Module 9: Cellular Energy Metabolism - Mechanisms of ATP generation

What is the electron transport chain (ETC)

• final step of cellular respiration

• uses donated e- from NADH FADH2 to make atp

• made up on 5 protein complexes embedded in inner membrane (named 1-5)

what are the baseline roles of the first four protein complexes in the ETC

1. to accept e- and shuttle them along

2. pump protons out of the mitochondrial matrix into the intermembrane space

what are the two electron carriers and what do they do

• coenzyme Q and cytochrome c

• shuttle the electrons between protein complexes

• e- do not diffuse well by themselves through membrane

  • charged and will not traverse through hydrophobic interior of membrane

• these carriers are lipid soluable

what in the proteins accept the e-?

• proteins themselves are not reversibly reduced and oxidized

  • there are several redox centres within them that are

• redox centres are categorized as:

  • coenzymes

  • Fe-S clusters

  • cytochromes

  • Cu

• each protein contains 2 or more combination of these types of redox centers

Electron pathway from NADH

first donates to complex 1 → complex 3 → complex 4

electron pathway fro FADH2

is a prosthetic group that is covalently boound to the enzyme succinate dehydrogenase (apart of complex 2), thus starts at:

complex 2 → 3 → 4

where do the two electron carriers shuttle electrons

• conenzyme Q: through membrane to complex 3

• cytochrome c: electrons from complex 3 to complex 4

why do the electrons move/shuttling?

due to reduction potential

• NADH has low affinity for electrons (reduction potential)

• O2 (final e- accepter) has high affinity

• all the redox centers between these two have increasing reduction potential, hence there is an attraction gradient where the e- is pulled through

complex 1

• takes e- from NADH and gives it to coE Q (ubiquinone)

• it also pumps protons across the membrane → gives it to ubiquinone → becomes QH2 (ubiquinol)

• complex 1 uses electron gradient of e- movement to pump 4H+ from matrix (inside) to intermembrane space (outside)

  • creates proton gradient for ATP

complex 2

succinate to ubiquinone (CoQ)

• recall: succinate from the citric acid cycle

• FAD accepts two e- from succinate (in citric acid cycle)

• e- → via iron-sulfur centers to ubiquinone → reduced QH2 (in ETC)

• no proton transport

complex 3

ubiquinone (CoQ) to cytrochrome c

• uses two e- from QH2 to reduce two molecules of cytochrome c

• 4 H+ transferred to intermembrane space

complex 4

• 4 e- used to reduce one oxygen molecule into two H2O molecules

• 4 H+ picked up from the matrix in process

• other 4H+ passed from matrix to intermembrane space

what are the numbers of protons pumped from each complex

after e- pass through protein complex with a redox center lower in reduction potential to a redox center with a higher reduction potential, E released

• complexes harness that E to drive a proton pump mechanism: matrix → intermembrane space

• complex 1 and 3: 4 protons for each pair of e- that passes through

• complex 4: only 2 for each pair of e-

• complex 2: none

what is the effect of shuttling e- and proton pumping

chemiosmotic gradient across the inner mitochondrial membrane

• where high conc of protons outside the inner membrane relative to inside matrix

  • protons can only be moved through proteins as membrane is impermeable to charged compounds

• gradient itself is called electrochemical (due to both electrical - charges, chemical - pH change)

• potential energy is stored in the chemiosmotic gradient

  • which can be converted to ATP

how is ATP synthesized through oxidative phosphorylation

ADP is turned to ATP through oxidative phosphorylation

• catalyzed by complex 5 (F1-F0 atp synthase)

• complex 5 is composed of multiple subunits

  • F0 subunit: proton channel that spans the inner mitrochondrial membrane

    • responsible for allowing protons to enter the matrix

  • F1: bulbous portion of the complex on the matrix side on inner membrane

    • comprises of the ATP synthase enzyme responsible for making ATP (catalyzes hydrolysis of ATP)

How is ADP → ATP in regards to the subunits of complex 5

• F0: protons pass through from outside to inside of matrix → drives atp synthase in f1 subunit

• F1: atp synthase has ADP + Pi present in the active site

  • proton going through F0 drives conformational change in the active site, driving the catalysis of ATP

Yield of ATP per NADH and FADH2

• one NADH pumps 10 protons, yielding 3 ATP

• one FADH2 pumps 6 protons, yielding 2 ATP

how many ATP generated from the breakdown of glucose

• note: GTP is equiv to ATP

• total: 38 per glucose molecule that is oxidized

• the pyruvate to atp is in the pyruvate dehydrogenase reaction

What do electrons from the cytosolic (happening in cytosol, not mitochondria) NADH must do before getting to the ETC

• NADH generated inside mitochondria through pyruvate dehydrogenae rxn and citric acid cycle will have easy access to the ETC

• NADH generated from glycolysis (in cytosol) will not have access to ETC

  • e- must be transported through proteins as it is relatively impermeable

  • no NADH transporter → 2 mechanisms where NADH donates electron to intermediate which goes past membrane:

    • glycerol-3-phosphate shuttle

    • malate-aspartate shuttle