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)

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.
 ...
PMID:Characterization of the heme in human cystathionine beta-synthase by X-ray absorption and electron paramagnetic resonance spectroscopies. 1095 45

Human cystathionine beta-synthase is a hemeprotein that catalyzes a pyridoxal phosphate (PLP)-dependent condensation of serine and homocysteine into cystathionine. Biophysical characterization of this enzyme has led to the assignment of the heme ligands as histidine and cysteinate, respectively, which has recently been confirmed by crystal structure determination of the catalytic core of the protein. Using site-directed mutagenesis, we confirm that C52 and H65 represent the thiolate and histidine ligands to the heme. Conversion of C52 to alanine or serine results in spectral properties of the resulting hemeprotein that are consistent with the loss of a thiolate ligand. Thus, the Soret peak blue-shifts from 428 to 415 and 417 nm in the ferric forms of the C52S and C52A mutants, respectively, and from 450 to 423 nm in the ferrous states of both mutants. Addition of CO to the dithionite-reduced ferrous C52 mutants results in spectra with Soret peaks at 420 nm. EPR spectroscopy of the ferric C52 variants reveals the predominance of a high-spin species. The H65R mutant, a variant described in a homocystinuric patient, has Soret peaks at 424, 421, and 420 nm in the ferric, ferrous, and ferrous CO states, respectively. EPR spectroscopy reveals predominance of the low-spin species. Both C52A and C52S mutations lead to protein with substoichiometric heme (19% with respect to wild type); however, the PLP content is comparable to that of wild-type enzyme. The heme and PLP contents of the H65R mutant are 40% and 75% that of wild-type enzyme. These results indicate that heme saturation does not dictate PLP saturation in these mutant enzymes. Both H65 and C52 variants display low catalytic activity, revealing that changes in the heme binding domain modulate activity, consistent with a regulatory role for this cofactor.
 ...
PMID:Effects of heme ligand mutations including a pathogenic variant, H65R, on the properties of human cystathionine beta-synthase. 1192 27