Glycolysis - Biochemistry

in this lesson we're going to talk about something called glycolysis so what is glycolysis well if we break down the word glyco plus lysis what does the word lysis mean lysis means to split apart and glyco is associated with the word sugar so glycolysis you're splitting apart a glucose molecule glucose is a six carbon molecule and in glycolysis glucose will be split apart into two molecules of pyruvate and each molecule of pyruvate represents a three carbon molecule and so basically you're taking glucose and breaking it apart in half so to speak and that's the basic idea behind glycolysis and in this process some of the energy that's released as glucose converts into two molecules of pyruvate is captured in the form of atp and nadh so glycolysis is an energy producing process now let's discuss the net reaction of glycolysis so glycolysis takes place in the cytosol of the cell and in the net reaction we're going to start with a molecule of glucose which will react with two nad plus molecules and two adp units or adenosine diphosphate with two inorganic phosphate ions and this is going to produce two molecules of pyruvate plus two molecules of nadh going from nadplus to nadh is that a reduction process or is that an oxidation process what would you say anytime you add a hydrogen to a molecule you're reducing the molecule so that is a reduction process now in addition to forming the two nadh molecules two hydrogen ions will form and two units of atp will be produced so we're converting adp to atp by the addition of a phosphate unit and then we'll also get two water molecules so that's the net reaction of glycolysis now glycolysis can be broken up into two phases and it occurs in 10 steps so in the first phase which represents steps one to five this is known as the investment phase and the second phase which occurs through steps six to ten is known as the payoff phase now when you hear the word investment what comes to your mind what do you think about when i hear the word investment i think of the stock market maybe you want to invest a thousand dollars in stock so you can sell it for a higher price maybe two thousand dollars and so when you think of the word investment it carries the idea that you have to buy first in order that you can sell for more later in the case of glycolysis you need to put in energy so that you can get more out later and in the first phase it's endothermic so to speak it consumes energy and in the second phase it's going to produce energy but more energy than what you put in in the first phase you need to put in two molecules of atp to get the process going but in the second phase you're going to get four molecules of atp in addition you're also going to get two molecules of nadh so that gives us a net gain of two molecules of atp and two molecules of nadh so now let's go over the 10 steps of glycolysis so here we have step one of glycolysis and on the left we're going to begin with a molecule of glucose that doesn't look like a u so let's fix that now what's happening in this reaction what is the difference between the structure on the left and the structure on the right notice that a hydroxyl group is replaced with a phosphate group now what do you think is the name of the product of the first step of glycolysis what is the name of this molecule well let's find out this is carbon 1 2 3 4 5 6 so we have a phosphate group attached to carbon 6 and the substrate is the glucose molecule so this is going to be called glucose 6 phosphate now what is this process called the process of adding a phosphate group to a molecule this process is known as phosphorylation we're phosphorylating glucose to create g6p glucose 6-phosphate now where does the phosphate come from the phosphate comes from a molecule of atp which is converted to a molecule of adp atp is known as adenosine triphosphate and adp is adenosine diphosphate so atp has three phosphate units attached to it and adp has two phosphate units attached to it so going from atp to adp it gives up a phosphate group and there is an enzyme that helps us to transfer that phosphate group from the atp molecule to the glucose acceptor molecule giving us glucose 6-phosphate now what is the name of that enzyme that enzyme is called hexokinase so a kinase enzyme catalyzes the transfer of a phosphate group from atp to another acceptor molecule and the word hexo is due to the fact that the acceptor molecule glucose contains six carbons hexo mean six now there's something else that's needed in order for this process to work and it's a cofactor it's the magnesium two plus ion so you need this ion in order for this reaction to work now is this a reversible reaction or an irreversible reaction can the reaction only go to the right or can it go in both directions now under intracellular conditions the standard free energy change for this reaction at least according to my textbook some numbers can change over time it's negative 16. 7 kilojoules per mole and i'm running out of space here so if you see me writing very small it's because there's not much space left but that's the standard free energy change for this reaction and because it's relatively large for intracellular conditions this reaction is irreversible so if you see a single arrow that means it can only go in one direction it's irreversible if you see a double arrow that means that it's reversible and the standard free energy change will be lower if you ignore the negative sign so if you have a very large negative delta g value it's going to be irreversible however if it's very small it will be reversible now one thing i do want to mention before moving on to step two is i think i wrote delta g naught is equal to negative 16. 7 kilojoules per mole for the first step of glycolysis this should be delta g prime not now if you compare it to delta g which is negative 34 kilojoules per mole the reason why it's different is due to the ph values delta g prime knot is based on a ph of 7 because most cells they exist in environment with a ph of 7 where the h plus concentration is one times ten to the minus seven and so this represents the biochemical standard free energy change now the one that chemists and people who deal with physics they tend to use this for this one the ph is zero and the h plus concentration is one molar or one mole per liter and so from a biochemical perspective this is not very useful because most cells they don't exist at a ph of 0.

