Structural Biochemistry/Cytochrome b6f Complex

The Cytochrome b₆f complex, also known as plastoquinol-plastocyanin reductase, is an energy transducing, hetero-oligomeric, dimeric enzyme found the thylakoid membranes of such organisms as the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. The enzyme acts as a mediator for the transfer of electrons between the reaction center complexes of photosystem II (PSII) and photosystem I (PSI) that lie inside the membrane. The reaction by which this occurs is shown below:

Structure of the Cytochrome b₆f complex from Chlamydomonas reinhardtii

$\ {Plastoquinol-1\ +\ 2\ oxidized\ plastocyanin\leftrightarrow \ plastoquinone\ +\ 2\ reduced\ plastocyanin}$

Mechanism of Reaction

During this process, the transfer of electrons from one side of the membrane to another results in a electrochemical potential gradient across the membrane, determined to be approximately 250mV, with the positive end of the gradient residing on the side of the membrane to which the protons are being transferred. Plastonquinol-1 (QH₂) and Plastocyanin (Pc) act as mobile redox carriers for either a clycic or non-cyclic electron transfer.

In the non-cyclic reaction, water is fed through the PSII enzyme to produce plastoquinol (QH₂) which is reduced to plastocyanin (Pc) by the cytochrome b₆f complex. Then, Pc is fed through the PSI enzyme resulting in the reduced form of Nicotinamide adenine dinucleotide phosphate, NADPH.

In the cyclic reaction,

QH₂ + 2Pc(Cu²⁺) + 2H⁺ → Q + 2Pc(Cu⁺) + 4H⁺

Pc transfers its electrons through an electron transport chain by means of electron bifurcation. This mechanism used in this cycle is commonly referred to as the Q Cycle.

Cytochrome b₆f complex (plastoquinol—plastocyanin reductase), Q cycle

The Q Cycle (seen right), is a process in which the positively charged side of the complex binds with QH₂ and is oxidized by a Fe-S center on the complex to form a semiquinone. This releases 2H⁺ in the positively charged side of the complex. Next, e^- are transfered by an electron transport chain to the Fe-S center to Pc, and the semiquinone follows by transferring its e^- to the heme bp of the cytochrome b6 complex. The heme bp then transfer its e^- to the heme bn of the enzyme which, in turn, reduces Q and produces SQ.

The cycle then continues (shown in the illustration as the "second half") by the binding of a second QH₂ to the positively charged side of the complex. By means of a high-low electron transport chain, an e^- is used to reduce an additional oxidized Pc, followed by the transfer of another e^- from the heme bn on the Cytochrome b₆f complex to the SQ molecule. Here, Q^2- is completely reduced and accepts 2 H⁺ from the initially negatively charged side of the membrane to for QH₂. And finally, QH₂ diffuses into the membrane, completing the cycle.

Structure

The structure of the cytochrome b₆f complex varies depending on the organism, but retains strong structural and compositional similarities and functions in essentially the same way. For example, the M. laminosus and C. reinhardtii complexes are structurally very similar, are both comprised of the same number of polypeptide subunits, and share a similar sequence identity in their photosynthetic electron transport (Pet) protein A-D subunits. These structures are both considered to be homodimers, of which each monomer is comprised of 8 polypeptide subunits. Of these 8 subunits, 4 of the largest of the them are considered as a separate subunit of the complex, as defined by Hurt & Hauska. According to Hurt & Hauska, these 4 subunits consist of c-type cytochrome f (PetA), cytochrome b6 (PetB), the ISP (PetC), and subunit IV (PetD, suIV), which is related to the C-terminal half of bc1 cytochrome b. All 8 subunits combine to form a bundle of 13 alpha helices inside the lipid bilayer of the membrane. Each monomer also contains a chain of six separate redox prosthetic groups to drive electron and proton transfer. The size of the enzyme is approximately twice the width of the membrane bilayer, and mass spectrometry has determine the typical molecular weight of the molecules to be on the order of 10⁵ g/mol.