Cytochrome c_6A is a funnel for thiol oxidation in the thylakoid lumen

Abstract

Cytochrome c 6A is a dithio-cytochrome recently discovered in land plants and green algae, and believed to be derived from the well-known cytochrome c 6 . The function of cytochrome c 6A is unclear. We propose that it catalyses the formation of disulphide bridges in thylakoid lumen proteins in a single-step disulphide exchange reaction, with subsequent transfer of the reducing equivalents to plastocyanin. The haem group of cytochrome c 6A acts as an electron sink, allowing rapid resolution of a radical intermediate formed during reoxidation of cytochrome c 6A . Our model is consistent with previously published data on mutant plants, and the likely evolution of the protein.

The role of a disulfide bridge in the stability and folding kinetics of Arabidopsis thaliana cytochrome c6A

“…To aid with determining the kinetic mechanism and to ascertain the contribution to overall protein stability, the two Cys residues forming the disulfide bond have been replaced with Ser residues to create the C67/73S double variant. The results of this study are strongly in favour of the disulfide bond stabilizing the conformation of the LIP and support a recent proposal that the LIP may serve as a recognition site/motif for a protein partner [7], rather than in a catalytic role [11,12].…”

“…This feature is not found in cyt c 6 , c 6B or c 6C and is thus unique to cyt c 6A members [5]. No conformational change in the disulfide bond or the LIP upon heme oxidation state change has been detected [10] and a number of hypotheses suggesting a catalytic role for this disulfide containing LIP to support possible cellular function have been put forward [11,12].…”

“…More recently allosteric disulfide bonds have been identified that are able to control function by triggering a conformational change in the protein upon thiol oxidation [32,49,50]. Cyt c 6A is unique in the cyt c 6 family in that it is the only member, so far identified, to contain a disulfide bond and in combination with the low E m has led to a number of interesting hypothesis relating to the function of cyt c 6A in plants that incorporate catalytic and redox activity of the disulfide [11,12]. A disulfide bond, however, is also present in other Class I cyt c family members [51,52].…”

Thioredoxin-mediated reduction of the photosystem I subunit PsaF and activation through oxidation by the interaction partner plastocyanin

“…However, under oxidizing conditions, Pc ox may reform the disulfide bridge, activating the system. Another possibility is that PsaF is oxidized by cytochrome c 6A , which, as recently suggested [11], may be oxidized by Pc. In addition one may hypothesize that oxidation of Cys residues in the PsaN subunit, which is a known Trx target [7,8], can lead to structural changes that strengthen the binding of Pc to PSI.…”

“…It is more likely that Pc with a pI of 3.8 interacts with PsaF 0 instead of Trx-6His. The observed slow reduction of Pc ox is not unexpected since two molecules of Pc ox are required to fully oxidize a thiol pair and since the first electron transfer presumably is thermodynamically unfavorable [11].…”