Biosynthesis of proteodermatan sulfate in cultured human fibroblasts

Biosynthesis and secretion of proteodermatan sulfate produced by cultured human skin fibroblasts were investigated employing immunological procedures. During an incubation period of 10 min in the presence of [3H]leucine, two core protein forms of Mr = 46,000 and 44,000, respectively, were synthesized. They were converted to mature proteodermatan sulfate with a half-time of approximately 12 min. Fifty per cent of total mature proteodermatan sulfate were found in the culture medium after a 35-min chase. Six to eight per cent remained associated with the cell layer after a chase of 6 h. In the presence of tunicamycin, fibroblasts synthesized a single core protein of Mr = 38,000 that was converted to mature proteodermatan sulfate and secreted with similar kinetics as the N-glycosylated species. Subtle differences in the molecular size of core proteins were noted when cell-associated and secreted proteodermatan sulfate were degraded with chondroitin ABC lyase, but core proteins free of N-linked oligosaccharides were identical. Labeling with [3H]mannose revealed that secreted proteodermatan sulfate contains two or three complex-type or two complex-type and one high-mannose-type N-linked oligosaccharide chains. The N-glycans are bound to a 21-kDa fragment of the core protein. After incubation in the presence of [3H]glucosamine, the [3H]galactosamine/[3H]glucosamine ratio was 3.76 and 3.30 for secreted and cell-associated proteodermatan sulfate, respectively. Evidence for the presence of O-linked oligosaccharides could not be obtained. Small amounts of core protein free of dermatan sulfate chains were secreted when the cultures were treated with p-nitrophenyl-beta-D-xyloside.     

Biosynthesis of embryonic prealbumin by cultured human fibroblasts

Embryonic prealbumin (EPA) was found in human cultured fibroblasts by means of immunodiffusion and immunofluorescence. No quantitative or qualitative differences in the content and localization of this antigen were revealed in embryonic and adult fibroblasts. A conclusion has been made that EPA is the product of human cultured fibroblasts and can be used as a marker of this type of cells.     

Functional properties of the active core of human cystathionine beta-synthase crystals

Human cystathionine beta-synthase is a pyridoxal 5'-phosphate enzyme containing a heme binding domain and an S-adenosyl-l-methionine regulatory site. We have investigated by single crystal microspectrophotometry the functional properties of a mutant lacking the S-adenosylmethionine binding domain. Polarized absorption spectra indicate that oxidized and reduced hemes are reversibly formed. Exposure of the reduced form of enzyme crystals to carbon monoxide led to the complete release of the heme moiety. This process, which takes place reversibly and without apparent crystal damage, facilitates the preparation of a heme-free human enzyme. The heme-free enzyme crystals exhibited polarized absorption spectra typical of a pyridoxal 5'-phosphate-dependent protein. The exposure of these crystals to increasing concentrations of the natural substrate l-serine readily led to the formation of the key catalytic intermediate alpha-aminoacrylate. The dissociation constant of l-serine was found to be 6 mm, close to that determined in solution. The amount of the alpha-aminoacrylate Schiff base formed in the presence of l-serine was pH independent between 6 and 9. However, the rate of the disappearance of the alpha-aminoacrylate, likely forming pyruvate and ammonia, was found to increase at pH values higher than 8. Finally, in the presence of homocysteine the alpha-aminoacrylate-enzyme absorption band readily disappears with the concomitant formation of the absorption band of the internal aldimine, indicating that cystathionine beta-synthase crystals catalyze both beta-elimination and beta-replacement reactions. Taken together, these findings demonstrate that the heme moiety is not directly involved in the condensation reaction catalyzed by cystathionine beta-synthase.     

