Cystathionine beta-synthase mutants exhibit changes in protein unfolding: conformational analysis of misfolded variants in crude cell extracts

The disease-linked R336C mutation in cystathionine β-synthase disrupts communication with the PLP cofactor, yet maintains the enzyme's overall structural integrity

Cystathionine β-synthase (CBS) is a pyridoxal-phosphate (PLP)-dependent enzyme essential for the reverse transsulfuration pathway, where homocysteine and serine combine to form cystathionine, the immediate precursor of cysteine. Mutations in the CBS gene cause homocystinuria, a disorder associated with intellectual disability, multisystem complications, and reduced life expectancy. The CBS p.R336C mutation, linked to severe pyridoxine non-responsiveness, results in reduced enzyme activity, previously attributed to protein instability and weakened substrate and PLP binding. To clarify the effects of the pathological R336C mutation, we performed biochemical, biophysical, and crystallographic analyses, as well as molecular dynamics simulations. Our findings show that the R336C mutation minimally impacts the structural environment around residue 336, does not cause enzyme misfolding, and does not impair the binding of PLP or the allosteric activator S-adenosylmethionine (AdoMet) binding. Instead, the mutation induces subtle reorientations in nearby hydrophobic residues, including F185 and Y381, altering intramolecular contacts that perturb the interaction between asparagine 149 and the O3 oxygen of PLP. This alteration is known to potentially shift the tautomeric equilibrium of the PLP Schiff base from its catalytically active ketoenamine form to the inactive enolimine form, which aligns with the reduced activity of the R336C variant. Additionally, the R336C mutation disrupts intermolecular contacts between the catalytic core and Bateman module, altering the Bateman module's intrinsic mobility in the enzyme's basal state and potentially affecting the cavity opening required for catalysis. Importantly, the R336C variant retains the native enzyme's ability to assemble into polymeric chains in crystals, preserving its filament formation capacity.

A proactive genotype-to-patient-phenotype map for cystathionine beta-synthase

BACKGROUND

For the majority of rare clinical missense variants, pathogenicity status cannot currently be classified. Classical homocystinuria, characterized by elevated homocysteine in plasma and urine, is caused by variants in the cystathionine beta-synthase (CBS) gene, most of which are rare. With early detection, existing therapies are highly effective.

METHODS

Damaging CBS variants can be detected based on their failure to restore growth in yeast cells lacking the yeast ortholog CYS4. This assay has only been applied reactively, after first observing a variant in patients. Using saturation codon-mutagenesis, en masse growth selection, and sequencing, we generated a comprehensive, proactive map of CBS missense variant function.

RESULTS

Our CBS variant effect map far exceeds the performance of computational predictors of disease variants. Map scores correlated strongly with both disease severity (Spearman's ϱ = 0.9) and human clinical response to vitamin B6 (ϱ = 0.93).

CONCLUSIONS

We demonstrate that highly multiplexed cell-based assays can yield proactive maps of variant function and patient response to therapy, even for rare variants not previously seen in the clinic.

Crystal structure of cystathionine β-synthase from honeybee Apis mellifera

Cystathionine β-synthase (CBS), the key enzyme in the transsulfuration pathway, links methionine metabolism to the biosynthesis of cellular redox controlling molecules. CBS catalyzes the pyridoxal-5'-phosphate-dependent condensation of serine and homocysteine to form cystathionine, which is subsequently converted into cysteine. Besides maintaining cellular sulfur amino acid homeostasis, CBS also catalyzes multiple hydrogen sulfide-generating reactions using cysteine and homocysteine as substrates. In mammals, CBS is activated by S-adenosylmethionine (AdoMet), where it can adopt two different conformations (basal and activated), but exists as a unique highly active species in fruit fly Drosophila melanogaster. Here we present the crystal structure of CBS from honeybey Apis mellifera, which shows a constitutively active dimeric species and let explain why the enzyme is not allosterically regulated by AdoMet. In addition, comparison of available CBS structures unveils a substrate-induced closure of the catalytic cavity, which in humans is affected by the AdoMet-dependent regulation and likely impaired by the homocystinuria causing mutation T191M.

