Key regulators in the architecture of substrate access/egress channels in mammalian cytochromes P450 governing flexibility in substrate oxyfunctionalization.

Cytochrome P450s (CYP) represent a superfamily of b-type hemoproteins catalyzing oxifunctionalization of a vast array of endogenous and exogenous compounds. The present review focuses on assessment of the topology of prospective determinants in substrate entry and product release channels of mammalian P450s, steering the conformational dynamics of substrate accessibility and productive ligand orientation toward the iron-oxene core. Based on a generalized, CYP3A4-related construct, the sum of critical elements from diverse target enzymes was found to cluster within the known substrate recognition sites. The majority of prevalent substrate access/egress tunnels revealed to be of fairly balanced functional importance. The hydrophobicity profile of the candidates revealed to be the most salient feature in functional interaction throughout the conduits, while bulkiness of the residues imposes steric restrictions on substrate traveling. Thus, small amino acids such as prolines and glycines serve as hinges, driving conformational flexibility in ligand passage. Similarly, bottlenecks in the tunnel architecture, being narrowest encounter points within the CYP3A4 model, have a vital function in substrate selectivity along with clusters of aromatic amino acids acting as gatekeepers. In addition, peripheral patches in conduits may house determinants modulating allosteric cooperativity between remote and central domains in the P450 structure. Remarkably, the bulk critical residues lining tunnels in the various isozymes reside in helices B'/C and F/G inclusive of their interhelical turns as well as in helix I. This suggests these regions to represent hotspots for targeted genetic engineering to tailor more sophisticated mammalian P450s exploitable in industrial, biotechnological and medicinal areas.