[MUSIC] Hello, everyone. Welcome back to my Coursera class about Biochemical Principles of Energy Metabolism. This is our last session for week one, okay? So so far, I've been focusing on the importance of ATP as our energy currency. So number 1 is, ATP hydrolysis and Energy coupling. And second one is ATP hydrolysis and Mechanical work. For today I'm going to talk about ATP hydrolysis and molecular transport. So in the beginning, I'm going to introduce the membrane potential, the membrane potential. What is membrane potential? Membrane potential is the difference in electrical potential between inside and outside the cells, inside and outside the cell. There is voltage difference, membrane potential. The typical value of membrane potential of eukaryotic cells, depending on the cell types, roughly -40 mV to a -80 mV, okay? And this membrane potential is one of key feature of living cells. So all types of living cells establish and maintain a given amount of membrane potential. So when we measure the voltage difference by using volt meter, we put a microelectrode inside the cells. So obviously, compared to the inside of cells to the outside of cells, it's -40, -80 mV difference observed. Why inside is more negative compared to the outside? So one of the main reasons is the inside of cells there are many negatively charged organic molecules like nucleic acid or proteins. Proteins or amino acid, right, negatively charged acid, nucleic acid, proteins. Those organic compounds are negatively charged, so inside is relatively negative, right? The other major reason why this type of membrane potential is established, that is because of this molecule sodium-potassium ATPase. So again, ATPase, adenylpyrophosphatase, ATP hydrolyzing molecules. So what does sodium-potassium means? So, Sodium-production ATPase is a pump. It's a pump. So what types of pump? Pump sodium and pump potassium. But interesting thing is the direction of this ion transport is opposite. It can pump out sodium from the inside to the outside. It can pump in potassium ions from outside to the inside, by consuming ATP, all right? The thing is, when you see this diagram, what is unique in this diagram? What is the interesting point of this diagram? This yellow ball structure is a sodium potassium ATPase, right? So three sodium ions out, and two potassium ions in. The importance of this ion pump. Three potassium positively charged ions can be pumped out and two potassiums come in. That means unequal ion distribution by consuming ATP because this type of transport is so-called active transport. So by consuming the precious ATP, free energy, actively transport potassium in and then throughout the expanse of ATP, and we pump out three sodium ions. And this type of transport processes is active transport due to the hydrolysis of free energy. We can actively transport these events, these molecules. So again, ATP hydrolysis drives the unequal distribution of irons, right? This is structural sodium potassium ATPase pump solved by a protein crystallography technology. So why don't we begin with this number one. Okay, this protein is a pump. Step number one, this protein is ATP bound. And inside of cells are three sodium ions bind and then ATP hydrolyse. ATP hydrolyse into ADP. Okay, and remaining phosphate is still attached to this pump protein. And there is a huge conformational changes, okay? The linings of this ion pump facing the outside of cells can be opened up and the three sodium ions can be released. And then from the external environment, two potential ions can bind to this ion pump. And this binding event of potassium ions to ion pump can trigger the release of phosphate. It's phosphate released, and two potassium ions over here still captured and ATP molecules bind to this ion pump and then trigger, and again induce conformational changes. And two potassium ions can be released inside of cells. That's how sodium potassium ions can consume ATP and can mediate the unequal transport event of sodium and potassium across the membrane. So once this membrane potential is established, in particular of neurons or the other types of excitable cells like pancreatic beta cells. And those cells can further utilize this membrane potential changes for cell cell communication, which is called action potential. Before I introduce action potential, I want to further emphasize the importance of sodium potassium ATPase in terms of bioenergetics. Okay, just look at this sentence. 50% of energy in the human brain is utilized by this single protein sodium pottasium-ATPase pump to maintain resting membrane potential. 