Introduction

The human cystathionine β-synthase (hCBS) is natively a homotetrameric enzyme catalysing the folowing reaction, which is part of the biosynthesis of cystein, using homocystein and serine : IMAGE It is encoded by the CBS gene located on chromosom 21.

Structure

A CBS monomer is natively a 63 kDa protein made of 551 amino-acids and each of them binds two cofactors (the iron heme and the pyridoxal phosphate), as well as two substrates (homocysteine and serine). Hence, the CBS contains (from N-terminal to C-terminal):
- a heme binding site located in a hydrophobic pocket (residues 50-67)
- a pyridoxal phosphate (covalently linked to Lysine 119 amino group) situated in a highly conserved central catalytic domain (residues 70-382)
- a C-terminal regulatory domain called Bateman module composed of two CBS domains (CBS 1 and CBS 2) IMAGE ALICE

Cofactor

• Heme iron:

The heme is one of the two cofactors of hCBS. It is bound in an hydrophobic pocket composed of the residues 50-67. The iron atom is hexacoordinated with the sulfhydryl group of Cys52 and the Nε2 atom of His65 (axial coordination) and with the four nitrogen atoms of the heme. Although the heme is essential for activity in human CBS, its role still remains an enigma. It is supposed to act as a redox sensor or as a way to facilitate a correct folding.

• Pyridoxal phosphate:

Quaternary structure

The hCBS is natively a homotetrameric enzyme. It is suggested that two monomers form a dimer, and then two dimers form a tetramer.

• Dimer formation: link to the structure : two monomers shape a dimer through both hydrophobic and polar interactions within the catalytic core. Hydrophobic interactions particularly involve two Phe112 (one from each monomer) which interact with each other. There are no interactions between Bateman modules in a single dimer concerning hCBS.

• Tetramer formation involves the Bateman modules as well as the catalytic core of each dimer. Each oligomerization loop (loop 513-519) of a monomer of one dimer interacts with the catalytic core of a monomer of the other dimer. Those loops interact within a crevace (shaped by α-helix 5-6-12-15-16 and β-strands 5-6) of the catalytic core. The tetramer is an inactive form of the enzyme.

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Function

Disease

Relevance