PLP-dependent H(2)S biogenesis

Abstract

The role of endogenously produced H(2)S in mediating varied physiological effects in mammals has spurred enormous recent interest in understanding its biology and in exploiting its pharmacological potential. In these early days in the field of H(2)S signaling, large gaps exist in our understanding of its biological targets, its mechanisms of action and the regulation of its biogenesis and its clearance. Two branches within the sulfur metabolic pathway contribute to H(2)S production: (i) the reverse transsulfuration pathway in which two pyridoxal 5'-phosphate-dependent (PLP) enzymes, cystathionine β-synthase and cystathionine γ-lyase convert homocysteine successively to cystathionine and cysteine and (ii) a branch of the cysteine catabolic pathway which converts cysteine to mercaptopyruvate via a PLP-dependent cysteine aminotransferase and subsequently, to mercaptopyruvate sulfur transferase-bound persulfide from which H(2)S can be liberated. In this review, we present an overview of the kinetics of the H(2)S-generating reactions, compare the structures of the PLP-enzymes involved in its biogenesis and discuss strategies for their regulation. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

Figure 1

Scheme showing the various H₂S generating reactions catalyzed by enzymes in the reverse transsulfuration pathway (CBS and CGL) and the cysteine catabolic pathway (CAT/AAT and MST).

Figure 2

Overall folds for PLP enzymes involved in H₂S generation pathways. The structures of chicken AAT/CAT (A), Drosophila CBS (B) and human CGL (C) are shown. The subunits in each enzyme are shown in different colors and the PLP is shown in ball representation in navy. PDB files 1MAQ, 3PC2 and 3COG were employed to generate the structures in A, B and C, respectively.

Figure 3

Close-up of the active site structure of chicken CAT/AAT with a glutamate ketimine intermediate. The residues that interact with the O3′ and pyridine nitrogen of PLP, the lysine residue that forms the Schiff base in the internal aldimine, and the arginine residues that interact with the proximal and distal carboxylates of the glutamate ketimine are shown. One of the two arginine residues, marked with the asterisk, is donated by the adjacent subunit. The figure was generated using the PDB file 1MAQ and the residues follow the numbering for the chicken sequence.

Figure 4

Close-up of the active site structure of Drosophila CBS. A. Structure of the carbanion intermediate showing the close distance between K88 and Cα and the hydrogen bonding interactions between O3′ and the pyridine nitrogen of PLP. B. Structure of the aminoacrylate intermediate reveals that K88 has repositioned away from Cα and is engaged in hydrogen bonding interactions with the phosphate group of PLP. The figure was generated using the PDB files 3PC4 (A) and 3PC3 (B) and the numbering for the Drosophila sequence is reported.

Figure 5

Close-up of the active site structure of human CGL. The residues that interact with the O3′ and pyridine nitrogen of PLP, the Schiff-base forming lysine residue and the tyrosine that lids the pyridine ring are shown. The figure was generated using the PDB file 3COG and the residues follow the numbering for the human sequence.

Figure 6

The heme environment influences the tautomeric equilibrium at the PLP site in human CBS. In the ferric state, the active ketoenimine tautomer predominates in the active site. Formation of the ferrous-CO CBS form results in displacement of the endogenous C52 heme ligand by CO and is correlated with a shift to the inactive enolimine form of the internal aldimine. A possible route for communication between the two cofactors is via an alpha helix in which R266 at one end forms a salt bridge with the heme ligand, C52. At the other end, two threonine residues, T257 and T260 engage in hydrogen bonding interactions with the phosphate group of PLP.

Scheme 1

Reactions catalyzed in the CAT/AAT-MST Pathway

Scheme 2

Reactions catalyzed by CBS (5–8) and CGL (7–8)