Cystathionine β-synthase: Difference between revisions
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The human form of CBS is a homotetramer consisting of 63-kDa subunits each of 551 amino acids in length. Two cofactors, pyridoxal 5’-phosphate (PLP) and heme, and two substrates, L-homocysteine and L-serine, are bound to every monomer and the function is further allosterically regulated by S-adenosyl-L-methionine (AdoMet). | The human form of CBS is a homotetramer consisting of 63-kDa subunits each of 551 amino acids in length. Two cofactors, pyridoxal 5’-phosphate (PLP) and heme, and two substrates, L-homocysteine and L-serine, are bound to every monomer and the function is further allosterically regulated by S-adenosyl-L-methionine (AdoMet). | ||
The monomer has a complex domain structure as it is composed of three functional domains, two of which are regulatory and one is catalytic. The middle domain contains the catalytic core responsible for the pyridoxal 5’-phosphate-catalyzed reaction and is flanked by an N-terminal heme-binding domain (70 amino acids), which probably regulates the enzyme in response to redox conditions, and a C-terminal Bateman module consisting of two CBS domains (CBS1, CBS2), protein folding motifs also found in inosine 5’-monophosphate dehydrogenase, chloride channels and several other proteins in various organisms.<ref>PMID:11483494</ref> The C-terminal domain also contains a negative regulatory region that is responsible for allosteric activation of the enzyme by AdoMet. | The monomer has a complex domain structure as it is composed of three functional domains, two of which are regulatory and one is catalytic. The middle domain contains the catalytic core responsible for the pyridoxal 5’-phosphate-catalyzed reaction and is flanked by an N-terminal heme-binding domain (70 amino acids), which probably regulates the enzyme in response to redox conditions, and a C-terminal Bateman module consisting of two CBS domains (CBS1, CBS2), protein folding motifs also found in inosine 5’-monophosphate dehydrogenase, chloride channels and several other proteins in various organisms.<ref>PMID:11483494</ref> The C-terminal domain also contains a negative regulatory region that is responsible for allosteric activation of the enzyme by AdoMet. | ||
Truncation of N-terminal domain produces an enzyme that is still active albeit less so (19%) than the wild type. Truncation of C-terminal domain activates the enzyme by ~2-fold over full-length wild type. In addition, deletion of the Bateman module leads to a change in oligomerization from a tetrameric to a dimeric form of the enzyme | Truncation of N-terminal domain produces an enzyme that is still active albeit less so (19%) than the wild type. Truncation of C-terminal domain activates the enzyme by ~2-fold over full-length wild type. In addition, deletion of the Bateman module leads to a change in oligomerization from a tetrameric to a dimeric form of the enzyme.<ref>PMID:12686134</ref><ref>PMID:29630349</ref> | ||
The CBS tetramer of the full-length enzyme has a strong tendency to aggregate, which makes physical studies very difficult. Therefore recombinant human CBS comprising the amino acid residues 1±413 (C-terminal domain missing), which does not exhibit the aggregating properties, is described. | The CBS tetramer of the full-length enzyme has a strong tendency to aggregate, which makes physical studies very difficult. Therefore recombinant human CBS comprising the amino acid residues 1±413 (C-terminal domain missing), which does not exhibit the aggregating properties, is described. | ||
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== The Reaction Catalyzed by CBS == | == The Reaction Catalyzed by CBS == | ||
Cystathionine β-synthase catalyzes β-elimination and β-replacement reactions. In a typical situation it condensates L-serine and L-homocysteine to give cystathionine but there are also other possible substrates for this enzyme leading to different products. | Cystathionine β-synthase catalyzes β-elimination and β-replacement reactions. In a typical situation it condensates L-serine and L-homocysteine to give cystathionine but there are also other possible substrates for this enzyme leading to different products. | ||