Dioxygen reactivity and heme redox potential of truncated human cystathionine beta-synthase

Cystathionine beta-synthase (CBS) catalyzes the condensation of serine and homocysteine to cystathionine, which represents the committing step in the transsulfuration pathway. CBS is unique in being a pyridoxal phosphate-dependent enzyme that has a heme cofactor. The activity of CBS under in vitro conditions is responsive to the redox state of the heme, which is distant from the active site and has been postulated to play a regulatory role. The heme in CBS is unusual; it is six-coordinate, low spin, and contains cysteine and histidine as axial ligands. In this study, we have assessed the redox behavior of a human CBS dimeric variant lacking the C-terminal regulatory domain. Potentiometric redox titrations showed a reversible response with a reduction potential of -291 +/- 5 mV versus the normal hydrogen electrode, at pH 7.2. Stopped-flow kinetic determinations demonstrated that Fe(II)CBS reacted with dioxygen yielding Fe(III)CBS without detectable formation of an intermediate species. A linear dependence of the apparent rate constant of Fe(II)CBS decay on dioxygen concentration was observed and yielded a second-order rate constant of (1.11 +/- 0.07) x 10 (5) M (-1) s (-1) at pH 7.4 and 25 degrees C for the direct reaction of Fe(II)CBS with dioxygen. A similar reactivity was observed for full-length CBS. Heme oxidation led to superoxide radical generation, which was detected by the superoxide dismutase (SOD)-inhibitable oxidation of epinephrine. Our results show that CBS may represent a previously unrecognized source of cytosolic superoxide radical.     

Reaction mechanism and regulation of cystathionine beta-synthase

In mammals, cystathionine beta-synthase catalyzes the first step in the transsulfuration pathway which provides an avenue for the conversion of the essential amino acid, methionine, to cysteine. Cystathionine beta-synthase catalyzes a PLP-dependent condensation of serine and homocysteine to cystathionine and is unique in also having a heme cofactor. In this review, recent advances in our understanding of the kinetic mechanism of the yeast and human enzymes as well as pathogenic mutants of the human enzyme and insights into the role of heme in redox sensing are discussed from the perspective of the crystal structure of the catalytic core of the human enzyme.     

Structural insights into mutations of cystathionine beta-synthase

Cystathionine beta-synthase (CBS) is a unique heme-containing enzyme that catalyses a pyridoxal 5'-phosphate (PLP)-dependent condensation of serine and homocysteine to give cystathionine. Deficiency of CBS leads to homocystinuria, an inherited disease of sulfur amino acid metabolism characterised by increased levels of homocysteine and methionine and decreased levels of cysteine. Presently, more than 100 CBS mutations have been described which lead to homocystinuria with different degrees of severity in the patients. We have recently solved the crystal structure of a truncated form of this enzyme, which enables us to correlate some of these mutations with the structure.     

Binding of pyridoxal 5'-phosphate to the heme protein human cystathionine beta-synthase

Cystathionine beta-synthase (CBS), a pyridoxal 5'-phosphate (PLP) dependent enzyme, catalyzes the condensation of serine and homocysteine to form cystathionine. Mammalian CBS was recently shown to be a heme protein. While the role of heme in CBS is unknown, catalysis by CBS can be explained solely by participation of PLP in the reaction mechanism. In this study, treatment of CBS with sodium borohydride selectively reduced the Schiff base but did not affect the heme. Purification and sequencing of the PLP-cross-linked peptide from a trypsin digest of the reduced enzyme revealed the evolutionarily conserved Lys119 to be the residue forming the Schiff base. Serine and hydroxylamine form an alpha-aminoacrylate and an oxime with PLP in CBS, respectively. The sulfhydryl-containing substrate, homocysteine, disturbs the heme environment but does not interact with PLP. In contrast to other PLP-dependent enzymes, CBS emits no PLP-related fluorescence when excited at 296 or 330 nm. PLP but not heme dissociates from the enzyme in the presence of hydroxylamine. The dissociation of PLP is a multistage process involving a short approximately 500 s lag phase, followed by a rapid inactivation and a slower PLP-oxime formation. PLP-free CBS exhibits a decrease of secondary structure as well as loss of CBS activity that can be only partially restored by PLP. This study constitutes the first comprehensive investigation of PLP interaction with a heme protein.     

