16 TheElectronTransportChain incomplete

The Electron Transport Chain

Outline

• brief review of the citric acid cycle

• the role of electron transport in metabolism • reduction potentials:

finally getting at the energy in electrons

• the organization of the electron transport complexes

• you have all mastered glycolysis and the citric acid cycle - you took an exam after all…

• out of glycolysis came a profit of two molecules of ATP and pyruvate • that pyruvate was used to make acetyl-CoA

- compliments of the PDC

• acetyl-CoA was fed into the citric acid cycle - CO2 was made and released as waste (exhaled) - one molecule of ATP was made from GTP

- but what is the main outcome of the citric acid cycle…? - what does the cycle make that we want most…?

• what we do with those ____________________ and how we use them to make ATP will be the focus of our next two lectures

- after Monday, we’ll be done with the metabolism of glucose - it only took eight lectures…

• I’ve already told you many times that with the _______________ we’ve harvested is… ENERGY - the question becomes where is that energy and how do we get it out?

• aerobic metabolism – the citric acid cycle, the electron transport chain and oxidative

phosphorylation – is probably the most efficient way to extract energy from food on this planet

• all aerobic metabolism occurs in the ________________________________ - including the electron transport chain

• anaerobic metabolism – glycolysis – takes place in the ___________________________

The Relationship Between the Structure of the Mitochondria and ATP Production

• the production of ATP in the mitochondria – by oxidative phosphorylation – is (obviously)

_____________________________ - why ‘obviously’…?

• the energy driving this process forward comes from the electron transport chain

- oxidative phosphorylation and the electron transport chain are ___________________ • here’s how it works in a nutshell…

• the electrons are passed from one protein complex to another through redox reactions (electron transport)

- and each time electrons are passed, energy is released (like a game of ‘hot potato’)

- this releases the energy a little bit at a time

• that released energy is used – little by little – simply to pump protons (H+) against their

concentration gradient across the inner mitochondrial membrane – _____________ TRANSPORT • this results in a ‘proton gradient’ across the inner membrane

• this is stored potential energy

- these protons want to ___________ BADLY

• when they do, the energy released will be used to make ADP into ATP

• this process – the coupling of released energy from the transport of electrons to the pumping of protons to form a gradient to making ATP from the energy stored in that proton gradient – is

called _____________________________ COUPLING (we’ll come back to this more later) • but, our goal for today’s lecture is to get all of the electrons that we put on reduced NAD+ and

FAD during glycolysis and the citric acid cycle passed along as a hot potato all the way to oxygen – ELECTRON TRANSPORT CHAIN

- where electrons go, H+ follows - oxygen will be reduced to H2O

- NAD+ and FAD will be regenerated and recycled

Reduction Potentials in the Electron Transport Chain

• there is a change in ____________ as electrons are moved from one electron carrier to another • consider two electron carriers:

- our old friend NAD+ - a newcomer: coenzyme Q

• both are electron carriers; both can exist in oxidized or reduced forms • which way would electrons prefer to move…?

- do they like to move from NAD to CoQ or from CoQ to NAD…? - dunno… not yet at least…

• the direction of movement preferred by electrons is measured by REDUCTION

________________________ for each of the carriers involved - this is a property or trait for each carrier – it cannot change!

• as a rule: a molecule with a ___________ reduction potential will be reduced (electrons will

move towards it) if it is paired with a molecule having a __________ reduction potential

• how reduction potentials are calculated experimentally is covered in the text - feel free to educate yourself if you’re interested

- I will not be covering that material, nor will it be on any exam • once these values are calculated, they’re known – permanently

- traits are permanent; so are reduction potentials • let’s take a look…

• just for giggles…

- what was the pH of the reaction when the experiments were done to determine these reduction potentials…?

• in general, reactions near the top will occur if they are paired with reactions lower down • remember, these are all reductions

- if we are doing REDOX reactions one molecule is reduced, but the other is…? - so what should we do?

• we ________________ the _____________ of the oxidation so, let’s consider the oxidation of NADH coupled to the reduction of oxygen to water

• +0.320 + 0.816 = 1.136V - what the hell does that mean?! • for redox reactions…

ΔG=-nFΔE

• n = # of moles of electrons transferred • F = Faraday’s constant

96.485kJ/Vmol

• ΔE = what we just did…

• you don’t need to know any of the math for this course… • you just need to know this:

• for redox reactions… ΔG=-nFΔE

• what will the sign of ΔG be if ΔE is positive?

• and so, _________ give __________ and are, thus, ______________________ • the full reduction of CoQ coupled to the oxidation of cytochrome a…?

0.060 + -0.290 = -0.230 - ain’t gonna happen!!!

• electrons moving from NADH to CoQ vs.

from CoQH2 to NAD

• this is why the flow of electrons moves in only one direction in the electron transport chain - the universe demands it!

