The Effect of Bacteriochlorophyll gOxidation on Energy and Electron Transfer in ReactionCenters from Heliobacterium modesticaldum.

The heliobacteria are a family ofstrictly anaerobic, Gram-positive,photoheterotrophs in the Firmicutes. They make use of a homodimerictype I reaction center (RC) that contains ∼20 antenna bacteriochlorophyll(BChl) gmolecules, a special pair of BChl g′ molecules (P800), two 81-OH-Chl aFmolecules (A0),a [4Fe–4S] iron–sulfur cluster (FX), anda carotenoid (4,4′-diaponeurosporene). It is known that inthe presence of light and oxygen BChl gis convertedto a species with an absorption spectrum identical to that of Chl a. Here, we show that main product of the conversion is81-OH-Chl aF. Smaller amountsof two other oxidized Chl aFspecies arealso produced. In the presence of light and oxygen, the kinetics ofthe conversion are monophasic and temperature dependent, with an activationenergy of 66 ± 2 kJ mol–1. In the presenceof oxygen in the dark, the conversion occurs in two temperature-dependentkinetic phases: a slow phase followed by a fast phase with an activationenergy 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 asthe antenna Chl’s. However, the loss of P800photooxidiationand flavodoxin reduction is not linear with the loss of BChl g. In anaerobic RCs, the charge recombination between P800+and FX–at 80K 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. TransientEPR data show that the line width of P800+increasesas BChl gis converted to Chl aFand the rate of electron transfer from A0to FX, as estimated from the net polarization generated by singlet–tripletmixing during the lifetime of P800+A0–, is unchanged. The transient EPR data also showthat conversion of the BChl gresults in increasedformation of triplet states of both BChl gand Chl aF. The nonlinear loss of P800photooxidiationand flavodoxin reduction, the biphasic backreaction kinetics, andthe increased EPR line width of P800+are allconsistent with a model in which the BChl g′/BChl g′ and BChl g′/Chl aF′ special pairs are functional but theChl aF′/Chl aF′ special pair is not. [ABSTRACT FROM AUTHOR]