Harpers

96 / CHAPTER 12NADHSuccinateComplex IFMN, FeSUncouplers –Malonate–Complex II––FADFeSPiericidin AAmobarbitalRotenoneCarboxinTTFAH 2 SCOCN –BALAntimycin AComplex IIIComplex IVQ Cyt b, FeS, Cyt c 1 Cyt cCyt a Cyt a 3Cu CuO 2–––Uncouplers–Oligomycin––Oligomycin–ADP + P i ATP ADP + P i ATP ADP + P i ATPFigure 12–7. Proposed sites of inhibition (− ) of the respiratory chain by specific drugs, chemicals, and antibiotics.The sites that appear to support phosphorylation are indicated. BAL, dimercaprol. TTFA, an Fe-chelatingagent. Complex I, NADH:ubiquinone oxidoreductase; complex II, succinate:ubiquinone oxidoreductase; complexIII, ubiquinol:ferricytochrome c oxidoreductase; complex IV, ferrocytochrome c:oxygen oxidoreductase. Other abbreviationsas in Figure 12–4.tribution of the hydrogen ions is used to drive themechanism responsible for the formation of ATP (Figure12–8).The Respiratory Chain Is a Proton PumpEach of the respiratory chain complexes I, III, and IV(Figures 12–7 and 12–8) acts as a proton pump. Theinner membrane is impermeable to ions in general butparticularly to protons, which accumulate outside themembrane, creating an electrochemical potential differenceacross the membrane (∆µ + H ).This consists of achemical potential (difference in pH) and an electricalpotential.A Membrane-Located ATP SynthaseFunctions as a Rotary Motor to Form ATPThe electrochemical potential difference is used to drivea membrane-located ATP synthase which in the presenceof P i + ADP forms ATP (Figure 12–8). Scatteredover the surface of the inner membrane are the phosphorylatingcomplexes, ATP synthase, responsible forthe production of ATP (Figure 12–1). These consist ofseveral protein subunits, collectively known as F 1 ,which project into the matrix and which contain thephosphorylation mechanism (Figure 12–8). These subunitsare attached to a membrane protein complexknown as F 0 , which also consists of several protein subunits.F 0 spans the membrane and forms the protonchannel. The flow of protons through F 0 causes it to rotate,driving the production of ATP in the F 1 complex(Figure 12–9). Estimates suggest that for each NADHoxidized, complex I translocates four protons and complexesIII and IV translocate 6 between them. As fourprotons are taken into the mitochondrion for each ATPexported, the P:O ratio would not necessarily be a completeinteger, ie, 3, but possibly 2.5. However, for simplicity,a value of 3 for the oxidation of NADH + H +and 2 for the oxidation of FADH 2 will continue to beused throughout this text.Experimental Findings Supportthe Chemiosmotic Theory(1) Addition of protons (acid) to the externalmedium of intact mitochondria leads to the generationof ATP.(2) Oxidative phosphorylation does not occur in solublesystems where there is no possibility of a vectorialATP synthase. A closed membrane must be present inorder to achieve oxidative phosphorylation (Figure 12–8).(3) The respiratory chain contains components organizedin a sided manner (transverse asymmetry) as requiredby the chemiosmotic theory.