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Background:The H 2 S-generating human enzyme cystathionine ␤-synthase (CBS) is inhibited by NO ⅐ and CO. Results: NO ⅐ binds to the ferrous heme in human CBS much more quickly than CO and much more tightly than currently thought. Conclusion:Results support the physiological role of NO ⅐ in CBS regulation. Significance: CBS may integrate the cross-talk among NO ⅐ , CO, and H 2 S, major modulators in human (patho)physiology.

Protein folding in the cell is a tightly regulated process, involving a series of proteins, from molecular chaperones to proteases that assist the folding process and monitor the quality of the final product. Despite this control, genetic or sporadic factors may compromise protein folding and the folded state resulting in the formation of non-native misfolded, destabilised, aggregated or fibrillar species. These are hallmarks of the so-called protein conformational disorders, in which the altered protein conformations result in cell toxicity, functional deficiency or lead to dominant negative effects. Examples of such pathologies include neurodegenerative and metabolic disorders. In recent years, it has become clear that several different small chemical compounds such as osmolytes, protein inhibitors, ligands and cofactors exert a chemical chaperoning effect and are able to rescue folding and trafficking defects, minimising or partly overcoming the pathological consequences of protein misfolding. Here we review the different types of chemical chaperones and provide a structural and energetic rationale for their action. Examples of chemical chaperoning are overviewed and discussed on the basis of the reported effects exerted by chemical compounds at different stages of the protein folding process and protein conformational states.

Galactose-1-phosphate uridylyltransferase (GALT) is a key enzyme in galactose metabolism, particularly important in the neonatal period due to ingestion of galactose-containing milk. GALT deficiency results in the genetic disorder classic galactosemia, whose pathophysiology is still not fully elucidated. Whereas classic galactosemia has been hypothesized to result from GALT misfolding, a thorough functional–structural characterization of GALT most prevalent variants was still lacking, hampering the development of alternative therapeutic approaches. The aim of this study was to investigate the structural–functional effects of nine GALT mutations, four of which account for the vast majority of the mutations identified in galactosemic patients. Several methodologies were employed to evaluate the mutations' impact on GALT function, on the protein secondary and tertiary structures, and on the aggregation propensity. The major structural effect concerns disturbed propensity for aggregation, particularly striking for the p.Q188R variant, resulting from the most frequent (∼60%) allele at a worldwide scale. The absence of major effects at the secondary and tertiary structure levels suggests that the disturbed aggregation results from subtle perturbations causing a higher and/or longer exposure of hydrophobic residues in the variants as compared to WT GALT. The results herein described indicate a possible benefit from introducing proteostasis regulators and/or chemical/pharmacological chaperones to prevent the accumulation of protein aggregates, in new avenues of therapeutic research for classic galactosemia.

Cystathionine ␤-synthase (CBS) is a key enzyme in human (patho)physiology with a central role in hydrogen sulfide metabolism. The enzyme is composed of a pyridoxal 5-phosphate-binding catalytic domain, flanked by the following two domains: a heme-binding N-terminal domain and a regulatory C-terminal domain binding S-adenosyl-L-methionine (AdoMet). CO or NO ⅐ binding at the ferrous heme negatively modulates the enzyme activity. Conversely, AdoMet binding stimulates CBS activity. Here, we provide experimental evidence for a functional communication between the two domains. We report that AdoMet binding significantly enhances CBS inhibition by CO. Consistently, we observed increased affinity (ϳ5-fold) and faster association (ϳ10-fold) of CO to the ferrous heme at physiological AdoMet concentrations. NO ⅐ binding to reduced CBS was also enhanced by AdoMet, although to a lesser extent (ϳ2-fold higher affinity) as compared with CO. Importantly, CO and NO ⅐ binding was unchanged by AdoMet in a truncated form of CBS lacking the C-terminal regulatory domain. These unprecedented observations demonstrate that CBS activation by AdoMet puzzlingly sensitizes the enzyme toward inhibition by exogenous ligands, like CO and NO ⅐ . This further supports the notion that CBS regulation is a complex process, involving the concerted action of multiple physiologically relevant effectors.

