Chapter 14.1 and 14.2: Glycolysis and Feeder Pathways CHEM 7784 Biochemistry Professor Bensley

CHAPTER 14.1 and 14.2 Glycolysis Today’s Objectives: To learn and understand the Process of harnessing energy from glucose via glycolysis Various pathways by which carbohydrates other than glucose enter glycolysis

Central Importance of Glucose Glucose is an excellent fuel Glucose is a versatile biochemical precursor Four major pathways of glucose utilization FIGURE 14-1 Major pathways of glucose utilization. Although not the only possible fates for glucose, these four pathways are the most significant in terms of the amount of glucose that flows through them in most cells.

Glycolysis: The Big Picture Anaerobic process carried out by all cells but at different rates Converts hexose to two pyruvates Generates 2 ATP and 2 NADH For certain cells in the brain and eye, glycolysis is the only ATP generating pathway Glucose + 2 ADP + 2 NAD+ + 2Pi  2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2H20

Glycolysis: Importance Glycolysis is a sequence of ten enzyme-catalyzed reactions by which glucose is converted into pyruvate Two phases: First phase converts glucose to two G-3-P Second phase produces two pyruvate molecules Three possible fates for pyruvate

Glycolysis: The Preparatory Phase FIGURE 14-2a The two phases of glycolysis. For each molecule of glucose that passes through the preparatory phase (a), two molecules of glyceraldehyde 3-phosphate are formed; both pass through the payoff phase (b). Pyruvate is the end product of the second phase of glycolysis. For each glucose molecule, two ATP are consumed in the preparatory phase and four ATP are produced in the payoff phase, giving a net yield of two ATP per molecule of glucose converted to pyruvate. The numbered reaction steps are catalyzed by the enzymes listed on the right, and also correspond to the numbered headings in the text discussion. Keep in mind that each phosphoryl group, represented here as P, has two negative charges (—PO32–).

Glycolysis: The Payoff Phase FIGURE 14-2b The two phases of glycolysis. For each molecule of glucose that passes through the preparatory phase (a), two molecules of glyceraldehyde 3-phosphate are formed; both pass through the payoff phase (b). Pyruvate is the end product of the second phase of glycolysis. For each glucose molecule, two ATP are consumed in the preparatory phase and four ATP are produced in the payoff phase, giving a net yield of two ATP per molecule of glucose converted to pyruvate. The numbered reaction steps are catalyzed by the enzymes listed on the right, and also correspond to the numbered headings in the text discussion. Keep in mind that each phosphoryl group, represented here as P, has two negative charges (—PO32–).

STEP 1 - The Hexokinase Reaction The first step, phosphorylation of glucose, is catalyzed by hexokinase in eukaryotes, and by glucokinase in prokaryotes This process uses the energy of ATP

STEP 2 - Phosphohexose Isomerization An aldose can isomerize into ketose via an enediol intermediate Overall – Glucose-6-Phosphate is converted to Fructose-6-Phosphate

FIGURE 14-4 (part 1) The phosphohexose isomerase reaction FIGURE 14-4 (part 1) The phosphohexose isomerase reaction. The ring opening and closing reactions (steps 1 and 4) are catalyzed by an active-site His residue, by mechanisms omitted here for simplicity. The proton (pink) initially at C-2 is made more easily abstractable by electron withdrawal by the adjacent carbonyl and nearby hydroxyl group. After its transfer from C-2 to the active-site Glu residue (a weak acid), the proton is freely exchanged with the surrounding solution; that is, the proton abstracted from C-2 in step 2 is not necessarily the same one that is added to C-1 in step 3.

FIGURE 14-4 (part 2) The phosphohexose isomerase reaction FIGURE 14-4 (part 2) The phosphohexose isomerase reaction. The ring opening and closing reactions (steps 1 and 4) are catalyzed by an active-site His residue, by mechanisms omitted here for simplicity. The proton (pink) initially at C-2 is made more easily abstractable by electron withdrawal by the adjacent carbonyl and nearby hydroxyl group. After its transfer from C-2 to the active-site Glu residue (a weak acid), the proton is freely exchanged with the surrounding solution; that is, the proton abstracted from C-2 in step 2 is not necessarily the same one that is added to C-1 in step 3.