Dynamics of carbon monoxide binding to cystathionine beta-synthase

Cystathionine beta-synthase (CBS) condenses homocysteine, a toxic metabolite, with serine in a pyridoxal phosphate-dependent reaction. It also contains a heme cofactor to which carbon monoxide (CO) or nitric oxide can bind, resulting in enzyme inhibition. To understand the mechanism of this regulation, we have investigated the equilibria and kinetics of CO binding to the highly active catalytic core of CBS, which is dimeric. CBS exhibits strong anticooperativity in CO binding with successive association constants of 0.24 and 0.02 microm(-1). Stopped flow measurements reveal slow CO association (0.0166 s(-1)) limited by dissociation of the endogenous ligand, Cys-52. Rebinding of CO and of Cys-52 following CO photodissociation were independently monitored via time-resolved resonance Raman spectroscopy. The Cys-52 rebinding rate, 4000 s(-1), is essentially unchanged between pH 7.6 and 10.5, indicating that the pK(a) of Cys-52 is shifted below pH 7.6. This effect is attributed to the nearby Arg-266 residue, which is proposed to form a salt bridge with the dissociated Cys-52, thereby inhibiting its protonation and slowing rebinding to the Fe. This salt bridge suggests a pathway for enzyme inactivation upon CO binding, because Arg-266 is located on a helix that connects the heme and pyridoxal phosphate cofactor domains.     

Plasma albumin cysteinylation is regulated by cystathionine beta-synthase

High homocysteine (Hcy) levels are a well-known independent risk factor for endothelial damage in atherosclerosis. We examined whether a rat intestinal model of ischemia-reperfusion was associated with high Hcy and with the modification of plasma albumin into cysteinylated species (CysAlb). The three treatment groups were as follows: midline abdominal incision (group A, n=10), followed by ligation of the superior mesenteric artery for a period of 2h (group B, n=3), and followed by reperfusion for 1h (group C, n=10). Hcy levels were 2.5-fold higher in group C than group A (p<0.05). 100% and 73.44+/-0.04% of Alb were modified into Cys species in groups C and B, respectively, compared to 51.2% in group A. A cystathionine beta-synthase (CBS) deficient mouse model, known to have high plasma Hcy levels, was also used to determine the extent of CysAlb. Hcy levels, %CysAlb, and %HcyAlb were 180.1+/-45.7 microM, 0%, and 23.4+/-4.4% in CBS deficient mice, while in control mice, those values were 5.7+/-1.8 microM, 24.2+/-4.1%, and 0%, respectively (p<0.05). High CysAlb and Hcy levels were observed in a rat model of bowel ischemia/reperfusion while high HcyAlb and Hcy levels with no CysAlb were observed in the CBS deficient mice. CysAlb may serve as a biomarker for the severity of gut ischemia, and high Hcy may explain endothelial damage associated with this model. Additionally, active CBS is essential for the formation of CysAlb.     

Structure of human cystathionine beta-synthase: a unique pyridoxal 5'-phosphate-dependent heme protein

Cystathionine beta-synthase (CBS) is a unique heme- containing enzyme that catalyzes a pyridoxal 5'-phosphate (PLP)-dependent condensation of serine and homocysteine to give cystathionine. Deficiency of CBS leads to homocystinuria, an inherited disease of sulfur metabolism characterized by increased levels of the toxic metabolite homocysteine. Here we present the X-ray crystal structure of a truncated form of the enzyme. CBS shares the same fold with O-acetylserine sulfhydrylase but it contains an additional N-terminal heme binding site. This heme binding motif together with a spatially adjacent oxidoreductase active site motif could explain the regulation of its enzyme activity by redox changes.     