they exist at a ph of 7. so the delta g values that i'm going to write in the future will be the biochemical delta g standard values now let's move on to step two of glycolysis so on the left we have g6p glucose 6 phosphate and what is the name of the molecule on the right side so take a minute pause the video see if you can come up with the name of the molecule so notice the location of the phosphate group it's still on carbon 6. now we don't have a glucose base unit but looking at this what type of sugar do you think this is glucose has a six membered ring but fructose has a five membered ring so the name of this particular molecule is called fructose 6 phosphate because we still have the phosphate group on carbon 6.

now the delta g value for this particular reaction the biochemical delta g value it's positive 1. 7 kilojoules per mole so based on that value would you say that this particular reaction is reversible or irreversible now if we look at the magnitude of this number it's about 10 times less than the value that was in step one which was 16. 7 so because this is so much more smaller we're going to say that this reaction is reversible now this reaction is facilitated with the magnesium two plus cofactor so we need that as well now looking at the reactant and the product for this step what is the relationship between the two what is the relationship between glucose 6-phosphate and fructose 6-phosphate well the phosphates group they haven't changed so they're still the same the only difference is going from glucose to fructose and glucose and fructose they're basically isomers of each other so with that in mind what type of enzyme do we need if we want to create an isomer if we want to catalyze a rearrangement reaction what type of enzyme will help us to do that so for rearrangement reactions you need an isomerase i think i said that wrong isomerase enzyme specifically you need a phospho hexose isomerase enzyme now why the word phosphohexose well this word is associated with the reactant if you look at the reactant it's a hexose it has six carbons and it has a phosphate group so it doesn't make sense that this is a phospho hexose isomerase and it also works for the right side as well because it can go this way the product is also a hexose fructose is a six carbon sugar and it has a phosphate group attached to it now let's move on to step three of glycolysis on the left side we have fructose 6 phosphate and what is the product on the right side what is the name of that molecule so this is carbon 1 2 3 4 5 6.

do you notice any differences between the reactant and the product so notice that a hydroxyl group was replaced with a phosphate group so in a product we have two phosphate groups we have one on carbon one and on carbon six so therefore this is going to be called fructose 1 6 biphosphate so i'm going to write bp for biphosphate now if you recall from step one what is the name of the process where we're adding a phosphate group to a molecule what is that process called so that process is known as phosphorylation this process step 3 is known as the phosphorylation of fructose 6-phosphate into fructose 1-6 biphosphate now where does the phosphate come from so based on step one it's going to come from an atp molecule and so when atp loses a phosphate group it transforms into adp now like step one we're gonna need a magnesium two plus ion to facilitate this reaction now what about the enzyme for this reaction what type of enzyme do we need what type of enzyme catalyzes the transfer of a phosphate group onto a fructose substrate based on step one we know that a kinase enzyme will catalyze the transfer of a phosphate group to a molecule now looking at the acceptor molecule it's fructose and it has a phosphate group on it so this particular enzyme is appropriately called phospho fructo kinase in short it's also called the pfk1 enzyme so that's the abbreviation for it and it should be a one here too it's phosphofructokinase one now let's talk about the reversibility of this reaction the standard free energy change for this reaction the biochemical version is negative 14. 2 kilojoules per mole so based on this value would you say that this reaction is reversible or irreversible now this value is pretty close to the value in step one which was negative 16. 7 so this particular reaction is not reversible under intracellular conditions it proceeds in the forward direction now let's move on to step 4 of glycolysis the cleavage of fructose 1 6 by phosphate so notice that we get two molecules a ketone and an aldehyde so what do you think are the names for these molecules a three carbon aldehyde with hydroxyl groups is known as glyceraldehyde and if you count the carbons the aldehyde has the highest priority so that's carbon one you'll find that the phosphate group is located on carbon 3.