The reaction for synthesis of cystathionine starts with displacement of the internal aldimine between the enzyme active site lysine and pyridoxal 5’-phosphate (E-PLP) by the incoming L-serine. The serine external aldimine adduct (E-PLP-L-ser) forms an aminoacrylate intermediate (E-PLP-aa) that reacts with the incoming second substrate, such as L-homocysteine, to form the (L,L)-cystathionine external aldimine, which is then displaced by the active site lysine to regenerate the active enzyme | The reaction for synthesis of cystathionine starts with displacement of the internal aldimine between the enzyme active site lysine and pyridoxal 5’-phosphate (E-PLP) by the incoming L-serine. The serine external aldimine adduct (E-PLP-L-ser) forms an aminoacrylate intermediate (E-PLP-aa) that reacts with the incoming second substrate, such as L-homocysteine, to form the (L,L)-cystathionine external aldimine, which is then displaced by the active site lysine to regenerate the active enzyme.<ref>PMID:29630349</ref> | ||
The type of reaction mechanisms used by the CBS is known as a double displacement or ping-pong mechanism. The rate determining step in the reaction is hydrolysis of the external aldimine of cystathionine | The type of reaction mechanisms used by the CBS is known as a double displacement or ping-pong mechanism. The rate determining step in the reaction is hydrolysis of the external aldimine of cystathionine.<ref>PMID:15890029</ref> | ||
[[Image:Reactions catalyzed by CBS.png|600px|left Reactions catalyzed by CBS. (A) Canonical reaction of CBS. (B) CBS reactions that generate or utilize H2S.]] | [[Image:Reactions catalyzed by CBS.png|600px|left Reactions catalyzed by CBS. (A) Canonical reaction of CBS. (B) CBS reactions that generate or utilize H2S.]] | ||
[[Image:Proposed enzymatic mechanism for the CBS.png|600px|left Proposed enzymatic mechanism for the CBS reaction with absorption maxima. Possible quinonoid intermediates between E-serine (E-PLP-L-ser) and E-aminoacrylate (E-PLPaa) or between E-aminoacrylate and E-cystathionine are not shown. The growing consensus among those working on fold type-II PLP-dependent enzymes is that the ring nitrogen does not undergo protonation during the catalytic cycle in these enzymes. However, at pH 6.5 the expectation is that the ring nitrogen is protonated as shown.]] | [[Image:Proposed enzymatic mechanism for the CBS.png|600px|left Proposed enzymatic mechanism for the CBS reaction with absorption maxima. Possible quinonoid intermediates between E-serine (E-PLP-L-ser) and E-aminoacrylate (E-PLPaa) or between E-aminoacrylate and E-cystathionine are not shown. The growing consensus among those working on fold type-II PLP-dependent enzymes is that the ring nitrogen does not undergo protonation during the catalytic cycle in these enzymes. However, at pH 6.5 the expectation is that the ring nitrogen is protonated as shown.]] | ||
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[[Image:Homocysteine metabolic pathway.png|600px|left Homocysteine metabolic pathway. Homocysteine sits at the intersection of the remethylation and transsulfuration pathways. In the remethylation pathway, THF is converted to N5,N10-methylene tetrahydrofolate and then to MTHF by methylenetetrahydrofolate reductase (Mthfr). The methyl group is donated to Hcy and in the presence of methionine synthase (Mtr), and B12 is converted to methionine. Methionine is used in many methyl transfer reactions. When the diet is replete with methionine, Hcy is converted, via the transsulfuration pathway, to cystathionine by cystathionine β-synthase (Cbs) and then converted to cysteine via the action of cystathionase (Cth, Cse) in the presence of B6. Cysteine is converted to several beneficial downstream products.]] | [[Image:Homocysteine metabolic pathway.png|600px|left Homocysteine metabolic pathway. Homocysteine sits at the intersection of the remethylation and transsulfuration pathways. In the remethylation pathway, THF is converted to N5,N10-methylene tetrahydrofolate and then to MTHF by methylenetetrahydrofolate reductase (Mthfr). The methyl group is donated to Hcy and in the presence of methionine synthase (Mtr), and B12 is converted to methionine. Methionine is used in many methyl transfer reactions. When the diet is replete with methionine, Hcy is converted, via the transsulfuration pathway, to cystathionine by cystathionine β-synthase (Cbs) and then converted to cysteine via the action of cystathionase (Cth, Cse) in the presence of B6. Cysteine is converted to several beneficial downstream products.]] | ||
