Before jumping into the specifics of Oxidative Phosphorylation, let's take a look at some of the players and a general overview of what's going to be going on. Now, we're going to be dealing with some new electron carriers, stuff that we haven't seen before. But the principle is very similar to that of NAD or FAD, where it's a molecule; some of these are quite big compared to NAD and FAD and can very easily accept and donate electrons. You guys know the drill. So, quinones are going to be one of these types of molecules, and these are lipid-soluble, which is an interesting feature about them. They're going to exist, and that's what this cue is right here. They're going to exist within the membrane and diffuse through the inside of the membrane between protein complexes. An interesting note about quinones, they contain an isoprene chain that's part of how they stay anchored within the membrane and they're capable of carrying 2 electrons. Now, cytochromes contain a porphyrin ring with an iron in the center and they can only carry 1 electron at a time. Interestingly, cyanide and carbon monoxide, those poisons that kill you, actually kill you by blocking electron flow at cytochrome a, which is at the very end, in fact, let me just go down. It's at the very end of the electron transport right before we take oxygen and form water, but we'll get to that later. So, moving on, we're going to see a bunch of iron-sulfur complexes. Iron in the iron-sulfur proteins is complexed by cysteine residues in the protein and sulfur. Additionally, these proteins can carry 1 to 2 electrons, but some can actually carry up to 4. It depends on how many irons there are; basically, one iron can take 1 electron. The iron-sulfur proteins that can carry 4 electrons have 4 irons in them. Now, if we trace the path of our electrons, we are going to start off either with NADH or FADH2, right? Those are our two starting points for electron transport. And we are going to drop our electrons off at complex 1 with NADH, or complex 2 with FADH2. From complex 1, the electrons, or rather complex 1 or 2, the electrons are going to be picked up by a quinone. This is actually ubiquinone. Ubiquinone is going to drop them off in complex 3. Now in complex 3, cytochrome B is going to pick up the electrons. They're going to go through iron-sulfur complex and eventually be picked up by cytochrome C1. Then cytochrome C, which is a protein that's actually hanging out on the periplasmic side of the membrane. So just to be clear, this is the periplasmic side, or the intermembrane space, if you will. And over here, we have the matrix. So cytochrome C is actually on the plasmic side. It's going to pick up those electrons and drop them off at the final complex, complex 4, where cytochrome A is going to get them before they join up with oxygen to form water. So looking again at our image here, NADH will drop off its electrons at complex I, which will oxidize it. I should say reoxidize it to NAD+, and FADH2 will drop its off in complex II, which will reoxidize it to FAD. Now actually, this FAD is part of complex 2. Here it's kind of depicted as if it's, you know, coming and going freely from any, or similar to NADH, but it's actually part of the protein complex. We're going to talk more about this in a second. And you might also remember that when we were talking about the citric acid cycle, I said that the reaction that produces FADH2 is actually occurring in a protein complex that's part of the electron transport chain. Well, here we are. So let's just take a look at the overall process that's occurring. From moving from complex 1 to complex 4, we have NADH and we are actually going to use 11 protons from the matrix and half of an O2. So this is a weird way to put it. Really we just mean 1 oxygen atom. Now the result of this electron transport from complex 1 to complex 4 is an NAD+, right? Ten protons pumped into the intermembrane space. We'll be talking more about this momentarily and the formation of a water molecule, 6 protons also half of an O2, and we wind up with FAD. Only 6 protons pumped this time and one water molecule. Now you might remember that there is a difference in the amount of ATP produced by an NADH through electron transport and FADH2. This here is what accounts for that difference. The fact that electrons that come from NADH will result in more protons pumped into the intermembrane space whereas FADH2, results in 4 fewer protons. Why this is significant to the amount of ATP produced is something that we will get to momentarily. So with that, let's flip the page.
Review 4: Amino Acid Oxidation, Oxidative Phosphorylation, & Photophosphorylation
Oxidative Phosphorylation 1