The Effect of Bacteriochlorophyll g Oxidation on Energy and Electron Transfer in Reaction Centers from Heliobacterium modesticaldum

The heliobacteria are a family of strictly anaerobic, Gram-positive, photoheterotrophs in the Firmicutes. They make use of a homodimeric type I reaction center (RC) that contains ~20 antenna bacteriochlorophyll (BChl) g molecules, a special pair of BChl g′ molecules (P₈₀₀), two 8¹-OH-Chl a_F molecules (A₀), a [4Fe-4S] iron-sulfur cluster (F_X), and a carotenoid (4,4′-diaponeurosporene). It is known that in the presence of light and oxygen BChl g is converted to a species with an absorption spectrum identical to that of Chl a. Here, we show that main product of the conversion is 8¹-OH-Chl a_F. Smaller amounts of two other oxidized Chl a_F species are also produced. In the presence of light and oxygen, the kinetics of the conversion are monophasic and temperature dependent, with an activation energy of 66 ± 2 kJ mol^-1. In the presence of oxygen in the dark, the conversion occurs in two temperature-dependent kinetic phases: a slow phase followed by a fast phase with an activation energy of 53 ± 1 kJ mol^-1. The loss of BChl g′ occurs at the same rate as the loss of Bchl g; hence, the special pair converts at the same rate as the antenna Chls. However, the loss of P₈₀₀ photooxidiation and flavodoxin reduction is not linear with the loss of BChl g. In anaerobic RCs, the charge recombination between P₈₀₀⁺ and F_X^- at 80 K is monophasic with a lifetime of 4.2 ms, but after exposure to oxygen, an additional phase with a lifetime of 0.3 ms is observed. Transient EPR data show that the line width of P₈₀₀⁺ increases as BChl g is converted to Chl a_F and the rate of electron transfer from A₀ to F_X, as estimated from the net polarization generated by singlet-triplet mixing during the lifetime of P₈₀₀⁺A₀^-, is unchanged. The transient EPR data also show that conversion of the BChl g results in increased formation of triplet states of both BChl g and Chl a_F. The nonlinear loss of P₈₀₀ photooxidiation and flavodoxin reduction, the biphasic backreaction kinetics, and the increased EPR line width of P₈₀₀⁺ are all consistent with a model in which the BChl g′/BChl g′ and BChl g′/Chl a_F′ special pairs are functional but the Chl a_F′/Chl a_F′ special pair is not.