A Clinically Relevant Variant of the Human Hydrogen Sulfide-Synthesizing Enzyme Cystathionine β-Synthase: Increased CO Reactivity as a Novel Molecular Mechanism of Pathogenicity?

The human disease classical homocystinuria results from mutations in the gene encoding the pyridoxal 5′-phosphate- (PLP-) dependent cystathionine β-synthase (CBS), a key enzyme in the transsulfuration pathway that controls homocysteine levels, and is a major source of the signaling molecule hydrogen sulfide (H₂S). CBS activity, contributing to cellular redox homeostasis, is positively regulated by S-adenosyl-L-methionine (AdoMet) but fully inhibited upon CO or NO• binding to a noncatalytic heme moiety. Despite extensive studies, the molecular basis of several pathogenic CBS mutations is not yet fully understood. Here we found that the ferrous heme of the reportedly mild p.P49L CBS variant has altered spectral properties and markedly increased affinity for CO, making the protein much more prone than wild type (WT) CBS to inactivation at physiological CO levels. The higher CO affinity could result from the slightly higher flexibility in the heme surroundings revealed by solving at 2.80-Å resolution the crystallographic structure of a truncated p.P49L. Additionally, we report that p.P49L displays impaired H₂S-generating activity, fully rescued by PLP supplementation along the purification, despite a minor responsiveness to AdoMet. Altogether, the results highlight how increased propensity to CO inactivation of an otherwise WT-like variant may represent a novel pathogenic mechanism in classical homocystinuria.

Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency

Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine. Patients can present to many different specialists and diagnosis is often delayed. Severely affected patients usually present in childhood with ectopia lentis, learning difficulties and skeletal abnormalities. These patients generally require treatment with a low-methionine diet and/or betaine. In contrast, mildly affected patients are likely to present as adults with thromboembolism and to respond to treatment with pyridoxine. In this article, we present recommendations for the diagnosis and management of CBS deficiency, based on a systematic review of the literature. Unfortunately, the quality of the evidence is poor, as it often is for rare diseases. We strongly recommend measuring the plasma total homocysteine concentrations in any patient whose clinical features suggest the diagnosis. Our recommendations may help to standardise testing for pyridoxine responsiveness. Current evidence suggests that patients are unlikely to develop complications if the plasma total homocysteine concentration is maintained below 120 μmol/L. Nevertheless, we recommend keeping the concentration below 100 μmol/L because levels fluctuate and the complications associated with high levels are so serious.

Deletion of AS87_03730 gene changed the bacterial virulence and gene expression of Riemerella anatipestifer

Riemerella anatipestifer is an important pathogen of waterfowl, which causes septicemia anserum exsudativa in ducks. In this study, an AS87_03730 gene deletion R. anatipestifer mutant Yb2ΔAS87_03730 was constructed to investigate the role of AS87_03730 on R. anatipestifer virulence and gene regulation. By deleting a 708-bp fragment from AS87_03730, the mutant Yb2ΔAS87_03730 showed a significant decreased growth rate in TSB and invasion capacity in Vero cells, compared to wild-type strain Yb2. Moreover, the median lethal dose (LD₅₀) of Yb2ΔAS87_03730 was 1.24 × 10⁷ colony forming units (CFU), which is about 80-fold attenuated than that of Yb2 (LD₅₀ = 1.53 × 10⁵ CFU). Furthermore, RNA-Seq analysis and Real-time PCR indicated 19 up-regulated and two down-regulated genes in Yb2ΔAS87_03730. Functional analysis revealed that 12 up-regulated genes were related to “Translation, ribosomal structure and biogenesis”, two were classified into “Cell envelope biogenesis, outer membrane”, one was involved in “Amino acid transport and metabolism”, and the other four had unknown functions. Polymerase chain reaction and sequence analysis indicated that the AS87_03730 gene is highly conserved among R. anatipestifer strains, as the percent sequence identity was over 93.5%. This study presents evidence that AS87_03730 gene is involved in bacterial virulence and gene regulation of R. anatipestifer.

