Cytochromes: Reactivity of the “dark side” of the heme.

This research describes the unique binding behaviors of certain heme-proteins, specifically, the Alcaligenes xylosoxidans cytochrome c’ and cardiolipin-bound horse heart cytochrome c. The study focuses on the interactions with nitric oxide (NO) and carbon monoxide (CO), and how these interactions could be crucial for understanding the functional roles of heme-proteins.

Interactions of Heme-Proteins

• Heme proteins typically feature a proximal ligand – most commonly, a histidine (His) residue – and a distal ligand, where different molecules can bind.
• In the common model of heme-protein biochemistry, molecules such as nitric oxide (NO) bind as the distal ligand to the iron atom of the heme. The result can be either a hexa-coordinated adduct – found in hemoglobin and myoglobin where His and NO are the proximal and distal ligands – or a penta-coordinated adduct, as seen in soluble guanylate cyclase, with NO as the sole axial ligand.

Unique Binding Patterns

• The research breaks from the traditional model by studying the ferrous derivatives of Alcaligenes xylosoxidans Cytochrome c’ (Axcyt c’) and cardiolipin-bound horse heart cytochrome c (CL-hhcyt c) and discovering unique binding patterns.
• In these cases, NO binds to what is termed the “dark side” of the heme, on the proximal side, replacing the endogenous ligand, His. This behavior is significantly different from traditional heme protein models.
• Cardiolipin-free horse heart cytochrome c (CL-free hhcyt c), on the other hand, behaves more like ferrous myoglobin, with NO binding on the heme’s distal side.

Implications of the Binding Patterns

• The researchers noticed that the ferrous form of CL-hhcyt c also binds to CO on the heme distal side, with His as the proximal axial ligand. Moreover, it exhibits peroxidase activity.
• In contrast, CL-free hhcyt c does not bind to CO and shows no peroxidase activity.
• The differential binding suggests that heme-proteins could use both sides of the heme for ligand discrimination. The choice of binding side could be influenced by allosteric modulation, meaning that the protein structure can change its shape to modulate its function.
• The study illuminates a more nuanced understanding of heme-protein biochemistry, highlighting the potential versatility of these proteins.