Solvent-accessible cysteines in human cystathionine beta-synthase: crucial role of cysteine 431 in S-adenosyl-L-methionine binding

Cystathionine beta-synthase (CBS) is a tetrameric heme protein that catalyzes the PLP-dependent condensation of serine and homocysteine to cystathionine. CBS occupies a crucial regulatory position between the methionine cycle and transsulfuration. Human CBS contains 11 cysteine residues that are highly conserved in mammals but completely absent in the yeast enzyme, which catalyzes an identical reaction, suggesting a possible regulatory role for some of these residues. In this report, we demonstrate that in both the presence and absence of the CBS allosteric regulator S-adenosyl-l-methionine (AdoMet), only C15 and C431 of human CBS are solvent accessible. Mutagenesis of C15 to serine did not affect catalysis or AdoMet activation but significantly reduced aggregation of the purified enzyme in vitro. Mutagenesis of C431 resulted in a constitutively activated form of CBS that could not be further activated by either AdoMet or thermal activation. We and others have previously reported a number of C-terminal CBS point mutations that result in a decreased or abolished response to AdoMet. In contrast to all of these previously investigated CBS mutants, the C431 mutant form of CBS was unable to bind AdoMet, indicating that either this residue is directly involved in AdoMet binding or its absence induces a conformational change that destroys the integrity of the binding site for this regulatory ligand.     

Human cystathionine beta-synthase is a target for sumoylation

Cystathionine beta-synthase (CBS) catalyzes the first irreversible step in the transsulfuration pathway and commits the toxic metabolite, homocysteine, to the synthesis of cysteine. Mutations in CBS are the most common cause of severe hereditary hyperhomocysteinemia. The molecular basis of the organ-specific pathologies associated with CBS deficiency is unknown as is the significance of the reported interaction between CBS and Huntingtin protein. In this study, we have used the yeast two-hybrid approach to screen for proteins that interact with CBS and have identified several components of the sumoylation pathway including Ubc9, PIAS1, PIAS3, Pc2, and RanBPM. We demonstrate that CBS is modified by the small ubiquitin-like modifier-1 protein (SUMO-I) under both in vitro and in vivo conditions. Deletion analysis of CBS indicates that the C-terminal regulatory domain is required for interaction with proteins in the sumoylation machinery. Sumoylated CBS is present in the nucleus where it is associated with the nuclear scaffold. The discovery that CBS is a target of sumoylation adds another layer to the complex regulation of this enzyme and reveals a previously unknown residence for this protein, i.e., in the nucleus.     

Mapping peptides correlated with transmission of intrasteric inhibition and allosteric activation in human cystathionine beta-synthase

Cystathionine beta-synthase plays a key role in the intracellular disposal of homocysteine and is the single most common locus of mutations associated with homocystinuria. Elevated levels of homocysteine are correlated with heart disease, Alzheimer's and Parkinson's diseases, and neural tube defects. Cystathionine beta-synthase is modular and subjected to complex regulation, but insights into the structural basis of this regulation are lacking. We have employed hydrogen exchange mass spectrometry to map peptides whose motions are correlated with transmission of intrasteric inhibition and allosteric activation. The mass spectrometric data provide an excellent correlation between kinetically and conformationally distinguishable states of the enzyme. We also demonstrate that a pathogenic regulatory domain mutant, D444N, is conformationally locked in one of two states sampled by the wild type enzyme. Our hydrogen exchange data identify surfaces that are potentially involved in the juxtaposition of the regulatory and catalytic domains and form the basis of a docked structural model for the full-length enzyme.     