In general, erythrocytes are highly permeable to water, urea and glycerol.However, expression of aquaporin isoforms in erythrocytes appears to be species characteristic. In the present study, human (hRBC) and bovine (bRBC) erythrocytes were chosen for comparative studies due to their significant difference in membrane glycerol permeability.Osmotic water permeability (P f ) at 23 ºC was (2.89 0.37) 10 -2 and (5.12 0.61) 10 -2 cm s -1 for human and bovine cells respectively, with similar activation energies for water transport. Glycerol permeability (P gly ) for human ((1.37 0.26) 10 -5 cm s -1 ) differed in three orders of magnitude from bovine erythrocytes ((5.82 0.37) 10 -8 cm s -1 ) that also showed higher activation energy for glycerol transport.When compared to human, bovine erythrocytes showed a similar expression pattern of AQP1 glycosylated forms on immunoblot analysis, though in slight higher levels, which could be correlated with the 1.5-fold larger P f found. However, AQP3 expression was not detectable. Immunofluorescence analysis confirmed the absence of AQP3 expression in bovine erythrocyte membranes.In conclusion, lack of AQP3 in bovine erythrocytes points to the lipid pathway as responsible for glycerol permeation and explains the low glycerol permeability and high E a for transport observed in ruminants.

To study the interaction between two different subunits of the heteromeric human phenylalanine hydroxylase (hPAH), present in hyperphenylalaninemic (HPA) compound heterozygous patients, heteroallelic hPAH enzymes were produced. A dual vector expression system was used (PRO Bacterial Expression System) in which each mutant subunit was expressed from a separate compatible vector, with different epitope tags, in a single bacterial host. Experimental conditions were selected in order that each plasmid produced equivalent levels of mutant subunits. In this study, we demonstrated that both subunits were expressed and that the purified heteroallelic enzymes, were catalytically active. As expected, the produced proteins displayed enzymatic activities levels lower than the predicted catalytic activity, calculated by averaging in vitro PAH activities from both alleles, and were strongly dependent on the proteins subunit composition. The obtained data suggest that interactions between the studied hPAH subunits, namely the I65T, R261Q, R270K and V388M, and the wild-type protein occurred. As postulated, this phenomenon could be a source of phenotypic variation in genetic diseases involving multimeric proteins.

A reduced response of cystathionine beta-synthase (CBS) to its allosteric activator S-adenosylmethionine (SAM) has been reported to be a cause of CBS dysfunction in homocystinuria patients. In this work we performed a retrospective analysis of fibroblast data from 62 homocystinuria patients and found that 13 of them presented a disturbed SAM activation. Their genotypic background was identified and the corresponding CBS mutant proteins were produced in E. coli. Nine distinct mutations were detected in 22 independent alleles: the novel mutations p.K269del, p.P427L, p.S500L and p.L540Q; and the previously described mutations p.P49L, p.C165Rfs*2, p.I278T, p.R336H and p.D444N. Expression levels and residual enzyme activities, determined in the soluble fraction of E. coli lysates, strongly correlated with the localization of the affected amino acid residue. C-terminal mutations lead to activities in the range of the wild-type CBS and to oligomeric forms migrating faster than tetramers, suggesting an abnormal conformation that might be responsible for the lack of SAM activation. Mutations in the catalytic core were associated with low protein expression levels, decreased enzyme activities and a higher content of high molecular mass forms. Furthermore, the absence of SAM activation found in the patients' fibroblasts was confirmed for all but one of the characterized recombinant proteins (p.P49L). Our study experimentally supports a deficient regulation of CBS by SAM as a frequently found mechanism in CBS deficiency, which should be considered not only as a valuable diagnostic tool but also as a potential target for the development of new therapeutic approaches in classical homocystinuria.