Chaperone therapy for homocystinuria: the rescue of CBS mutations by heme arginate

Classical homocystinuria is caused by mutations in the cystathionine β-synthase (CBS) gene. Previous experiments in bacterial and yeast cells showed that many mutant CBS enzymes misfold and that chemical chaperones enable proper folding of a number of mutations. In the present study, we tested the extent of misfolding of 27 CBS mutations previously tested in E. coli under the more folding-permissive conditions of mammalian CHO-K1 cells and the ability of chaperones to rescue the conformation of these mutations. Expression of mutations in mammalian cells increased the median activity 16-fold and the amount of tetramers 3.2-fold compared with expression in bacteria. Subsequently, we tested the responses of seven selected mutations to three compounds with chaperone-like activity. Aminooxyacetic acid and 4-phenylbutyric acid exhibited only a weak effect. In contrast, heme arginate substantially increased the formation of mutant CBS protein tetramers (up to sixfold) and rescued catalytic activity (up to ninefold) of five out of seven mutations (p.A114V, p.K102N, p.R125Q, p.R266K, and p.R369C). The greatest effect of heme arginate was observed for the mutation p.R125Q, which is non-responsive to in vivo treatment with vitamin B(6). Moreover, the heme responsiveness of the p.R125Q mutation was confirmed in fibroblasts derived from a patient homozygous for this genetic variant. Based on these data, we propose that a distinct group of heme-responsive CBS mutations may exist and that the heme pocket of CBS may become an important target for designing novel therapies for homocystinuria.

Insights into the regulatory domain of cystathionine Beta-synthase: characterization of six variant proteins

Cystathionine beta-synthase (CBS) catalyzes the formation of cystathionine from homocysteine and serine. CBS is allosterically activated by S-adenosylmethionine (SAM), which binds to its C-terminal regulatory domain. Mutations in this domain lead to variants with high residual activity but lacking SAM activation. We characterized six C-terminal CBS variants (p.P427L, p.D444N, p.V449G, p.S500L, p.K523Sfs*18, and p.L540Q). To understand the effect of C-terminal mutations on the functional/structural properties of CBS, we performed dynamic light scattering, differential scanning fluorimetry, limited proteolysis, enzymatic characterization, and determination of SAM-binding affinity. Kinetic data confirm that the enzymatic function of these variants is not impaired. Although lacking SAM activation, the p.P427L and p.S500L were able to bind SAM at a lower extent than the wild type (WT), confirming that SAM binding and activation can be two independent events. At the structural level, the C-terminal variants presented various effects, either showing catalytic core instability and increased susceptibility toward aggregation or presenting with similar or higher stability than the WT. Our study highlights as the common feature to the C-terminal variants an impaired binding of SAM and no increase in enzymatic activity with physiological concentrations of the activator, suggesting the loss of regulation by SAM as a potential pathogenic mechanism.

Energetics-based methods for protein folding and stability measurements

Over the past 15 years, a series of energetics-based techniques have been developed for the thermodynamic analysis of protein folding and stability. These techniques include Stability of Unpurified Proteins from Rates of amide H/D Exchange (SUPREX), pulse proteolysis, Stability of Proteins from Rates of Oxidation (SPROX), slow histidine H/D exchange, lysine amidination, and quantitative cysteine reactivity (QCR). The above techniques, which are the subject of this review, all utilize chemical or enzymatic modification reactions to probe the chemical denaturant- or temperature-induced equilibrium unfolding properties of proteins and protein-ligand complexes. They employ various mass spectrometry-, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)-, and optical spectroscopy-based readouts that are particularly advantageous for high-throughput and in some cases multiplexed analyses. This has created the opportunity to use protein folding and stability measurements in new applications such as in high-throughput screening projects to identify novel protein ligands and in mode-of-action studies to identify protein targets of a particular ligand.

