, 10 enhance and 17 inhibit FIH activity. Comparison with results for AspH reveals that 2OG derivatives selectively enhance or inhibit FIH or AspH. Comparison of FIH structures complexed with 2OG derivatives to those for AspH provides insight into the basis of the observed selectivity. 2-Oxo acid derivatives have potential as drugs, for use in biomimetic catalysis, and in functional studies. The results suggest that the in vivo activity of 2OG oxygenases may be regulated by natural 2-oxo acids other than 2OG.