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Review
. 2017 May 9:6:662.
doi: 10.12688/f1000research.11015.1. eCollection 2017.

The catalytic function of cytochrome P450 is entwined with its membrane-bound nature

Carlo Barnaba  1 , Katherine Gentry  1 , Nirupama Sumangala  1 , Ayyalusamy Ramamoorthy  1
Affiliations
Review

The catalytic function of cytochrome P450 is entwined with its membrane-bound nature

Carlo Barnaba et al. F1000Res. .

Abstract

Cytochrome P450, a family of monooxygenase enzymes, is organized as a catalytic metabolon, which requires enzymatic partners as well as environmental factors that tune its complex dynamic. P450 and its reducing counterparts-cytochrome P450-reductase and cytochrome b 5 -are membrane-bound proteins located in the cytosolic side of the endoplasmic reticulum. They are believed to dynamically associate to form functional complexes. Increasing experimental evidence signifies the role(s) played by both protein-protein and protein-lipid interactions in P450 catalytic function and efficiency. However, the biophysical challenges posed by their membrane-bound nature have severely limited high-resolution understanding of the molecular interfaces of these interactions. In this article, we provide an overview of the current knowledge on cytochrome P450, highlighting the environmental factors that are entwined with its metabolic function. Recent advances in structural biophysics are also discussed, setting up the bases for a new paradigm in the study of this important class of membrane-bound enzymes.

Keywords: NADPH-P450 reductase; cytochrome P450; cytochrome b5; membrane; sold-state NMR; structure.

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Conflict of interest statement

Competing interests: The authors declare that they have no competing interests.No competing interests were disclosed.No competing interests were disclosed.No competing interests were disclosed.No competing interests were disclosed.No competing interests were disclosed.

Figures

Figure 1.
Figure 1.. The catalytic cycle of cytochrome P450.
The binding of substrate R-H (1) causes a decrease of the redox potential of about 100 mV, which allows the first electron transfer from cytochrome P450-reductase (CPR) (2). The reduction of Fe 3+ to Fe 2+ makes suitable O 2 binding (3), which now can accept a second electron from either CPR or cytochrome b 5 (cyt b 5) (4), to form a hydroperoxyl intermediate known as compound 0. The O 2 −2 complex reacts with surrounding protons to form the highly reactive oxyferryl intermediate, also known as compound I (5). The Fe-ligated O atom (6) is transferred to the substrate forming a hydroxylated form of the substrate (7). The product is finally released (8), replaced by a molecule of water. Three uncoupling reactions are shown as dashed lines, with the respective products: the autoxidation shunt O 2 −2, the peroxide shunt (H 2O 2), and the oxidase shunt (H 2O).
Figure 2.
Figure 2.. Model structures of cytochrome P450 with cytochrome b 5 (left) and cytochrome P450 with cytochrome P450 NADPH-reductase (right) in a lipid bilayer.
The models were constructed from the crystallographic or solution-state nuclear magnetic resonance (NMR) structures of the soluble domains of the proteins. The structure, dynamics, and topology of the invisible transmembrane domains of the full-length cytochrome proteins have been determined for the first time by static solid-state NMR experiments on magnetically aligned bicelles. These studies reported that the helical structures of the transmembrane domains are significantly tilted away from the lipid bilayer normal and undergo motion in the millisecond (or slower) timescale that is far slower than that of the residues in the soluble domain , , .
Figure 3.
Figure 3.. First high-resolution structure of the membrane-bound cytochromes-b5-P450 complex revealing electron transfer pathway.
( A) Three-dimensional structure of the membrane-bound rabbit cytochrome P4502B4–cytochrome b 5 complex reproduced from Ahuja et al. . It was obtained by using the high-resolution (solution- and solid-state) nuclear magnetic resonance (NMR) structure of membrane-bound rabbit cytochrome b 5 (PDB code is 2M33), crystal structure of the soluble domain of cytochrome P4502B4 (PDB code is 1SUO), and NMR and mutational constraints to obtain the structure of the membrane-bound complex. ( B) Electron transfer pathway revealed by HARLEM in the complex structure. PDB, Protein Data Bank.
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