CBS

Cystathionine-β-synthase (CBS) is a key enzyme in the transsulfuration pathway metabolism of in vivo homocysteine (Hcy). The decreased activity of CBS leads to an abnormal increase of Hcy, resulting in Hyperhomocyseinemia (HHcy). HHcy is involved in the formation of atherosclerosis (AS) and is a new independent risk factor for cardiovascular diseases. Since the discovery, that CBS deficiency (congenital or homozygotes of cystinuria) patients often had AS in early years and died soon, has been found by Mc Cully in the 1960s, the study of relationship between CBS with HHcy and AS continues to deepen.

Structure

CBS is localized to the cytoplasm and is a homotetramer composed by 63 kDa subunits. CBS is a haemoglobin protein belonging to the β family of pyridoxal phosphate (PLP)-dependent enzymes. Each subunit binds to the two cofactors: haemoglobin and PLP. Human CBS includes a haemoglobin-binding region, a highly conserved catalytic domain, and a regulatory domain. Haemoglobin binds to the first 70 amino acid residues at the N-terminus, where Cys52 and His65 are heme iron-binding residues. The highly conserved catalytic domain is located at amino acid residues 40 to 413 and can form a 45 kDa active center. The Lys119 residue in this region is a PLP-binding residue, and PLP is a requisite for the CBS catalyzed reaction. The regulatory domain consisting of 140 amino acid residues at the C-terminus plays an important role in the formation of human CBS tetramers and the activation of the enzyme. This region contains a self-inhibitory region of adenosylmethionine (AdoMet) binding site at amino acid residues 421 to 468. AdoMet is an important regulatory hub in methionine metabolism. It is an allosteric activator of CBS and, also an allosteric inhibitor of 5,10-methylenetetrahydrofolate reductase (MTHFR) and betaine-homocysteine methyltransferase (BHMT) mediated catalytic reaction.

Catalytic Mechanism

Hcy is a sulfur-containing amino acid, which is formed by the demethylation of methionine, and its metabolism has two pathways of transsulfuration and methylation. CBS is a key enzyme in the transsulfuration pathway. Under the participation of PLP as a coenzyme, CBS catalyzes the β-substitution reaction, which mediates the condensation of Hcy and serine to generate cystathionine. Cystathionine is further converted to cysteine α-ketobutyric acid by the action of cystathionine-γ-lyase (CSE). In the human body, about 53% of Hcy is irreversibly converted into cysteine by CBS and CSE. The methylation pathway is that Hcy re-synthesizes methionine with the aid of folic acid and VitB₁₂. The above two pathways mainly rely on AdoMet concentration to coordinate roughly balance, and the decrease of CBS activity may lead to HHcy caused by the failure of the transsulfuration pathway.

Influencing Factors

First, allosteric activators. AdoMet is an allosteric activator of CBS that increases its activity by 2 to 5 fold. The activation mechanism may be related to the spatial conformational change of self-inhibitory regions in the C-terminal regulatory domain where AdoMet induces. Second, the cofactors. Haemoglobin is a redox sensor, and its redox state can result in changes of CBS activity. Therefore, cofactors that can cause changes in the redox state of hemoglobin can affect CBS activity. Third, gene mutations of CBS. Mutations in the CBS gene can cause changes in the stability of the enzyme, the binding of the enzyme to the cofactor and the substrate, and the disruption of the regulation of the allosteric agents, thereby affecting the enzyme activity. So far, 132 CBS gene mutations have been found, mostly in exon 3 and exon 8. Most of them are missense mutations, followed by deletion mutations, insertion mutations, and splicing mutations.

Related Diseases

Mutations in the CBS gene cause the decrease of CBS activity and the formation of HHcy, and HHcy participates in the formation of AS. At present, most clinical and epidemiological studies have shown that the level of tHcy is related to the extent of atherosclerosis in carotid artery, coronary artery, and peripheral artery. It can participate in the formation of AS from the following aspects. First, it damages endothelial cells. In the presence of metal ions (Fe³⁺ or Cu²⁺), Hcy undergoes autooxidation through sulfhydryl groups. And the generated reactive oxygen such as hydrogen peroxide (H₂O₂), superanion (O_2-), and OH^-, etc. induce oxidative stress and cause endothelial cell damage. Second, it stimulates proliferation of vascular smooth muscle cells. In the vascular smooth muscle cell (VSMC) medium in the presence of catalase, the addition of 1.0 mmol/L Hcy resulted in a 1.5-fold increase in DNA synthesis and a 2-fold increase in VSMC. Its cellular and molecular mechanisms may be related to Hcy-induced H₂O₂-independent pathway activation and activation of mitogen-activated kinase (MAPK). As the activated MAPK causes a cell proliferation response, promotes the synthesis of VSMC collagen, thereby accelerating the progression of AS. Third, it activates platelets and promote thrombosis. Hcy activates platelets, causing a decrease in platelet concentration-dependent L-arginine uptake, resulting in decreased NO synthesis. Hcy can also inhibit adenosine diphosphate (ADP) enzyme activity, reduce ADP degradation, resulting in increased ADP aggregation, enhanced platelet adhesion and aggregation, and thus promote thrombosis. Fourth, activation of inflammatory factors. AS is a chronic inflammatory disease. Cell culture experiments showed that Hcy enhances the expression of monocyte chemoattractant protein (MCP-1), MCP-1 then enhances monocyte-endothelium binding and the filling of subcutaneous space to accelerate AS development.