The proton circuits for mitochondria, chloroplasts and photosynthetic bacteria are basically similar, having evolved from a common ancestor. All can be represented by the same diagramatic proton circuit:


Respiratory chain

N is NAD:ubiquinone oxidoreductase (Complex I, or NADH dehydrogenase)

M is ubiquinol:cytochrome c oxidoreductase (Complex III, or bc-complex)

O is cytochrome c oxidase (Complex IV)

The ATP synthase is similar in all the sytems, although its proton stoichiometry in chloroplasts may be different.

The transport systems shown are for the adenine nucleotide transporter, which exchanges ATP- for ADP, and the phosphate transporter, which exchanges H2PO4- for OH-

(Various other dehydrogenases eg. succinate:ubiquinone oxidoreductase, omitted for clarity)

n = 4 H+ / 2e; m = 2 H+ / 2e; o = 4 H+ / 2e; q = 1 H+ / ATP; a = 3 H+ / ATP

Overall reaction:

H+ + NADH + 10 H+matrix + ½O2 ==> NAD+ + 10 H+cyto + H2O

10 H+cyto + 2.5ADPcyto + 2.5Picyto ==>10 H+matrix + 2.5ATPcyto

Purple photosynthetic bacteria

Cyclic photosynthetic chain

N and O are photochemical reaction center (ferrocytochrome c2:ubiquinone photo-oxidoreductase)

M is ubiquinol:ferricytochrome c2 oxidoreductase (bc1-complex)

n = 2 H+ / 2e; m = 2 H+ / 2e; a = 3 H+ / ATP

Overall reaction:

2hv + 4H+cytoplasm ==> 4H+periplasm

1.33ADP + 1.33Pi + 4H+periplasm==> 1.33ATP + 4H+cytoplasm

(Bacteria also have a respiratory chain like that of the mitochondria, with several alternative oxidases)
(Membrane transport carriers use proton gradient to accumulate substrates, exchange ions)

Chloroplasts (and cyanobacteria)

Non-cyclic photosynthetic chain

N is photosystem II (water:plastoquinone photo-oxidoreductase)

M is plastoquinol:plastocyanin oxidoreductase (bf-complex)

O is photosystem I (plastocyanin:ferredoxin photo-oxidoreductase)

n = 2 H+ / 2e; m = 2 H+ / 2e; o == 2 H+ / 2e-; a = 3 (or possibly 4) H+ / ATP

Overall reaction:

8hv + 2H2O + 2NADP+ + 12H+stromal ==> O2 + 2H+ + 2NADPH + 12H+lumenal

4ADP + 4Pi + 12H+lumenal ==> 4ATP + 12H+stromal

(Membrane transport systems in chloroplasts carry electro-neutral exchange of triose phosphate for Pi; bacterial membrane carriers in cyanobacteria use proton gradient to accumulate substrates, exchange ions)

©Copyright 1996, Antony Crofts, University of Illinois at Urbana-Champaign,