Biochemistry Lecture 9

Glycolysis and Catabolism Glycolysis is a sequence of enzyme-catalyzed reaction by which glucose is converted into pyruvate Pyruvate can be further aerobically oxidized Pyruvate can be used as a precursor in biosynthesis

Central Importance of Glucose Glucose is an excellent fuel Yields good amount of energy upon oxidation(-2840 kJ/mole) Can be efficiently stored in the polymeric form Many organisms and tissues can meet their energy needs on glucose only Glucose is a versatile biochemical precursor Bacteria can use glucose to build the carbon skeletons of: All the amino acids Membrane lipids Nucleotides in DNA and RNA Cofactors needed for the metabolism

FIGURE 14-1 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.

FIGURE 14-2 The two phases of glycolysis FIGURE 14-2 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–).

FIGURE 14-2a The two phases of glycolysis 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–).

FIGURE 14-2b The two phases of glycolysis 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: Hexokinase

Step 2: Phosphohexose isomerase

Phosphohexose Isomerase Mechanism FIGURE 14-4 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: Phosphofructokinase

Step 4: Adolase

Adolase Mechanism FIGURE 14-5 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

FIGURE 14-2 The two phases of glycolysis FIGURE 14-2 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. Glyceraldehyde 3-phosphate dehydrogenase

FIGURE 14-7 The glyceraldehyde 3-phosphate dehydrogenase reaction.

Step 2. Phosphoglycerate Kinase

Step 3. Phosphoglycerate mutase

Step 4. Enolase

Step 5. Pyruvate Kinase

How does food enter this process? FIGURE 14-10 Entry of dietary glycogen, starch, disaccharides, and hexoses into the preparatory stage of glycolysis.

Anaerobic Exercise

“Anaerobic Exercise” in Yeast