Among the pathological states that have been mentioned in relation with eHcy are cardiovascular disorders, atherosclerosis, myocardial infarction, stroke, minimal cognitive impairment, dementia, Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, epilepsy, and eclampsia | Among the pathological states that have been mentioned in relation with eHcy are cardiovascular disorders, atherosclerosis, myocardial infarction, stroke, minimal cognitive impairment, dementia, Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, epilepsy, and eclampsia.<ref>PMID:25324876</ref> All these observations indicates that Hcy, and especially eHcy, exerts direct toxic effects on both the vascular and nervous systems. | ||
Mutations in the gene encoding CBS resulting in abnormalities of its function with all the consequences are usually referred as of homocystinuria. The mutations can alter either mRNA or enzyme stability, activity, binding of PLP and heme, or impair allosteric regulation | Mutations in the gene encoding CBS resulting in abnormalities of its function with all the consequences are usually referred as of homocystinuria. The mutations can alter either mRNA or enzyme stability, activity, binding of PLP and heme, or impair allosteric regulation.<ref>PMID:15087459</ref> To date there have been over 100 mutations described in this gene.<ref>PMID:12686134</ref> | ||
Deficiency in the activity of the enzyme caused by insufficient supplementation of enzymatic cofactores leads to accumulation of L-homocysteine, hyperhomocysteinemia, with similar but moderate indications like homocystinuria. | Deficiency in the activity of the enzyme caused by insufficient supplementation of enzymatic cofactores leads to accumulation of L-homocysteine, hyperhomocysteinemia, with similar but moderate indications like homocystinuria. | ||
Involvement in such a wide number of apparently unrelated diseases may be caused by affecting a very basic biological process central to a variety of diseases. There are two major hypotheses about how this could be accomplished. | Involvement in such a wide number of apparently unrelated diseases may be caused by affecting a very basic biological process central to a variety of diseases. There are two major hypotheses about how this could be accomplished. | ||
The first one has centered on the relationship between homocysteine and oxidative stress. Homocysteine itself has been shown to cause increased oxidative stress on cells, both through direct effects (e.g., the production of hydrogen peroxide by oxidation of homocysteine to homocystine) and indirect effects (e.g., reduction of glutathione peroxidase). In addition, it is estimated that as much as 50% of the cellular antioxidant glutathione is produced from homocysteine by conversion through the transsulfuration pathway. | The first one has centered on the relationship between homocysteine and oxidative stress. Homocysteine itself has been shown to cause increased oxidative stress on cells, both through direct effects (e.g., the production of hydrogen peroxide by oxidation of homocysteine to homocystine) and indirect effects (e.g., reduction of glutathione peroxidase). In addition, it is estimated that as much as 50% of the cellular antioxidant glutathione is produced from homocysteine by conversion through the transsulfuration pathway. | ||
A second popular hypothesis suggests that eHcy affects the control of biologically important methylation reactions by causing a build-up of S-adenosyl-L-homocysteine (AdoHcy) which is a competitive inhibitor of S-adenosyl-L-methionine (AdoMet) binding for methyltransferase enzymes. As methyltransferases are involved in a variety of important biological processes, inhibition of this class of enzymes could have extremely diverse effects on the organism | A second popular hypothesis suggests that eHcy affects the control of biologically important methylation reactions by causing a build-up of S-adenosyl-L-homocysteine (AdoHcy) which is a competitive inhibitor of S-adenosyl-L-methionine (AdoMet) binding for methyltransferase enzymes. As methyltransferases are involved in a variety of important biological processes, inhibition of this class of enzymes could have extremely diverse effects on the organism.<ref>PMID:15890029</ref> | ||
</StructureSection> | </StructureSection> | ||
== References == | == References == | ||
<references/> | <references/> | ||