The role of surface electrostatics on the stability, function and regulation of human cystathionine β-synthase, a complex multidomain and oligomeric protein

Human cystathionine β-synthase (hCBS) is a key enzyme of sulfur amino acid metabolism, controlling the commitment of homocysteine to the transsulfuration pathway and antioxidant defense. Mutations in hCBS cause inherited homocystinuria (HCU), a rare inborn error of metabolism characterized by accumulation of toxic homocysteine in blood and urine. hCBS is a complex multidomain and oligomeric protein whose activity and stability are independently regulated by the binding of S-adenosyl-methionine (SAM) to two different types of sites at its C-terminal regulatory domain. Here we study the role of surface electrostatics on the complex regulation and stability of hCBS using biophysical and biochemical procedures. We show that the kinetic stability of the catalytic and regulatory domains is significantly affected by the modulation of surface electrostatics through noticeable structural and energetic changes along their denaturation pathways. We also show that surface electrostatics strongly affect SAM binding properties to those sites responsible for either enzyme activation or kinetic stabilization. Our results provide new insight into the regulation of hCBS activity and stability in vivo with implications for understanding HCU as a conformational disease. We also lend experimental support to the role of electrostatic interactions in the recently proposed binding modes of SAM leading to hCBS activation and kinetic stabilization.

The interplay between protein stability and dynamics in conformational diseases: the case of hPGK1 deficiency

Conformational diseases often show defective protein folding efficiency in vivo upon mutation, affecting protein properties such as thermodynamic stability and folding/unfolding/misfolding kinetics as well as the interactions of the protein with the protein homeostasis network. Human phosphoglycerate kinase 1 (hPGK1) deficiency is a rare inherited disease caused by mutations in hPGK1 that lead to loss-of-function. This disease offers an excellent opportunity to explore the complex relationships between protein stability and dynamics because of the different unfolding mechanisms displayed towards chemical and thermal denaturation. This work explores these relationships using two thermostable mutants (p.E252A and p.T378P) causing hPGK1 deficiency and WT hPGK1 using proteolysis and chemical denaturation. p.T378P is degraded ~30-fold faster at low protease concentrations (here, the proteolysis step is rate-limiting) and ~3-fold faster at high protease concentrations (where unfolding kinetics is rate-limiting) than WT and p.E252A, indicating that p.T378P is thermodynamically and kinetically destabilized. Urea denaturation studies support the decrease in thermodynamic stability and folding cooperativity for p.T378P, as well as changes in folding/unfolding kinetics. The present study reveals changes in the folding landscape of hPGK1 upon mutation that may affect protein folding efficiency and stability in vivo, also suggesting that native state stabilizers and protein homeostasis modulators may help to correct folding defects in hPGK1 deficiency. Moreover, detailed kinetic proteolysis studies are shown to be powerful and simple tools to provide deep insight into mutational effects on protein folding and stability in conformational diseases.

Conformational properties of nine purified cystathionine β-synthase mutants

Protein misfolding due to missense mutations is a common pathogenic mechanism in cystathionine β-synthase (CBS) deficiency. In our previous studies, we successfully expressed, purified, and characterized nine CBS mutant enzymes containing the following patient mutations: P49L, P78R, A114V, R125Q, E176K, R266K, P422L, I435T, and S466L. These purified mutants exhibited full heme saturation, normal tetrameric assembly, and high catalytic activity. In this work, we used several spectroscopic and proteolytic techniques to provide a more thorough insight into the conformation of these mutant enzymes. Far-UV circular dichroism, fluorescence, and second-derivative UV spectroscopy revealed that the spatial arrangement of these CBS mutants is similar to that of the wild type, although the microenvironment of the chromophores may be slightly altered. Using proteolysis with thermolysin under native conditions, we found that the majority of the studied mutants is more susceptible to cleavage, suggesting their increased local flexibility or propensity for local unfolding. Interestingly, the presence of the CBS allosteric activator, S-adenosylmethionine (AdoMet), increased the rate of cleavage of the wild type and the AdoMet-responsive mutants, while the proteolytic rate of the AdoMet-unresponsive mutants was not significantly changed. Pulse proteolysis analysis suggested that the protein structure of the R125Q and E176K mutants is significantly less stable than that of the wild type and the other mutants. Taken together, the proteolytic data shows that the conformation of the pathogenic mutants is altered despite retained catalytic activity and normal tetrameric assembly. This study demonstrates that the proteolytic techniques are useful tools for the assessment of the biochemical penalty of missense mutations in CBS.