• and so, this is the way it works

• a series of redox reactions done in succession - each having a reasonable –ΔG

- each releasing a reasonable amount of energy (no explosion) • this is why the ORDER of redox reactions matters

- it is to achieve these specific ΔG’s

• that released energy will be used to pump protons against their concentration gradient - that gradient will be used to make ATP

• so, let’s play hot potato - let’s pass some electrons

The Organization of the Electron Transport Chain

• there are four protein complexes that are part of the electron transport chain –

_______________________ COMPLEXES

Complex I

• also called _____________-CoQ OXIDOREDUCTASE

- in a nutshell, it transfers electrons from NADH to ______________________ Q - it is an integral protein of the inner mitochondrial membrane

- it contains two components that can serve as electron carriers:

- a ‘___________________________’ (FMN) and an ‘iron-sulfur cluster’

- these are simply things that can be reduced and/or oxidized (i.e., things that can hold electrons)

• first electrons are transferred from NADH to the ___________ (FMN) portion of the flavoprotein (the FMN is covalently bound to the complex)

- in other words, FMN is reduced while NAD is oxidized

• next, FMN gives up its electrons (it is oxidized back to its original form) and passes them to an

iron-sulfur cluster (Fe-S) via _____________________

• lastly, the Fe-S cluster is oxidized and CoQ is reduced to ___________

• so, for complex I, the electrons are passed from NADH to FMN to Fe-S clusters to CoQ • let’s discuss some broader implications…

• complex I is one place where we will pump protons

• the redox reactions I just discussed release a total of _______kJ/mol of energy – that’s a lot… - some of that energy is used to pump protons

- but what protons…?

• remember when I told you that, in biochemical reactions, where electrons go protons always follow…?

- well… I lied … I’m sorry…

… but, it helped you out in exam 2…

• in biochemical reactions, where electrons go protons almost always follow – but, not when

considering __________________________

• NADH + H+ _{gives up its electrons and protons to FMN}

• FMNH2 gives up its electrons to Fe-S clusters, but Fe-S clusters don’t want the ______________ - so they get pumped across the inner mitochondrial membrane instead

• then CoQ is reduced and becomes CoQH2 - WAIT!!!

- the Fe-S clusters didn’t have any protons to give to CoQ, so what happened – where did they come from?

- from the mitochondrial ___________________

• anyone want to take a guess as to how this effects the proton gradient…? • one last point that will become important in a few minutes…

- CoQ is ______________

- it can move around in the inner mitochondrial membrane

• but before we continue following the electrons on CoQ, we must first discuss complex II • complex II will also transfer electrons to CoQ

Complex II

• also called ______________________ CoQ-OXIDOREDUCTASE • also bound to the inner mitochondrial membrane

• also called succinate dehydrogenase • also part of the citric acid cycle…

• those electrons we left on FADH2 will now be transferred to CoQ - FADH2 is oxidized; its electrons are passed to Fe-S clusters

- this time, there is not enough energy generated by the redox reactions to pump the protons out of the matrix

- but when the Fe-S clusters pass the electrons to CoQ, CoQ also takes on protons, contributing to the concentration gradient (just like before)

• that “middle-man” is a CYTOCHROME…

• CYTOCHROMES are proteins that contain a ____________ group, but they do not carry oxygen

- the iron in these _______________ groups can be oxidized and/or reduced

• for our purposes, cytochromes are just another electron _____________________ - another thing to play hot potato with

• what makes cytochromes special is that they are a protein FAMILY - different cytochromes have different reduction potentials

- and so, cytochromes can pass electrons amongst themselves and still result in –ΔG’s

Complex III

• also called _________________-cytochrome c oxidoreductase • also bound to the inner mitochondrial membrane

• catalyzes the transfer of electrons from CoQH2 to cytochrome c • CoQ can carry two electrons; iron can only carry one

- so two cytochromes are needed for every one CoQH2 if we’re going to oxidize CoQH2 and reduce cytochromes

• also, cytochromes cannot carry protons, so here again the protons from CoQH2 will be pumped

out of the matrix and into the ____________________________________ space • so, let’s follow the (complicated) path of these electrons

• CoQ is unique for electron carriers in that it can pass __________________________________ - first, one electron is passed to Fe-S clusters of the complex and then on to cytochrome c - then another CoQH2 passes a single electron to cytochrome c

- leaving two ______________ molecules, each half ____________________

• it appears as though, at this point, one _____________ is oxidized while the other is reduced (with a single electron) to generate CoQH2 and CoQ

Complex IV

• also called cytochrome c ______________________ • also a part of the inner mitochondrial membrane • catalyzes the final steps of the electron transport chain

- the transfer of electrons from cytochrome c to ______________________ • it also pumps protons (directly as a pump)

• it’s quite simple…

• cytochrome c transfers its electron to a pair of cytochrome ___ proteins that make up a

complex called cytochrome _________________

- ___________________ is involved as temporary holding place for electrons • cytochrome oxidase is oxidized directly by oxygen (why we breathe…)

- oxygen itself is _____________________ and becomes water (H2O) • reducing oxygen to water also contributes to the proton gradient…

- how…?

• at this point we’ve milked all the energy we possibly can out of glucose

• that energy is now stored as a proton gradient DYING to flow across the inner mitochondrial membrane

- but it can’t; not without a channel

• next, we’ll open the floodgates and let those protons flow - and as they do, we’ll harness that energy and make ATP • but, that will all have to wait until Monday…

Summary

• as electrons are passed along in the electron transport chain energy is released

– the released energy from the transport of electrons is ____________________ to the pumping of protons to form a gradient

- this gradient will be used to make ATP

– __________________________________ COUPLING

• a molecule with a ___________ reduction potential will be reduced if it is paired with a

molecule having a ____________ reduction potential

• there is a change in _______________ as electrons are moved from one electron carrier to another

____________ give ____________

• we also discussed how gradients can be influenced by removing _______________ and not necessarily pumping them