so this is called glyceraldehyde 3-phosphate and this one is called dihydroxyacetone phosphate a three carbon ketone is known as acetone now is this reaction reversible the delta g value for this reaction is positive 23. 8 kilojoules per mole now this is a high number and it's positive so that indicates that it favors the reactant on the left side now keep in mind this is the biochemical delta g value and not the the other one that's based on chemistry and physics it turns out that this particular reaction reversible now we're going to talk more about that later but the enzyme that catalyzes this reaction is known as the atylase enzyme which is used to catalyze a reversible aldol condensation reaction which is what we have in step four now let's talk about the reversibility of the reactions in glycolysis now for those of you who just want to know which steps are irreversible for those of you who prefer memorization steps 1 3 and 10 are irreversible the rest are reversible so if you know that you don't have to go any further now for those of you who want to understand it let's take the chemist view of delta g so this is delta g naught notice that when you have a very high negative value the reaction is irreversible it's product favored and if you look at the other values they're very small they could be positive or negative and those are the reversible reactions when delta g is close to zero now let's say if you're given the biochemical delta g value and you want to use that to determine if it's reversible or not notice that all the ones with a positive value are reversible this four that have a negative value and three of these are irreversible so this is the only exception you have to watch out for but if you're given the biochemical delta g value then if you get a positive delta g for glycolysis it's going to be reversible now let's move on to step 5 of glycolysis so this is the last step of the investment phase in this step we're going to convert dihydroxy acetone phosphate which i'm going to call dap into glyceraldehyde 3-phosphate now this is a reversible reaction and looking at the reactant and the product what's the chemical relationship between them notice that they both have three carbons five hydrogen atoms six oxygen atoms and a phosphorus atom so because they have the same number of atoms but a different structure these two species are known as isomers so this is a rearrangement reaction and what type of enzyme catalyzes a rearrangement reaction if you recall it's an isomerase enzyme now let's focus on the substrate molecule that we have here so we have a three carbon ketos sugar or just keto sugar with a phosphate group attached to it so the enzyme that we need to catalyze this reaction is going to be called triose because we have a three carbon sugar triose phosphate isomerase now it's important to understand that as we get into step six of glycolysis the beginning of the payoff phase two molecules of g3p enter that phase so if you need to calculate the number of atp molecules and nadh molecules in that phase that's produced you need to double everything so let me give you an overview so in step four we produced two different molecules dihydroxyacetone phosphate and g3p now in step five this molecule dihydroxy acetone phosphate gets converted to g3p so if we start from our original glucose molecule at the end of step 5 we have two g3p molecules so going from step 6 to step 10 we need to double everything since we have two of these molecules keep that in mind now let's move on to step six of glycolysis the beginning of the payoff phase so on the left we have g3p glyceraldehyde 3-phosphate and what do you think we have on the right side so we still have a 3-carbon molecule but it's not going to be called glyceraldehyde because we don't have an aldehyde functional group so this is an aldehyde function group as you can see we have a phosphate now now instead of the hydrogen we do have an oxygen and so this is an aldehyde and this is a carboxylate group so instead of glyceraldehyde we now have the name glycerate now we also have two phosphate groups attached to this molecule we have a phosphate on carbon 1 and on carbon 3.