NO and Heme Proteins: Cross-Talk between Heme and Cysteine Residues

Abstract

Heme proteins are a diverse group that includes several unrelated families. Their biological function is mainly associated with the reactivity of the heme group, which-among several other reactions-can bind to and react with nitric oxide (NO) and other nitrogen compounds for their production, scavenging, and transport. The S-nitrosylation of cysteine residues, which also results from the reaction with NO and other nitrogen compounds, is a post-translational modification regulating protein activity, with direct effects on a variety of signaling pathways. Heme proteins are unique in exhibiting this dual reactivity toward NO, with reported examples of cross-reactivity between the heme and cysteine residues within the same protein. In this work, we review the literature on this interplay, with particular emphasis on heme proteins in which heme-dependent nitrosylation has been reported and those for which both heme nitrosylation and S-nitrosylation have been associated with biological functions.

Keywords: S-nitrosylation; cysteine; heme; heme proteins; nitric oxide; nitrosation.

Figure 1

Structures of heme a, b, c, and o. The pyrrol rings are in green. The axial ligands mentioned in this work are reported.

Figure 2

NO bound to an iron ion of a heme group (a) and to a Cys residue as nitrosothiol (SNO) (b).

Figure 3

Possible reactions of NO with the heme group (a) and Cys residues (b). The reaction numbers (red circles) refer to the reactions reported in the text.

Scheme 1

Hydroxide-mediated reductive nitrosylation of Fe(III) heme b.

Scheme 2

S-nitrosylation coupled with transnitrosylation of proteins.

Scheme 3

Metal-to-Cys S-nitrosylation.

Figure 4

Three-dimensional structures of S-nitrosylated forms of (a) liganded human Hb (PDB 1BUW) with the α chains in cyan, and the β chains in yellow; (b) blackfin tuna Mb (PDB 2NRM) with the globin chain in green; (c) C. lectularius NP (PDB 1Y21) with the protein chain in blue. In all figures, the heme is in red; the binding of NO with Cys is highlighted in stick and balls with Cys in orange, NO in blue and red. In the enlargements, the heme and the residues relevant to S-nitrosylation are reported; the distance between the iron and the Cys is measured in Å.

Scheme 4

Simplified model of the conformation-dependent nitrosylation and denitrosylation of Hb according to Stamler and coworkers.

Scheme 5

Simplified model of the catalytic cycle of Ascaris Hb, proposed in [65].

Scheme 6

Proposed k_ox reaction of NOS. Simplified from [78].

Scheme 7

Reactions of NO with the heme group and proximal Cys of C. lectularius NP.