Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:4.2.1.22 (cystathionine beta-synthase)
 965 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Cystathionine beta-synthase (CBS) catalyzes the condensation of homocysteine and serine to cystathionine-an irreversible step in the eukaryotic transsulfuration pathway. The native enzyme is a homotetramer or multimer of 63-kDa (551 amino acids) subunits and is activated by S-adenosyl-l-methionine (AdoMet) or by partial cleavage with trypsin. Amino-terminal analysis of the early products of trypsinolysis demonstrated that the first cleavages occur at Lys 30, 36, and 39. The enzyme still retains the subunit organization as a tetramer or multimer composed of 58-kDa subunits. Analysis by electrospray ionization mass spectrometry showed that further trypsin treatment cleaves CBS in its COOH-terminal region at Arg 413 to yield 45-kDa subunits. This 45-kDa active core is the portion of CBS most conserved with the evolutionarily related enzymes isolated from plants, yeast, and bacteria. The active core of CBS forms a dimer of approximately 85 kDa. The dimer is about twice as active as the tetramer. It binds both pyridoxal 5'-phosphate and heme cofactors but is no longer activated by AdoMet. Further analysis suggests that the dissociation of CBS to dimers causes a decrease in enzyme thermostability and a threefold increase in affinity toward the sulfhydryl-containing substrate-homocysteine. We found that the COOH-terminal region, residues 414-551, is essential for maintaining the tetrameric structure and AdoMet activation of the enzyme. The inability of the active core to form multimeric aggregates has facilitated its crystallization and X-ray diffraction studies.
 ...
PMID:Trypsin cleavage of human cystathionine beta-synthase into an evolutionarily conserved active core: structural and functional consequences. 967 31

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.
 ...
PMID:Binding of pyridoxal 5'-phosphate to the heme protein human cystathionine beta-synthase. 1005 42