Molecular basis of peptide recognition in metallopeptidase Dug1p from Saccharomyces cerevisiae

N2-Acetylornithine deacetylase functions as a Cys-Gly dipeptidase in the cytosolic glutathione degradation pathway in Arabidopsis thaliana

Glutathione (GSH) is required for various physiological processes in plants, including redox regulation and detoxification of harmful compounds. GSH also functions as a repository for assimilated sulfur and is actively catabolized in plants. In Arabidopsis, GSH is mainly degraded initially by cytosolic enzymes, γ-glutamyl cyclotransferase, and γ-glutamyl peptidase, which release cysteinylglycine (Cys-Gly). However, the subsequent enzyme responsible for catabolizing this dipeptide has not been identified to date. In the present study, we identified At4g17830 as a Cys-Gly dipeptidase, namely cysteinylglycine peptidase 1 (CGP1). CGP1 complemented the phenotype of the yeast mutant that cannot degrade Cys-Gly. The Arabidopsis cgp1 mutant had lower Cys-Gly degradation activity than the wild type and showed perturbed concentrations of thiol compounds. Recombinant CGP1 showed reasonable Cys-Gly degradation activity in vitro. Metabolomic analysis revealed that cgp1 exhibited signs of severe sulfur deficiency, such as elevated accumulation of O-acetylserine (OAS) and the decrease in sulfur-containing metabolites. Morphological changes observed in cgp1, including longer primary roots of germinating seeds, were also likely associated with sulfur starvation. Notably, At4g17830 has previously been reported to encode an N2-acetylornithine deacetylase (NAOD) that functions in the ornithine biosynthesis. The cgp1 mutant did not show a decrease in ornithine content, whereas the analysis of CGP1 structure did not rule out the possibility that CGP1 has Cys-Gly dipeptidase and NAOD activities. Therefore, we propose that CGP1 is a Cys-Gly dipeptidase that functions in the cytosolic GSH degradation pathway and may play dual roles in GSH and ornithine metabolism.

Cloning, expression and characterization of metalloproteinase HypZn from Aspergillus niger

A predicted metalloproteinase gene, HypZn, was cloned from Aspergillus niger CGMCC 3.7193 and expressed in Pichia pastoris GS115, and the physicochemical characteristics of recombinant HypZn were investigated after separation and purification. The results showed that the specific activity of the purified HypZn reached 1859.2 U/mg, and the optimum temperature and pH value of HypZn were 35°C and 7.0, respectively. HypZn remained stable both at 40°C and at pH values between 5.0 and 8.0. The preferred substrate of HypZn was soybean protein isolates, and the Km and Vmax values were 21.5 μmol/mL and 4926.6 μmol/(mL∙min), respectively. HypZn was activated by Co2+ and Zn2+ and inhibited by Cu2+ and Fe2+. The degree of soybean protein isolate hydrolysis reached 14.7%, and the hydrolysates were of uniform molecular weight. HypZn could tolerate 5000 mM NaCl and completely lost its activity after 30 min at 50°C. The enzymological characterizations indicated that HypZn has great application potential in the food industry, especially in fermented food processing.

Molecular mechanism of selective substrate engagement and inhibitor disengagement of cysteine synthase

O-acetyl serine sulfhydrylase (OASS), referred to as cysteine synthase (CS), synthesizes cysteine from O-acetyl serine (OAS) and sulfur in bacteria and plants. The inherent challenge for CS is to overcome 4 to 6 log-folds stronger affinity for its natural inhibitor, serine acetyltransferase (SAT), as compared with its affinity for substrate, OAS. Our recent study showed that CS employs a novel competitive-allosteric mechanism to selectively recruit its substrate in the presence of natural inhibitor. In this study, we trace the molecular features that control selective substrate recruitment. To generalize our findings, we used CS from three different bacteria (Haemophilus, Salmonella, and Mycobacterium) as our model systems and analyzed structural and substrate-binding features of wild-type CS and its ∼13 mutants. Results show that CS uses a noncatalytic residue, M120, located 20 Å away from the reaction center, to discriminate in favor of substrate. M120A and background mutants display significantly reduced substrate binding, catalytic efficiency, and inhibitor binding. Results shows that M120 favors the substrate binding by selectively enhancing the affinity for the substrate and disengaging the inhibitor by 20 to 286 and 5- to 3-folds, respectively. Together, M120 confers a net discriminative force in favor of substrate by 100- to 858-folds.

Carnosine modulates glutamine synthetase expression in senescent astrocytes exposed to oxygen-glucose deprivation/recovery

Carnosine is believed to be neuroprotective in cerebral ischemia. However, few reports concern its function on senescent astrocytes during cerebral ischemia. The aim of this study was to investigate the effects of carnosine on cell damage and glutamine synthetase (GS) expression in D-galactose-induced senescent astrocytes exposed to oxygen-glucose deprivation/recovery (OGD/R). The results showed that OGD/R caused massive cell damage and a significant decrease in GS expression both in the young and senescent astrocytes. The GS expression level was partly recovered whereas it continued to decline in the recovery stage in the young and senescent astrocytes, respectively. Decreased GS expression significantly inhibited glutamate uptake and glutamine production and release. Carnosine prevented the cell damage, rescued the expression of GS and reversed the glutamate uptake activity and glutamine production in the senescent astrocytes exposed to OGD/R. The modulatory effect of carnosine on GS expression was partly antagonized by pyrilamine, a selective histamine H₁ receptors antagonist, but not bestatin. Bisindolylmaleimide II, a broad-spectrum inhibitor of PKC could also reverse the action of carnosine on GS expression. Thus, histamine H₁ receptors and PKC pathway may be involved in the modulatory action of carnosine in GS expression in the senescent astrocytes exposed to OGD/R.

Evidence for an essential role of intradimer interaction in catalytic function of carnosine dipeptidase II using electrospray-ionization mass spectrometry

Carnosine dipeptidase II (CN2/CNDP2) is an M20 family metallopeptidase that hydrolyses various dipeptides including β-alanyl-L-histidine (carnosine). Crystallographic analysis showed that CN2 monomer is composed of one catalytic and one dimerization domains, and likely to form homodimer. In this crystal, H228 residue of the dimerization domain interacts with the substrate analogue bestatin on the active site of the dimer counterpart, indicating that H228 is involved in enzymatic reaction. In the present study, the role of intradimer interaction of CN2 in its catalytic activity was investigated using electrospray-ionization time-of-flight mass spectrometry (ESI-TOF MS). First, a dimer interface mutant I319K was prepared and shown to be present as a folded monomer in solution as examined by using ESI-TOF MS. Since the mutant was inactive, it was suggested that dimer formation is essential to its enzymatic activity. Next, we prepared H228A and D132A mutant proteins with different N-terminal extended sequences, which enabled us to monitor dimer exchange reaction by ESI-TOF MS. The D132A mutant is a metal ligand mutant and also inactive. But the activity was partially recovered time-dependently when H228A and D132A mutant proteins were incubated together. In parallel, H228A/D132A heterodimer was formed as detected by ESI-TOF MS, indicating that interaction of a catalytic center with H228 residue of the other subunit is essential to the enzymatic reaction. These results provide evidence showing that intradimer interaction of H228 with the reaction center of the dimer counterpart is essential to the enzymatic activity of CN2.