Unit 6 - Oxidative Phosphorylation Flashcards
what are compounds that interfere with E coupling during oxidative phosphorylation?
- uncouplers
- phosphorylation inhibitors
- electron transport chain inhibitors
what do uncouplers do in oxidative phosphorylation? what is a classic example?
uncouple oxidation from phosphorylation
- stimulate O2 consumption in absence of ADP
- dissipate H+ gradient by providing alternate route for H+ reentry into matrix
- -if no gradient, synthase runs in reverse, hydrolyzing ATP and releasing it as heat
- dinitrophenol and natural uncoupler protein in brown adipose mitochondria are examples
what are properties of uncouplers?
weak acids (bind H+ when outside), hydrophobic (dissolve across IMM), delocalized charge (spread it over a number of atoms in the molecule in resonance-style; “soft charge”)
why is dinitrophenol good and bad as a weight loss supplement?
would sweat (ATP hydrolyzed and lost as heat) to lose weight, but is broken down by P450, and byproducts cause cataracts
what is the natural uncoupler PRO? what blocks it?
in mitochondria of brown adipose tissue
- involved in non-shivering thermogenesis (especially for hibernating animals) and weight regulation
- has H+ pore that is under careful hormonal regulation from NE
- blocked by ATP, ADP, GTP, and GDP
how is the natural uncoupler PRO activated?
NE –> AC –> cAMP –> PKA –> phosphorylated triglycerol lipase –> FFA –> opens channel
how do phosphorylation inhibitors work?
block ADP, but not uncoupler-stimulated respiration
- example: oligomycin binds to F1 and blocks H+ movement thru FoF1
- respiration slows b/c H+ no longer able to reenter matrix thru FoF1
- uncouplers can stimulate respiration, but no ATP is made
what is a natural phosphorylation inhibitor? when does it work?
the inhibitor PRO that protects against rapid ATP hydrolysis during ischemia
- normally does not bind to FoF1
- in absence of O2, pH of matrix drops and IP is protonated to change its conformation to a form that binds FoF1 tightly
- when O2 reintroduced, H+ are pumped out of matrix, IP is deprotonated, and allows ATP synthesis to resume
how do electron transport chain inhibitors work?
both ADP- and uncoupler-stimulated respiration are blocked (example with cyanide)
-cyanide competes with O2 for binding to heme a3 Fe and Cu B, but CN- cannot accept electrons that O2 could, so E flow stops
where do rotenone and antimycin A block?
both ETC inhibitors
-rotenone/amytal (barbituate) inhibits beginning of chain (Fe-S center from NADH dehydrogenase)
-antimycin A inhibits middle of chain (Fe-S center that goes from CoQ to cytochrome c1
antimycin A blocks middle of chain
what are the 2 components of the electrical gradient?
electrical and proton gradient
what is Mitchell’s chemiosmotic theory?
delocalized electrochemical gradient is a required intermediate in coupling exergonic redox RXNs to endergonic synthesis of ATP (uses common intermediate principle)
-membranes are impermeable to protons, and electrochemical gradient exists
what are “Mitchell’s loops”?
direct coupling mechanism for generating electrochemical gradient (“Q cycle” is one of them)
- Fe-S of dehydrogenase, and cytochrome Bh donate 1 electron each to Q
- Q gets 2 H+ from aqueous inside phase to become QH2
- QH2 diffuses closer to outside surface
- 1 electron is transferred to Fe-S of b/c1 complex to cytochrome c
- the other electron is transfered to cytochrome Bl that goes to Bh
- 2 H+ are released to outside, and Q is recycled back
what is the stoichiometry of the Q cycle?
since only one electron flows down to O2, and the other is recycled, it is 2 H+ per electron
how do proton pumps fit in with Mitchell’s loops?
loops cannot function in cytochrome oxidase b/c no H+ donor or acceptor, so need indirect coupling mechanism
- H+ transport is coupled to exergonic redox reactions indirectly through PRO conformational changes
- H+ binding sites are filled by matrix H+ before reduction can occur
- reduction causes sites to be exposed to outside surface, and H+ must dissociate before reoxidation can occur