50% which is a lot, right? So most of energy, ATP, you synthesize from the biochemical reactions we are going to study in a few weeks. Most of energy is consumed by this single enzyme, sodium-potassium ATPase pump, for what? To maintain resting membrane potential. So the neurons, so it's about this, the active transport event made by sodium potassium ATPase pump. Roughly -70 mV of resting membrane potential established along your axon, your neuron, your neuron, your axon. Okay, and then, upon the electrical stimuli, this potential can be reversed, a lot of sodium ions can be flushed in. Because of that, this mind's membrane potential is depolarized, can be positively positive 30 mV value can be made and then this change can be reversed. And this event is called action potential. And this action potential can be propagated from one point to the other point. That's how electrical signals can be relayed along your axons in your nervous system and this time is millisecond scale. So I'm not going to go further into this or the details about neurobiology or neurochemistry. What I want to emphasize is the establishment and restoration of this resting membrane potential requires a lot of energy and an equal ion distribution. And that biochemical event made possible by sodium potassium ATPase pump. So I'm going to introduce a very interesting aspect of sodium potassium ATPase. We are looking at a beautiful flower, so called the foxglove. Traditionally, in the field herbal medicine, the extract of this plant, the genus name is Digitalis. The Digitalis medicine can be used to treat congestive heart failure. So congestive heart failure means the cardio muscular contraction, the force is not right enough to support the pumping blood. Okay so, many people are really curious about the action mechanism of this Digitalis medicine. So what's going on? And later, biochemists and pharmacologists identified the action mechanism. This is how it works. So Digitalis extracts, one of the main components is this one, and inhibit, specific inhibitor of sodium potassium ATPase. These types of molecules from this foxglove extract can specifically block the dephosphorylation step of the enzyme. So overall, sodium-potassium unequal ion distribution cannot be made. So in that case, this Digitalis medicine block, this sodium-potassium ATPase pump. So intracellular sodium iron level is increased relatively, and then sodium calcium exchanging event activity also increased, and overall cytosolic calcium level is increased. You may remember the importance of calcium in terms of muscular contraction. In the previous session I introduced that the primary, the first event for skeletal muscular contraction and muscular contraction is the increase of cytoplasmic calcium level. When cytoplasmic calcium level is high enough functionally, the Digitalis medicine finally can improve the cardiac muscle contractility. Due to the toxicity and some severe side effect, this medicine Digitalis dependent medicine cannot be widely used now. But the thing is, sodium-potassium ATPase and related pharmacological action is one of active area of investigation. So take home message is very clear. Why ATP so special, reason number 3, ATP hydrolysis and molecular. In particular for today's session I emphasized ion, ion transport. So ATP hydrolysis and coupled protein conformational changes, structural changes can drive the transport of other molecules into or out of the cell. So one good example is sodium potassium ATPase, which is very essential for establishing and maintaining membrane potential. And other examples include calcium ATPase which is involved in restoring calcium inside the endoplasmic reticulum, the system. And the other one is vacuolar-type protein ATPase inside the cells. Some vacuoles, there are a lot of protons concentrated to digest other macro molecules. And the accumulation of these are protons made by these proton ATPase pump. And the last class in ATP-binding cassette transporter, ABC transporter. I'm going to spend like one more minute about this protein. So multidrug resistance MDR ABC transporter. This is structure, this is structure. So this is membrane and membrane spending region over here and some like a global domain and those domain is ATP-binding and hydrolysis. Hydrolysis. Okay, so what is multidrug resistance? Multidrug resistance is very, very interesting phenomena. In particular cancer cells, Or some pathogenic, Bacteria. Point is of multidrug resistance, and those cells somehow become resistant against drugs. Why is that? Because this multidrug resistant membrane protein ATP binding transporter just pump out those drugs. So that's one of the reasons why multidrug resistance is made possible by these membrane transporters. So the problem is this one. So many cancer cells showing this phenotype. So cancer patient they become resistant toward the anti-cancer drug they're taking. So very efficient anti-cancer therapeutics cannot be applied to this MDR cancer patient. So indeed one of MDR proteins, ABCB1 protein, is very specialized in terms of pump many types of anti-tumor drugs out of the cells. So these proteins ABCG2 is also called breast cancer resistance protein. Those proteins make cancer cells resist against cancer therapeutics, anti-cancer chemo therapeutics. So many groups of medical doctors and pharmacologists tried to develop how to manage this MDR ABC transporter in terms of improving the cancer therapeutics. So week number one is done. [COUGH] I believe I clearly introduced the concept of metabolism and the concept of free energy, and finally the characteristics of ATP and why the ATP is that special in terms of as the universal energy currency for the cell. So energy sources, like from food, during the catabolic reactions, a lot of ATP produced. We are going to study those by chemical details one by one during this course. And then that ATP can be used for biosynthesis of macro molecules and transport and muscular mechanical work. And throughout these reactions, ATP is being dephosphorylated and this ADP is constantly regenerated and throughout this ADPHB turnover cycle.
