Electronic coupling cytochrome

Tire most studied of all copper-containing oxidases is cytochrome c oxidase of mitochondria. This multisubunit membrane-embedded enzyme accepts four electrons from cytochrome c and uses them to reduce 02 to 2 H20. It is also a proton pump. Its structure and functions are considered in Chapter 18. However, it is appropriate to mention here that the essential catalytic centers consist of two molecules of heme a (a and a3) and three Cu+ ions. In the fully oxidized enzyme two metal centers, one Cu2+ (of the two-copper center CuA) and one Fe3+ (heme a), can be detected by EPR spectroscopy. The other Cu2+ (CuB) and heme a3 exist as an EPR-silent exchange-coupled pair just as do the two copper ions of hemocyanin and of other type 3 binuclear copper centers. 

Some insight into the understanding of these later intermediates comes from the observation that the fully oxidized enzyme may undergo a one-electron oxidation reaction, in which the electron donor is probably water and the acceptor ferricytochrome c. The overall product would be a one-electron oxidation product of the fully oxidized centre plus water. Presumably, one-electron reactions in the opposite direction can occur. The transfer of one electron from cytochrome a to the a3/CuB centre in compound C, plus one electron from a3 or CuB will allow a second concerted two-electron reaction with the formation of (Felv=02 CuB—OH-), and ESR visibility of the copper. In the next stage of the reaction antiferromagnetic coupling would be reintroduced. 

The artificial intelligence-superexchange method in which the details of the electronic structure of the protein medium are taken into account was used for estimating the electronic coupling in the metalloproteins (Siddarth and Marcus, 1993a,b,c). Fig.2.11 demonstrates a correlation of experimental and calculated ET rate constants for cytochrome c derivatives, modified by Ru complexes. The influence of the special mutual orientation of the donor and acceptor orbitals in Ru(bpy)2im HisX-cytochrome c on the rate of electron transfer was analyzed by the transition amplitude methods (Stuchebrukhov and Marcus, 1995). In this reaction the transferring electron in the initial and the final states occupies the 3d shell of the Fe atom and the 4d shell of Ru, respectively. It was shown that the electron is localized on t2g subshells of the metal ions. Due to the near-... 

Siddarth, P. and Marcus, R.A. (1993c) Correlation between theory and experiment in electron-transfer reaction in protein electronic couplings in modified cytochrome c and myoglobin derivatives. J. Phys. Chem. 97, 13078-13082. 

It had long been known that apoptosis and activation of caspase are coupled with mitochondrial permeabilization. However, until 1996, the molecular link between mitochondria and caspase activation had not been known. Dr. Wang s group at the University of Texas isolated activators of caspase 3 by using classical fractionation methods in 1996-1997. One protein that was required for activation of caspase 3 was a 15-kD protein with pink color, which was cytochrome c. This finding is rather surprising because cytochrome c is localized in mitochondria and has a well-established role as an electron carrier. Cytochrome c was later shown to be released... 

A point of special interest is that the electronic couplings for intramolecular ET reactions in His-58 and His-66 cytochromes are not enhanced by aromatic residues (Trp-59 and Tyr-67) in the intervening media (36). The correlation of ET rates with or does not preclude a coupling role for the ir orbitals of the aromatic groups in the pathway, but it does indicate that, in the Ru-modified cytochromes that we examined, they are no more efficient in mediating the coupling than is the (7-bonded framework. Hence, the presence of aromatic groups in the medium between redox sites does not necessarily result in faster ET than in a purely aliphatic medium (15-20, 37-41). 

Fig. 5. Schematic representation of the interrelationships between the metal centers at the active site of cytochrome c oxidase showing an electronically coupled iron-copper pair (left) and an electronically and magnetically coupled iron-copper pair (right) which interact weakly with each other (—).

FIGURE 17. Electron transfer pathways in FCSD. Through-space jumps are indicated hy dotted lines and paths along hydrogen bonds are indicated by dashed lines. Four paths (ln4) with decreasing electronic coupling are indicated. Fp and Cy indicate residues in the flavo-protein and the cytochrome subunits, respectively. 

The 19.6-A electron transfer from Cua to cytochrome a proceeds rapidly at low driving force (1.8 x 10 s AG° = —0.05 eV). Multiple electronic coupling pathways... 

Water is present in protein cavities as individual molecules, water chains, and clusters. Indeed, tightly bound waters can be resolved in X-ray crystallography experiments. Water molecules in larger cavities, especially those with a hydrophobic surface, are mobile and less readily resolved. In some proteins, such as the cytochrome b(f complex or cytochrome c oxidase, bound water molecules tend to form water chains. These water molecules provide hydrogen-bonded relays for proton transfer, and they may mediate donor-to-acceptor electronic coupling (2-6). 

Figure 2 Dependence of the mean square electronic coupling on distance between two porphyrin rings in the cytochrome self-exchange ET reaction (41). For each distance, system conformations were sampled using MD and the coupling was computed for each conformation at the extended Huckel level. The black line marked XEI(P, W) shows the water-mediated coupling for comparison, the red line marked XEI(P) shows the coupling computed for the same protein conformation in vacuum. Conformational snapshots typical for the three coupling regimes are shown.

The mechanism for the coupling of electron transfer from Q to cytochrome c to transmembrane proton transport is known as the Q cycle (Figure 18.17). The Q cycle also facilitates the switch from the two-electron carrier ubiquinol to the one-electron carrier cytochrome c. The cycle begins as ubiquinol (QH2) binds in the Q site. Ubiquinol transfers its... 

Proton-coupled electron transfer (PCET) reactions play a vital role in a wide range of chemical and biological processes. For example, PCET is required for the conversion of energy in photosynthesis [1] and respiration [2], In particular, the coupling between proton motion and electron transfer is involved in the pumping of protons across biological membranes in photosynthetic reaction centers [1] and in the conduction of electrons in cytochrome c [3]. In addition to biological processes, PCET is also important in electrochemical processes [4, 5] and in solid state materials [6]. 

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