The Mitochondria and Calcium Homeostasis Part 2

Well, welcome to this next video about

mitochondria

and

calcium

. Well, in the previous video what we have seen is that in the m of the

mitochondria

l membrane you have a channel that is a uni Porter, that is, the uni Porter of mitochondrial

calcium

, which allows calcium to move from the intermembrane space to the Matrix and then you also have another um another uh well, a pump this time it's a channel, this is a pump that will basically move the calcium against its electrochemical gradient because the calcium wants to go. towards the Matrix, its electrochemical gradient is pushing it towards the Matrix, so this is a pump because it moves it against its electrochemical gradient, it uses secondary active transport because it uses the concentration of sodium and the electrical gradient of sodium as well, so the sodium wants to enter in the Matrix and you're basically allowing sodium in in exchange for taking out calcium, so that's the exchange of calcium and sodium, okay, so now what I want to do is study this mitochondrial calcium unit a little bit more because that's new, we've studied the sodium calcium exchanger before, but this is a new, uh, new Beast, so we'll study it first and then what we'll do is see what actually happens if cytosolic calcium increases, how does that affect? uh the concentration of calcium in the mitochondria is okay, so first of all, this mitochondrial calcium unit, then what is the structure of the mitochondrial calcium uniporter?

So what we're looking at is the MCU, well, it's a dier basically, it's made up of two polypeptides and those two. Polypeptides have a membrane-spanning structure that resembles the membrane. They have a warm membrane that spans the Alpha Helix, a loop that fails to get through the membrane, and then another membrane that spans the Alpha Helix. This is a very typical structure for an ion channel. This is a single polypeptide, so it is a single subunit that forms the channel. Basically, you put two of these together to create the channel, so I'm going to draw the channel like this if we draw it with this sphere. like sorry, a cylinder, not a sphere, then the pores in the middle and basically you've made it made up of these two subunits, like, here's one of these subunits and, uh, that's one and then there's another one, a second one. . and you basically use two of these two proteins to make each of the halves, basically, this pink

part

is one of these, this orange

part

is one of these and they join together and form a pore basically that the calcium can move through well , OK?

So, that's the real pore. Now these channels have another interesting little thing which is that they can associate with auxiliary subunits that basically modulate their function, so the subunit here can basically associate with two other proteins and each subunit can do this. The orange subunit can associate with two proteins and the pink subunit can also associate with two proteins, so these two proteins are known as uh miku1 and Miku 2. Okay, let me put them in um, so this is uh mq2 , here is a smaller one that is closer to the mq2 subunit and uh this is mq1 uh mq1 let me write them bigger because they probably won't be visible under this bad light so this is miku1 m i c U1 and this is m M iu1 uh Mi iu2 more well mq2 right, so mq2 has is The usual function of Miku 2 is that if cytoc calcium is at its usual very low level, then miku2 does not have calcium bound and basically what mq2 does when it does not have calcium bound is it causes the subunit MCU here or the MCU protein. what constitutes this MCU channel, it represents half of it anyway, makes it adopt Clos confirmation, so basically when the calcium in the cytool is at its normal level of 100 nanometers, basically what that means is that Miku has no calcium United.

Miku's activity is then to make this channel close, so the MCU stays closed. For um, it is kept closed by mq2. Okay, right now, let me show you where the calcium binding domains are in both mq1 and mq2 R, so let's draw them. again here so basically in the cytotool portion of mq1 and Miku 2 what you have is one of these EF domains where they have a dime increase you have two EF uh EF hands pretty close together so I'll draw it like that but maybe I'm going to place them and attach them to the protein that way so that you have two of these EF domains uh hand on the cytosolic side of this miku2 protein, so remember that these proteins are going to be located on the inner mitochondrial membrane, like this, here's miku2 .

These EF hands will be up here, they'll basically be on um on this side instead of facing Lumin, there the parts will be facing the intermembrane space, but then of course the outer condal membrane is very permeable, so effectively the external mitoch the intermembrane space is um its calcium level is in balance with the um uh cytosolic calcium level very good now mcu1 mq1 also has two EF hand domains so here is mcu1 and it also has two EF hand domains okay then these are uh calcium binding domains and they're basically just uh polypeptides that form these loops and in this loop is where the calcium will bind there's a lot of uh acidic residues on that polypeptide there and those acidic residues donate their protons and then they get a negative charge. so it can interact very well with calcium, okay, now let's look at what happens when calcium rises in the cytosol, so if calcium rises in the cytoplasm of the cytosol, they are the same thing, then it will also rise in the intermembrane space. because of the permeability of the interexternal mitochondrial membrane, okay, so the calcium has gone up in this intermembrane space, here the calcium will then come and bind to these EF hand domains, so two calciums will bind to each of mq1 and mq2. so this is miku2 and this is miku1 here uh so two calcium ions are found one in each of EF's hands so this is mq1 here and this is mq2 right now when the calcium binds to mq2 it no longer inhibits the protein MCU and that causes the MCU protein to adopt an open confirmation, plus when calcium binds to mq1, mq1 actually triggers the MCU to adopt the open confirmation, so you have a double whammy that basically leads to opening this channel, so that when the calcium in the cytoplasm rises, MCU opens for the calcium to rise.

The MCU opens up so that the mitochondrial calcium uniporter opens up wide, okay? It's going to turn on and extrude all the calcium again and that would be great. The thing is, I told you that the maximum speed that this thing can operate at is 5000 calcium ions per second, that is, 5000 ions per second while the channels are what this thing can drive millions per second, so basically this will let calcium in faster than you can extrude it back so that the calcium transiently goes up in the matrix uh if the calcium goes up in the cytoplasm uh and obviously once the calcium goes back down in the cytoplasm it will stop coming in and gradually the Calcium-sodium exchanger will extrude it all again and that will destroy the signal, but when the calcium goes up in the cytool, that makes the calcium appear to also go up in the M matrix of the mitochondria. and it is due to the activation of this MCU channel uh by uh well by the binding of calcium to these EF hand domains of miku1 and miku2 sorry mq2 and miku1 and uh the binding of calcium to the EF hand domains of miku2 stops its inhibition and calcium binding to E The hand domains of miku1 cause it to actually activate the MCU protein and then that activates the MCU channel if both MCU proteins are activated, that's how the concentrations that change the calcium concentrations in the cytotool They are related to changing calcium concentrations. in the mitochondria