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Lecture 19

BIOC13H3 Lecture 19: BIOC13 Lecture 19

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Biological Sciences
Jason Brown

BIOC13 Lecture 19  Within the membrane domain region of complex 1 there are 3 domains present, they exist as half channels  At any given time only 1 half is ever open  The white channel is the open side, other side is shaded and closed  Lysine is present in the centre of these channels, and is able to bind and unbind protons  When we are pumping protons we are trying to move them from low concentration to high concentration, so the protons don’t really want to move against the gradient  The affinity that lysine has for protons when its facing the matrix side is quite high  Matrix side is where proton concentration is low  High affinity of lysine for protons on matrix side allows it to pick up the protons and when the piston operates the lysine is exposed to the other half channel, its affinity for the proton drops and the proton unbinds from the lysine even though there’s high proton conc  Piston is connected to the 3 channels, has a negative region so when e come to q, and causes q to be negative, and that charge repulsion is what pushes the piston forward and that causes the channels to switch sides  The movement of the piston is what’s ultimately going to be opening and closing the half channels  We notice there are only 3 of these channels, but complex 1 pumps 4 protons for every pair of electrons, so where does the forth come from?  The 3 protons are passed through a mechanical gate (proton gate) th  4 proton passes through a chemical gate  As e come to Q, a proton also comes, and is able to work its way through the channel (not sure how this works)  There are 2 different mechanisms for proton pumping being used  Q has a cyclical head region, and that’s the part that accepts and donates e  It has a lipophilic/hydrophobic tail which allows it to exist in the membrane  When Q binds complex 1, it does so in a way that the head is associated with the peripheral region of the enzyme, tail gets inserted in the hydrophobic membrane domain  The 3 inhibitors all have a similar structure to q, and act to bind the enzyme as how q will, but they aren’t able to accept electrons  One area to accommodate the head to get e, and another area to accommodate the tail  All have a cyclic head and a relatively long hydrophobic, lipophilic tail  Of these 3 inhibitors, the classical inhibitor is rotenone, natural product produced by plants, to inhibit complex 1 of organisms that eat the plant  One of the types of plants is barbasco (leaves contain rotenone) most is produced in roots  The roots can be used for fishing by putting it in water, then the rotenone diffuses into the water, and kills the fish, so doing poison fishing  But doesn’t it also contain the rotenone? But we are able to consume the small amount of rotenone because we can detoxify it and so its less harmful  Complex 2 is succinate dehydrogenase  Its function is to oxidize succinate and pass electrons from succinate to Q  It is the smallest of all the complexes of the ETC, comprised of 4 subunits (proteins)  Flavoprotein (has 1 RC, fad)  In blue we have IS clusters (3 ISC)  In magenta we have membrane anchor subunits  E flux through complex 2 is taken from succinate, accepted first by FAD (2e acceptor), passes one electron at a time to IS cluster, e go from there to Q  The B is a heme B group  The reduction potential of all these redox centers are all the same, this means that as e go from succinate to Q, there is hardly any free energy released  So no free energy there to pump electrons (or little free energy)  It is different from other complexes because it pumps no protons  Complex 1 and 2 are not the only ones that can take e and pass it to Q  There are others like G3P dehydrogenase, takes e from G3P and passes it to Q  We have a series of enzymes that oxidize fatty acids and e form that are passed to Q  Complex 1 and 2 and the others do the same thing, oxidize and give electrons to Q  Why haven’t we done the same thing with other complexes, like give them the name  It has to do with finding them historically, they were the first discovered  You can say that we should think of them as 1a and 1b and 1c and so on  We talked about why there are so may copies of 3 and 4 it can be
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