Biochemistry 401 lecture 21 Today we re going to talk about the electron transport chain We re going to begin with a discussion of how reducing equivalents get from the cytoplasm to the mitochondrial matrix so that they can be used in electron transport chain We re then going to talk about oxidative phosphorylation and we ll start with an overview of this process We ll talk specifically about the electron transport chain and about integral membrane electron carriers and mobile electron carriers that make up this electron transport chain We ll then follow electrons through the electron transport chain to see how this works And so how do reducing equivalents get from the cytoplasm into the mitochondrial matrix so that they can be used in the electron transport chain Well it s not actually the carrier that is transported just the electrons and so we re going to hand off electrons from one electron carrier to another We ll start with an electron carrier that is in the cytoplasm that s reduced and then we ll pass those electrons to a mitochondrial electron carrier and reduce that So the electrons will be passed from the cytoplasm to the mitochondria by way of electron carriers The electron carriers themselves don t cross the membrane but the electrons do There are two main ways that reducing equivalents get from the cytoplasm into the mitochondrial matrix This is the glycerol three phosphate shuttle and the malate aspartate shuttle The glycerol three phosphate shuttle uses cytoplasmic NADH and mitochondrial FAD Now this mitochondrial FAD is shown here as e FAD because this FAD is actually a prosthetic group that s bound into an enzyme and we ll look at this in a little while With the malateaspartate shuttle we begin with NADH carrying the electrons in the cytoplasm and we re going to pass those over to an NAD molecule that s in the mitochondrion We end up with NAD in the cytoplasm and NADH in the mitochondria So now let s look at how these transporters work This is the glycerol 3phosphate shuttle and it s known as the glycerol 3 phosphate shuttle because that is the molecule that s going to be oxidized to pass electrons to an electron carrier that has close links to the inner mitochondrion This is an enzyme that is intimately associated with the inner mitochondrial membrane It sits on the cytoplasmic face of the inner mitochondrial membrane facing the inter membrane space 1 Now this process is going to happen in three steps The first thing we re going to do is we re going to reduce dihydroxyacetone phosphate to yield glycerol 3phosphate and so the reaction is dihydroxyacetone phosphate plus NADH plus a proton yields NAD plus glycerol 3 phosphate Dihydroxyacetone phosphate is being reduced and NADH is being oxidized to NAD Now these electrons from glycerol 3 phosphate can be handed off to another electron carrier This is bound into an enzyme the mitochondrial glycerol 3 phosphate dehydrogenase We re going to reduce FAD that s bound in this enzyme to yield FADH 2 and in the process glycerol 3 phosphate will be oxidized to form dihydroxyacetone phosphate again But this shuttle doesn t stop here The electrons that are carried by FADH2 can be passed off to a mobile inner mitochondrial membrane electron carrier that is known as ubiquinone Now ubiquinone is sometimes called Q10 This is a mobile carrier that is lipophilic and shuttles back and forth in the inner mitochondrial membrane carrying electrons from one electron carrier to another Ubiquinone is often abbreviated Q this is the oxidized form of the electron carrier whereas QH2 ubiquinol is the reduced form We will look at this more closely in a little while This is the malate aspartate shuttle It is much more complex than the woman just looked at and we re going to follow this really slowly so that you can figure out what s going on The enzymes that are involved in this are a malate dehydrogenase in the cytoplasm an aspartate aminotransferase in the cytoplasm a malate dehydrogenase in the mitochondrion and an aspartate aminotransferase in the mitochondrion Now both of these mitochondrial enzymes are in the mitochondrial matrix And so we re going to start off with oxaloacetate in the cytoplasm and we re going to reduce that to form malate As we do this the electrons from NADH will be transferred to oxaloacetate to yield malate and NAD So oxaloacetate is reduced to malate and NADH is oxidized to NAD Malate will leave the cytoplasm and go into the mitochondrial matrix Once there it will be oxidized again to form oxaloacetate and reduce NAD to NADH and so now we have our electrons transferred from the cytoplasm into the mitochondrial matrix and that s what we want however what we don t want is we don t want a lot of oxaloacetate in the mitochondrial matrix because this might slow up the TCA cycle and so we re going to move this oxaloacetate out of the mitochondrial matrix But wait a 2 minute We can t do that because we can t transport oxaloacetate directly out of the mitochondrial matrix There s no transporter So we re going to use another one of our tricks and we re going to add an amino group to oxaloacetate Now remember at the beginning of the semester I said it s important to know which amino acids have four carbons and which have five and I said it s important to know which metabolites have four carbons and which have five and this is why Oxaloacetate has four carbons so does aspartate and so now what we re going to do is we re going to take an amino group from glutamate and transfer it to oxaloacetate When this happens oxaloacetate will be changed to aspartate There are 4 carbons in oxaloacetate and 4 in aspartate Aspartate can now leave the mitochondrial matrix by way of a transporter But let s go back to the mitochondrial matrix for minute We took the amino group off of glutamate this is the alpha amino group and when we do this we change glutamate into alphaketoglutarate 5 carbons in glutamate 5 carbons in alpha ketoglutarate This is the same intermediate that we saw in the TCA cycle so we don t want this to build up either and what we re going to do is we re going to transport this out of mitochondrial matrix by way of a transporter Once this alpha ketoglutarate is in the cytoplasm it can accept an amino group from aspartate to be turned back into glutamate to begin the cycle again and aspartate once it gives off that alpha amino group is changed from the 4 carbon aspartate into the 4 carbon ketoacid oxaloacetate and in this
View Full Document
Unlocking...