Why is heme present in mammalian cystathionine beta-synthase (CBS)? Human CBS is a remarkable enzyme that requires three different cofactors: S-adenosylmethionine (AdoMet), heme and pyridoxal 5'-phosphate (PLP). Enzyme activity is regulated by AdoMet, which binds to the autoinhibitory C-terminal domain and opens the active site. The reaction chemistry of CBS is attributed solely to the PLP. The heme is essential for maximal enzyme activity but its precise role is as yet unclear. Positioned at a branch point in sulfur metabolism, CBS is the only enzyme that irreversibly removes toxic homocysteine from the body. A defect in the CBS protein in humans is the most common cause of homocystinuria, an inherited metabolic disease characterized by elevated levels of homocysteine and decreased levels of cysteine and glutathione and a profoundly enhanced risk of thromboembolism and stroke. More than one hundred pathological variants of CBS are known in homocystinuric patients, many of which cluster in the AdoMet-associated and heme-associated regions of the protein. We will investigate the biochemical consequences of the absence of heme and of select mutations in the AdoMet- and heme-binding regions of CBS, in order to determine how the three cofactors interact in the physiological function of the enzyme. This project is an integrated, collaborative effort between the Kraus lab, with expertise in the physiology and molecular genetics of homocysteine metabolism, and the Burstyn lab, with expertise in heme protein structure and function. To accomplish our goal we will combine physiological, biochemical, and biophysical methods to understand the role of heme in CBS.
The Specific Aims for the continuation of this project are: 1) to test the hypothesis that the heme is critical for AdoMet regulation and functional stability of the human CBS protein, 2) to test the hypothesis that impaired AdoMet response correlates with aberrant behavior of heme or PLP, and 3) to test the hypothesis that the CBS heme stabilizes the active site PLP and that disruption of this interaction compromises CBS function.

Public Health Relevance

Some people have errors in the gene for an important protein, named CBS, whose job is to remove the molecule homocysteine from blood. These people are very likely to get blood clots or have strokes because their CBS protein does not work properly and their homocysteine levels are too high. This project looks at the errors in the CBS protein, and tries to learn from those errors how the protein normally works.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Research Project (R01)
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Special Emphasis Panel (ZRG1-GTIE-A (01))
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Kindzelski, Andrei L
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University of Colorado Denver
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Ereño-Orbea, June; Majtan, Tomas; Oyenarte, Iker et al. (2014) Purification, crystallization and preliminary crystallographic analysis of the catalytic core of cystathionine ?-synthase from Saccharomyces cerevisiae. Acta Crystallogr F Struct Biol Commun 70:320-5
Rivera-Rivera, Izarys; Román-Hernández, Giselle; Sauer, Robert T et al. (2014) Remodeling of a delivery complex allows ClpS-mediated degradation of N-degron substrates. Proc Natl Acad Sci U S A 111:E3853-9
Ereño-Orbea, June; Majtan, Tomas; Oyenarte, Iker et al. (2014) Structural insight into the molecular mechanism of allosteric activation of human cystathionine ?-synthase by S-adenosylmethionine. Proc Natl Acad Sci U S A 111:E3845-52
Pey, Angel L; Majtan, Tomas; Kraus, Jan P (2014) The role of surface electrostatics on the stability, function and regulation of human cystathionine ?-synthase, a complex multidomain and oligomeric protein. Biochim Biophys Acta 1844:1453-62
Majtan, Tomas; Pey, Angel L; Fernández, Roberto et al. (2014) Domain organization, catalysis and regulation of eukaryotic cystathionine beta-synthases. PLoS One 9:e105290
Pey, Angel L; Majtan, Tomas; Sanchez-Ruiz, Jose M et al. (2013) Human cystathionine ýý-synthase (CBS) contains two classes of binding sites for S-adenosylmethionine (SAM): complex regulation of CBS activity and stability by SAM. Biochem J 449:109-21
Ereno-Orbea, June; Majtan, Tomas; Oyenarte, Iker et al. (2013) Structural basis of regulation and oligomerization of human cystathionine *-synthase, the central enzyme of transsulfuration. Proc Natl Acad Sci U S A 110:E3790-9
Su, Yang; Majtan, Tomas; Freeman, Katherine M et al. (2013) Comparative study of enzyme activity and heme reactivity in Drosophila melanogaster and Homo sapiens cystathionine ?-synthases. Biochemistry 52:741-51
Majtan, Tomas; Kraus, Jan P (2012) Folding and activity of mutant cystathionine ýý-synthase depends on the position and nature of the purification tag: characterization of the R266K CBS mutant. Protein Expr Purif 82:317-24
Barr, Ian; Smith, Aaron T; Chen, Yanqiu et al. (2012) Ferric, not ferrous, heme activates RNA-binding protein DGCR8 for primary microRNA processing. Proc Natl Acad Sci U S A 109:1919-24

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