Glycolysis
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.
2Pyruvate + 2ATP +2NADH +2H+ +2H2O The three steps Glucose + 2Pi +2ADP +2NAD 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–). 2Pyruvate + 2ATP +2NADH +2H+ +2H2O
FIGURE 14-3 Three possible catabolic fates of the pyruvate formed in glycolysis. Pyruvate also serves as a precursor in many anabolic reactions, not shown here.
Glucose to Glucose 6 phosphate
Hexokinase
G6P to F6P Mg++ Phosphohexose isomerase DG’O=1.7 kJ/mol
FIGURE 14-4 The phosphohexose isomerase reaction 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.
F6P to F1,6, BP DG’o= -14.2 KJ/mol
F1,6BP to DHAP & G3P DG’O = 23.8kJ/mol
DHAP & GAP
FIGURE 14-6 Fate of the glucose carbons in the formation of glyceraldehyde 3-phosphate. (a) The origin of the carbons in the two three-carbon products of the aldolase and triose phosphate isomerase reactions. The end product of the two reactions is glyceraldehyde 3-phosphate (two molecules). (b) Each carbon of glyceraldehyde 3-phosphate is derived from either of two specific carbons of glucose. Note that the numbering of the carbon atoms of glyceraldehyde 3-phosphate differs from that of the glucose from which it is derived. In glyceraldehyde 3-phosphate, the most complex functional group (the carbonyl) is specified as C-1. This numbering change is important for interpreting experiments with glucose in which a single carbon is labeled with a radioisotope. (See Problems 6 and 9 at the end of this chapter.)
GAP to 1,3-BPG DG’O = 6.3kJ/mol
1,3-BPG to 3-PG DG’O = -18.5kJ/mol
3’PG to 2’PG Rearrangement shifts PO4 position from C3 to C2.
2PG to PEP Enol PO4
PEP to Pyruvate
Thermodynamics
Uses of Pyruvate Regeneration of NAD
Pyruvate to ethanol
Pyruvate to lactate
Recycling NAD
BOX 14-1 FIGURE 1 The anaerobic metabolism of glucose in tumor cells yields far less ATP (2 per glucose) than the complete oxidation to CO2 that takes place in healthy cells under aerobic conditions (~30 ATP per glucose), so a tumor cell must consume much more glucose to produce the same amount of ATP. Glucose transporters and most of the glycolytic enzymes are overproduced in tumors. Compounds that inhibit hexokinase, glucose 6-phosphate dehydrogenase, or transketolase block ATP production by glycolysis, thus depriving the cancer cell of energy and killing it.
Xxxxxxxx
Galactose
Glycogen to glucose (starvation)
xxxxx