Alternative
H2 Cycling Model Tutorial - Judy Wall
Judy Wall's tutorial on an alternative to the classical H2 cycling
model
Classical H2 Cycling Model: During oxidation of organic acids, hydrogen
cycling has been proposed to occur whereby the reducing equivalents
and protons produced (2H+ + 2e-) are substrates for cytoplasmic membrane
bound hydrogenases which generate hydrogen that then diffuses into
the periplasm. Periplasmic hydrogenases oxidize the hydrogen and
the electrons are recaptured by the c-type cytochromes and ultimately
return to the cytoplasm for use in sulfate reduction. The protons
left contribute to the proton gradient which drives ATP synthesis,
membrane, then use for sulfate reduction.
Alternative H2 Cycling Model: Lactate
is oxidized by the membrane-bound lactate dehydrogenase (LDH)
passing electrons to the putative pool of menaquinone (MK). MK
picks up cytoplasmic protons upon reduction and delivers them
to the periplasm while delivering electrons to a carrier such
as the quinone-interacting membrane bound oxidoreductase (QMO)
Pyruvate oxidation may result in delivery of electrons to a transmembrane
complex that pumps protons to the periplasmic face of the cytoplasmic
membrane. There tetraheme cytochrome c3 accepts the electrons
and simultaneously becomes protonated. If cytc3 cannot be oxidized
by HMC rapidly enough, H2 will be generated and subsequently
cycled by the NiFe Hase. Cytochrome c3 is proposed to deliver
e- to HMC (or another transmembrane conduit) and H+ to the ATPase.
Thus no free proton gradient need be established. |