The reaction of yeast cystathionine beta-synthase is rate-limited by the conversion of aminoacrylate to cystathionine

Our studies of the reaction mechanism of cystathionine beta-synthase from Saccharomyces cerevisiae (yeast) are facilitated by the spectroscopic properties of the pyridoxal phosphate coenzyme that forms a series of intermediates in the reaction of L-serine and L-homocysteine to form L-cystathionine. To characterize these reaction intermediates, we have carried out rapid-scanning stopped-flow and single-wavelength stopped-flow kinetic measurements under pre-steady-state conditions, as well as circular dichroism and fluorescence spectroscopy under steady-state conditions. We find that the gem-diamine and external aldimine of aminoacrylate are the primary intermediates in the forward half-reaction with L-serine and that the external aldimine of aminoacrylate or its complex with L-homocysteine is the primary intermediate in the reverse half-reaction with L-cystathionine. The second forward half-reaction of aminoacrylate with L-homocysteine is rapid. No primary kinetic isotope effect was obtained in the forward half-reaction with L-serine. The results provide evidence (1) that the formation of the external aldimine of L-serine is faster than the formation of the aminoacrylate intermediate, (2) that aminoacrylate is formed by the concerted removal of the alpha-proton and the hydroxyl group of L-serine, and (3) that the rate of the overall reaction is rate-limited by the conversion of aminoacrylate to L-cystathionine. We compare our results with cystathionine beta-synthase with those of related investigations of tryptophan synthase and O-acetylserine sulfhydrylase.     

High prevalence of a mutation in the cystathionine beta-synthase gene.

We found that a mutation previously described by Sebastio et al., involving a 68-bp insertion in the coding region of exon 8 of the cystathionine-beta-synthase (CBS) gene in a single patient with homocystinuria, is highly prevalent. In our control population, 11.7% (9/77) of the individuals were heterozygous carriers of this mutation. In contrast to the previous report, which assumed that the 68-bp insertion introduced a premature-termination codon and resulted in a nonfunctional CBS enzyme, we found that the presence of this mutation is not associated with hyperhomocysteinemia. Assay of CBS activity in transformed lymphocytes from individuals who were heterozygous or homozygous for this mutation showed normal activity. Furthermore, reverse-transcripion-PCR showed that individuals carrying this mutation have normal size mRNA. Our results suggest that the insertion creates an alternate splicing site, which eliminates not only the inserted intronic sequences but also the T833C mutation associated with this insertion. The net result is the generation of both quantitatively and qualitatively normal mRNA and CBS enzyme. Although the mutation does not seem to affect the activity of the CBS enzyme, the prevalence is somewhat increased in patients with premature coronary-artery disease, although the difference is not statistically significant.     

Characterization of the heme in human cystathionine beta-synthase by X-ray absorption and electron paramagnetic resonance spectroscopies

Human cystathionine beta-synthase is one of two key enzymes involved in intracellular metabolism of homocysteine. It catalyzes a beta-replacement reaction in which the thiolate of homocysteine replaces the hydroxyl group of serine to give the product, cystathionine. The enzyme is unusual in its dependence on two cofactors: pyridoxal phosphate and heme. The requirement for pyridoxal phosphate is expected on the basis of the nature of the condensation reaction that is catalyzed; however the function of the heme in this protein is unknown. We have examined the spectroscopic properties of the heme in order to assign the axial ligands provided by the protein. The heme Soret peak of ferric cystathionine beta-synthase is at 428 nm and shifts to approximately 395 nm upon addition of the thiol chelator, mercuric chloride. This is indicative of 6-coordinate low-spin heme converting to a 5-coordinate high-spin heme. The enzyme as isolated exhibits a rhombic EPR signal with g values of 2.5, 2.3, and 1.86, which are similar to those of heme proteins and model complexes with imidazole/thiolate ligands. Mercuric chloride treatment of the enzyme results in conversion of the rhombic EPR signal to a g = 6 signal, consistent with formation of the high-spin ferric heme. The X-ray absorption data reveal that iron in ferric cystathionine beta-synthase is 6-coordinate, with 1 high-Z scatterer and 5 low-Z scatterers. This is consistent with the presence of 5 nitrogens and 1 sulfur ligand. Together, these data support assignment of the axial ligands as cysteinate and imidazole in ferric cystathionine beta-synthase.