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Introduction to enzymes and catalysis

Video transcript

so today I want to talk to you about enzymes and how they're critically important pieces of cellular machinery but first let's review the idea that biochemical reactions happen in the body all the time almost every cellular process involves a biochemical reaction at one point or another you know the TCA cycle is actually just a series of different biochemical reactions in carbon metabolism DNA replication which needs to happen before a cell can go through mitosis is also just a series of reactions and this also applies to the expression of genes going from DNA to RNA to protein and we need enzymes because enzymes make all of these reactions go much faster and let's look at this idea a little more deeply and how a reaction will go on differently when it has an enzyme versus not having one at all so you may be familiar with the reaction where water and carbon dioxide can combine to form carbonic acid and this is a reversible reaction so it can go backwards and forwards now when people make soda or any carbonated beverage they'll start by pumping that soda can full of co2 and while some of that co2 will dissolve in the water and the can the soda may taking companies are able to get a lot more co2 in the water by using this reaction you know the abundant co2 will react with the water to form some carbonic acid in the can and when you go to open the can you'll hear a pop sound which is really just a bunch of co2 escaping but after that the soda will start to fizz really slowly and what's happening here is the carbonic acid that was made before is slowly dissociating back to carbon dioxide in water as co2 escapes and that extra co2 that's being made will come out of the soda solution and you'll see it as little bubbles floating around but what happens if you then take this person over here and he'll pick up a can of soda and take a drink that person might notice the soda will start fizzing a lot more once it hits his or her tongue and this is because humans have an enzyme in their blood in saliva called carbonic anhydrase and this makes the carbonic acid turn into carbon dioxide and water much more quickly so more co2 will come out of the can and it will fizz more and this is just one of the many examples of how enzymes make reactions go faster so how exactly do the enzymes make the reactions go faster though well they use a bunch of different catalytic strategies to push reactions along a little more quickly and I'm going to talk about a few of those strategies just to give you an idea of what enzymes are doing so first I'll mention acid base catalysis which happens when enzymes act like either acids or bases now remember that acids and bases are proton donors and acceptors and if you look at this type of reaction which if you remember from organic chemistry is a keto enol tautomerization reaction we have a proton moving from a carbon atom to an oxygen atom and since acids and bases are pretty good proton carriers they could both help with this reaction make it go a little more quickly by helping to move that proton around instead of this molecule doing it by itself our next catalytic strategy is covalent catalysis which happens when enzymes form a covalent bond with another molecule usually their target molecule remember that covalent bonds involves two molecules sharing electrons and looking at this reaction here we have a decarboxylation reaction going on which if you remember from organic chemistry is when a carboxy or co 2 group is being taken off a molecule and if you remember these reactions usually have a lot of electrons moving around so if we had a covalently bound enzyme that could hold on to some electrons be an electron carrier or what sound like people like to call an electron sink then that would definitely help this type of reaction move a little more quickly next we have electrostatic catalysis now if you remember DNA is a very negatively charged polymer because of all the negatively charged phosphate groups that we find in DNA so if an enzyme had a metal cation on it like magnesium we could use it to stabilize the negative charge found in DNA and make it a little easier to work with and DNA polymerase which is the enzyme that allows DNA replication to occur does exactly this and in order for it to help with DNA replication it needs to find a way to counteract all of the negative charge on DNA and magnesium ions totally come in handy there so the last catalytic strategy I want to mention in a little more general and it has to do with proximity and orientation effects remember that in order for two molecules to react with each other which is usually what enzymes help out with they need to physically collide at some point if we have molecule a and molecule B that will only react once they crash into each other and a lot of enzymes are able to bring two molecules close together so that these types of collisions happen more often making the two molecules react more quickly also remember that the orientation of the two colliding molecules in space is also really important if molecule a and molecule B collide but one of them is upside down or not in the correct position then the collision may not result in a successful reaction so enzymes also make sure that the two molecules will collide in the right orientation and all of this increases the frequency of collision in general but also helps to make sure that those collisions are successful and result in a reaction so what did we learn well first we learned that the role enzymes play is to make biochemical reactions happen more quickly and the next thing we talked about were four of the many different catalytic strategies that enzymes can use we talked about acid-base catalysis which helps with proton transfer we talked about covalent catalysis which helps with electron transfer we mentioned electrostatic catalysis which deals with stabilizing charge and finally proximity and orientation effects which increase the frequency of successful collisions between molecules that we want reacting together