GLYCOLYSIS Dear students: This presentation is intended to help you see the chemical reactions that make up the glycolysis pathway.

12 th graders: As you know, drawing structures lets you see what is happening. For example, glucose in glycolysis. LET’S DO IT! Draw the straight chain structure of glucose (click 1). Now, convert glucose into fructose by changing the top two carbons on the molecule (click 1). Note an –OH groups on C-6 where the first phosphate group attached to glucose (click 1). C-1 in fructose receives a second phosphate later in the pathway (click 1). The phosphates come from ATP and require kinase enzymes hexokinase and phosphofructokinase I [PFK-1]. Glucose-6-PO 4 is converted into fructose 6-PO 4 by an isomerase enzyme phosphoglucoisomerase [PGI]. See how structures show you what is happening. CH 2 OH HO-C-H H-C-OH CHO H-C-OH C=O CH 2 OH -P-O O OH -P-O O OH

CH 2 OH HO-C-H H-C-OH CHO H-C-OH C=O CH 2 OH -P-O O OH -P-O O OH The second phase of glycolysis is called the triose or payoff phase. In this phase the fructose 1,6-bisphosphate molecule (click 1) is cut in half (click 1). The top half, i.e., the first 3 carbons with the keto group become dihydroxyacetone phosphate (DHAP) (click 1). C4-C6, become glyceraldehyde-3-phosphate (click 1). Note that when the split occurs, the two bonding electron go to the top (click 2) and what was C-4 becomes a carbonium ion with a positive charge (click 1). The carbonium ion causes a rearrangement of electrons to form a carbonyl group (click 1). A proton is displaced in the final step (click 1). The glyceraldehyde-3-phosphate goes on to the next step (click 1). HO-C-H C=O CH 2 O -P-O O OH -P-O O OH CH 2 O H-C-OH + H CH 2 OPO 3 = HC=O Glyceraldehyde 3-PO 4 DHAP CH 2 OPO 3 = C=O CH 2 OH

In the triose or second phase the glyceraldehyde 3-PO 4 is converted into pyruvate. Glyceralaldehyde-3-PO 4 is in equilibrium with DHAP via the enzyme triose phosphate isomerase (TPI). This assures that all of the carbons of glucose will be converted into pyruvate, the final product of the pathway. The first step in the triose pathway is to oxidize C-1 of glyceraldehyde-3-PO 4. The glyceraldehyde-3-PO 4 then becomes 3-phosphoglyceric acid (3PGA) (click 2). The slanted arrows show C-1 before and after oxidation. There is concern, however. The cell must somehow devise a way to conserve the oxidation energy. Click 1 to see how the energy is conserved. Note that to preserve free energy the cell adds phosphate to C-1 simultaneously with the oxidation by NAD + (click 1). This creates a high energy phosphate bond (click 1) in the intermediate 1,3-bisphosphoglycerate (1,3-BPG) that is used to synthesize ATP from ADP (click 1). The enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes the first reaction (click 1) and phosphoglycerate kinase(PGK) catalyzes the second (click 1). Both of these reactions are freely reversible. Click 1 to go on. CHO H-C-OH CH 2 OPO 3 COO H-C-OH CH 2 OPO 3 C H-C-OH CH 2 OPO 3 ~OPO 3 O PO 4 NAD + NADH + H++ H+ ADP ATP Glyceraldehyde-3-PO 4 dehydrogenase Phosphoglycerate kinase

COO H-C-OH CH 2 OPO 3 3-PGA COO C=O CH 3 Pyruvate COO C~ CH 2 OPO3 PEP To end the triose phase, 3-phosphoglycerate is converted into pyruvate (click 1). By now you should be able to predict that the phosphate on C-3 of 3PGA will used to make ATP. This is not obvious at first because the phosphate ester in 3PGA is not high energy (click 1). Therefore, to make ATP this phosphate must be converted into one that is high energy. That high energy molecule is phosphoenolpyruvate (PEP) (click 1). Your chore now is figure how to make PEP from 3PGA. A clue comes when you see that the phosphate in PEP in on C-2. Therefore, you must first shift the phosphate on 3PGA to make 2PGA (click 1). This is done by a mutase enzyme, phosphoglycerate mutase (PGM). To make the phosphate in 2PGA high energy, a H 2 O must be removed from 2PGA to form an enol-phosphate bond (click 1). Enol-phosphate is a high energy phosphate, in fact one of the highest in terms of release of free energy (click 1). From PEP it is simply a matter to transfer the phosphate to ADP (click 1),which brings us to the end of the pathway with the formation of ATP and pyruvate. COO H-C-OPO 3 CH 2 OH 2-PGA Low energy High energy -H 2 O ADP ATP

What did you learn? Answer the following questions regarding glycolysis and energy. 1. What is the overall energy yield in ATP production when one molecule of glucose is converted to pyruvate? (click for answer). There is a net of 2 ATPs synthesized for each glucose molecule. Two ATPs are required to bring the glucose to the fructose 1,6-bisphosphate stage (-2). One ATP each is synthesized when glyceraldehyde-3-PO 4 goes to 3PGA and PEP to pyruvate (+2). Since the latter reaction occurs with each triose, the total is +4 for all the carbons of glucose (+4) = Based on free energy yield, predict the reactions that are irreversible or poorly reversible in the glycolysis pathway. (click for answer) There are 3. Glucose to glucose 6-PO 4 catalyzed by hexokinase; fructose 6-PO 4 to fructose 1,6 bisPO 4 catalyzed by phosphofructokinase-I, and PEP to pyruvate catalyzed by pyruvate kinase. All 3 are catalyzed by kinase enzymes and all three have very large negative delta G values (check your textbook for delta G values). 3. Why is the phosphoglycerate kinase reaction reversible? What about the other kinase reactions? (click for answer) The product, glycerate 1,3-bisPO 4 is high energy. Thus, it can yield ATP by transferring the phosphate group to ADP. The other kinase reactions all yield low energy phosphate esters as products and hence cannot be reversed. Reversing would mean forming a high energy ATP phosphoanhydride bond in ATP from a low energy phosphate ester compound and this is not energetically feasible.