BioPHYSICAL chemistry

Midpoint potentials

Notice that when the concentrations of the oxidized and reduced species are
equal, the potential Eis equal to E^0 , so this constant is usually termed the
oxidation/reduction midpoint potential, Em. Many biological half reactions
involve protons so the midpoint potential is defined at a pH of 7. The mid-
point potentials for biological molecules have been tabulated (Table 6.1)
and provide the basis for electron-transfer reactions, as discussed in the next
section. In general, increasing midpoint potentials correspond to a greater
affinity for an electron and hence increasing oxidation capability.
The potentials measured for biologically relevant oxidation/reduction
reactions cover a very large range. The primary donor of photosystem II,
P680, is the most oxidizing cofactor found in biology. The potential of P680
is high enough even to oxidize water. Notice that the midpoint potential
of P680 is larger than for its chemical component, chlorophyll a, in solu-
tion and much larger than the potential of the corresponding bacterial
electron donor, P870 (see Chapter 20). Cytochromes are proteins with
hemes as cofactors that serve as electron carriers, as for cytochrome c, or
as membrane proteins that are part of electron-transfer chains, such as
cytochrome f, which is part of the cytochrome b 6 fcomplex. Ubiquinones
serve as electron acceptors in different protein complexes, including the

CHAPTER 6 REDOX REACTIONS AND BIOENERGETICS 117

Table 6.1

Oxidation/reduction midpoint potentials for some biological reactions.

Redox reaction Em(V)

P680++e−↔P680 ~1.1

O 2 +4H++4e−↔2H 2 O0.82

Chl++e−↔Chl 0.78

Bchl++e−↔Bchl 0.64

P870++e−↔P870 0.50

Cyt f(Fe^3 +) +e−↔Cyt f(Fe^2 +)0.37

Cyt c(Fe^3 +) +e−↔Cyt c(Fe^2 +)0.25

Q +2e−+2H+↔QH 2 0.06

2H+(aq) +2e−→H 2 (gas) (standard conditions, pH 0) 0.00

FMNoxidized+2e−+2H+↔FMNH 2 −0.21

FAD +2e−+2H+↔FADH 2 −0.22

Glutathione +2e−+2H+↔2 glutathionereduced −0.23

NAD++2e−+H+↔NADH −0.32

2H+(aq) +2e−→H 2 (gas) (pH 7) −0.42

Ferredoxin (Fe^3 +) +e−↔ferredoxin (Fe^2 +) −0.43

Bchl, bacteriochlorophyll; Chl, chlorophyll; Cyt, cytochrome; FAD, flavin adenine

dinucleotide; FMN, flavin mononucleotide; NAD, nicotiamide adenine dinucleotide;

P680, primary donor of photosystem II; P870, primary donor of bacterial reaction

centers; Q, ubiquinone.