STEP 3 - The Second Priming Reaction; The First Commitment This is an irreversible step The product, fructose 1,6-bisphosphate is committed to become pyruvate and yield energy

STEP 4 - Aldolases Cleave 6-Carbon Sugars Step four is the cleavage of Fructose 1,6-Bisphosphate

FIGURE 14-5 (part 1) The class I aldolase reaction FIGURE 14-5 (part 1) The class I aldolase reaction. The reaction shown here is the reverse of an aldol condensation. Note that cleavage between C-3 and C-4 depends on the presence of the carbonyl group at C-2. A and B represent amino acid residues that serve as general acid (A) or base (B).

FIGURE 14-5 (part 2) The class I aldolase reaction FIGURE 14-5 (part 2) The class I aldolase reaction. The reaction shown here is the reverse of an aldol condensation. Note that cleavage between C-3 and C-4 depends on the presence of the carbonyl group at C-2. A and B represent amino acid residues that serve as general acid (A) or base (B).

FIGURE 14-5 (part 3) The class I aldolase reaction FIGURE 14-5 (part 3) The class I aldolase reaction. The reaction shown here is the reverse of an aldol condensation. Note that cleavage between C-3 and C-4 depends on the presence of the carbonyl group at C-2. A and B represent amino acid residues that serve as general acid (A) or base (B).

FIGURE 14-5 (part 4) The class I aldolase reaction FIGURE 14-5 (part 4) The class I aldolase reaction. The reaction shown here is the reverse of an aldol condensation. Note that cleavage between C-3 and C-4 depends on the presence of the carbonyl group at C-2. A and B represent amino acid residues that serve as general acid (A) or base (B).

FIGURE 14-5 (part 5) The class I aldolase reaction FIGURE 14-5 (part 5) The class I aldolase reaction. The reaction shown here is the reverse of an aldol condensation. Note that cleavage between C-3 and C-4 depends on the presence of the carbonyl group at C-2. A and B represent amino acid residues that serve as general acid (A) or base (B).

STEP 5 - Triose Phosphate Interconversion DAP is converted enzymatically to GAP

STEP 6 - Glyceraldehyde 3-Phosphate Dehydrogenase Reaction First step in the “Payoff Phase” of Glycolysis First energy-yielding step in glycolysis

FIGURE 14-7 (part 1) The glyceraldehyde 3-phosphate dehydrogenase reaction.

FIGURE 14-7 (part 2) The glyceraldehyde 3-phosphate dehydrogenase reaction.

FIGURE 14-7 (part 3) The glyceraldehyde 3-phosphate dehydrogenase reaction.

FIGURE 14-7 (part 4) The glyceraldehyde 3-phosphate dehydrogenase reaction.

FIGURE 14-7 (part 5) The glyceraldehyde 3-phosphate dehydrogenase reaction.

STEP 7 - First Substrate-Level Phosphorylation 1,3-bisphosphoglycerate is a high-energy compound that can donate the phosphate group to ADP to make ATP

STEP 8 - Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate This is a reversible isomerization reaction

Mechanism of the Phosphoglycerate Mutase Reaction FIGURE 14-8 (part 1) The phosphoglycerate mutase reaction.

FIGURE 14-8 (part 2) The phosphoglycerate mutase reaction.

STEP 9 - Dehydration of 2-Phosphoglycerate The goal here is to create a better phosphoryl donor Loss of phosphate from 2-phosphoglycerate would merely give a secondary alcohol with no further stabilization …

STEP 10 - Second Substrate-Level Phosphorylation … but loss of phosphate from phosphoenolpyruvate yields an enol that tautomerizes into ketone

Feeder Pathways for Glycolysis FIGURE 14-10 Entry of dietary glycogen, starch, disaccharides, and hexoses into the preparatory stage of glycolysis.