this chapter introduces glycolysis and its role in cellular energy production. glycolysis is basically the first step in breaking down glucose for use in energy production. in aerobic glycolysis, one molecule of glucose is broken down into two molecules of pyruvate, in the process forming 2 molecules of NADH and 2 molecules of ATP. in anaerobic glycolysis, one molecules of glucose produces lactate and 2 molecules of ATP. the reactions for aerobic glycolysis are as follows:
1. glucose is phophorylated to glucose-6-phosphate via hexokinase.
2. glucose 6-phophate is isomerized to fructose 6-phosphate via phosphoglucose isomerase.
3. fructose 6-phosphate is phosphorylated to fructose 1,6 bisphosphate via phosphofructokinase-1.
4. fructose 1,6 bisphosphate is cleaved into two triose phosphates via an aldolase.
5. the triose phophates isomerize into glyceraldehyde 3-phosphate (G3P).
6. G3P is oxidized by G3P dehydrogenase, which reduces NAD+ to NADH2 and transfers a high energy acyl phosphate bond onto G3P, forming 1,3 bisphophoglycerate.
7. 1,3 bisphosphoglycerate phosphorylates ADP via phosphoglycerate kinase and becomes 3 phosphoglycerate.
8. 3-phosphoglycerate is isomerized to 2-phosphoglycerate via phosphoglyceromutase.
9. 2-phosphoglycerate loses an H2O and becomes phosphoenolpyruvate (PEP) via enolase.
10. PEP phosphorylates another ADP via pyruvate kinase and becomes pyruvate.
reactions 1 through 4 are known as the preparative phase, in which glucose is basically phosphorylated twice and cleaved into two 3 carbon molecules. reactions 5-10 are known as the ATP generating phase, because the phosphate groups from the 3 carbon molecules are then transferred onto ADP. after the first phosphate group is transferred in reaction 7, reactions 8 and 9 basically transform the remaining phosphate group from a low energy to a high energy one so that it can also be transferred to ADP.
in reaction 6, NAD+ is used as an electron acceptor during the oxidation of G3P. thus, the NADH2 must be constantly reoxidized to NAD+ in order to maintain a source for transferring electrons. in aerobic glycolysis, this problem is solved by shuttling NADH2's electrons across the mitochondrial membrane (shuttling is needed because the membrane is impermeable to NAD), where they can be used to power oxidative phosphorylation. the most common shuttle is the G3P shuttle: NADH2 reduces DHAP to G3P via G3P dehydrogenase. G3P then diffuses across the mitochondrial membrane and transfers the electrons to FAD, forming FADH2, which can then donate its electrons to coenzyme Q in the electron transport chain. the other shuttle mechanism is the malate aspartate shuttle, in which NADH2 reduces oxaloacetate to malate, which is then shuttled across the mitochondrial membrane via a translocase, and then donates its electrons to mitochondrial NAD+, reforming oxaloacetate. oxaloacetate is then transaminated to aspartate, which can then be shuttled back out the mitochondria via another translocase, and once it is back, transaminated back into oxaloacetate and ready to accept electrons from NADH2 again.
the net ATP production from the G3P shuttle is 3 moles of ATP per mole of glucose, whereas the malate aspartate shuttle produces 5 moles of ATP. combining this with the ATP produced in the ATP-generating phase of glycolysis (2 ATP per glucose), and taking into account the pyruvate, which can be further oxidized in the mitochondria to produce an additional 12.5 ATP (25 ATP per glucose): this means in aerobic glycolysis, total ATP production for each molecule of glucose is either 3+2+25=30 when the G3P is in use or 5+2+25=32 when the malate aspartate shuttle is in use.
when oxygen is not readily available or mitochondria are not present (as in red blood cells, where oxidative metabolism might interfere with hemoglobin's interaction with oxygen, or in the eye, where mitochondria would deflect incoming light), or if cellular demands for energy exceeds that which is produced via aerobic processes, then anaerobic glycolysis is used. in order to regenerate the NAD+ to serve as the electron acceptor in reaction 6, NADH2 reduces pyruvate into lactic acid, which dissociates into lactate and proton (which has the potential to decease body pH). pyruvate therefore cannot be further oxidized and therefore the energy production in anaerobic glycolysis is quite lower, requiring tissues to work 15 times as fast to produce the same energy output. whereas in aerobic glycolysis the total energy production can be up to 32 moles of ATP per mole of glucose, in anaerobic glycolysis, the total energy production is only 2 moles of ATP (from the ATP-generating phase of glycolysis)
questions
1. describe the process of glycolysis and what it produces.
2. what are the two phases of glycolysis and what happens in each?
3. what is the first step in glycolysis and what does it produce?
4. what are the next steps in the preparative phase of glycolysis?
5. describe the oxidation of G3P.
6. what is substrate level phosphorylation?
7. where is the first phosphorylation of ADP?
8. how is the low energy phosphate bond in 3 phosphoglycerate transformed into a high energy phosphate bond that can be transferred to ADP?
9. what is the final reaction of glycolysis?
10. what is the total free energy change from glycolysis?
11. why does NADH need to be reoxidized and what are the two pathways in which this is accomplished?
12. what are the two fates for pyruvate?
13. why are reducing equivalent shuttles required?
14. describe the G3P shuttle.
15. describe the malate-aspartate shuttle.
16. describe anaerobic glycolysis.
17. what is the ATP production of the G3P vs. the malate-aspartate shuttle?
18. describe the difference of ATP production possible in aerobic vs. anaerobic glycolysis.
19. how is anaerobic glycolysis related to body pH?
20. when do cells or tissues use anaerobic glycolysis vs. aerobic?
answers
1. glycolysis is the splitting of glucose into pyruvate and production of 4 ATP molecules per molecule of glucose.
2. the preparative phase, in which glucose is phosphorylated twice by ATP and cleaved into two triose phosphates, and the ATP-generating phase, in which the triose phosphates produce 2 ATP each.
3. phosphorylation of glucose yields glucose-6-phosphate via hexokinases.
4. isomerization of glucose 6-phosphate via phosphoglucose isomerase to fructose 6-phosphate, phosphorylation to fructose 1,6 bisphosphate via phosphofructokinase-1 (PFK-1), and cleavage into two triose molecules (glyceraldehyde 3-phosphate) via aldolase.
5. G3P is oxidized by glyceraldehyde-3-phosphate dehydrogenase, which has a cysteine residue near the active site which accepts a high energy acyl phosphate and transfers it to G3P. in this process, NADH and a high energy phosphate bond are formed.
6. the formation of a high energy phosphate bond without oxygen via direct transfer of a phosphate bond from a high energy intermediate.
7. 1,3 bisphosphoglycerate transfers its phosphate group to ADP via phosphoglycerate kinase, forming 3 phosphoglycerate.
8. the low energy phosphate bond in 3-phosphoglycerate is first shifted to the middle carbon via phosphoglyceromutase, and then transformed into a higher energy phosphate bond via enolase, which removes a water molecule and turns the compound into phosphoenolasepyruvate.
9. transfer of the high energy phosphate bond phospho enol pyruvate to ADP, turning the compound into pyruvate.
10. -22kcal
11. NADH needs to be reoxidized in order to continually accept electrons during the oxidation of G3P via G3P dehydrogenase. when oxygen is available, reducing equivalents are shuttled across the mitochondrial membrane to the electron chain. when oxygen is not available, NADH transfers its electrons to pyruvate, which is then reduced to lactate.
12. pyruvate can be reduced to lactate in anaerobic glycolysis, or can be converted into acetyl coenzyme A and further oxidized in aerobic glycolysis.
13. because the mitochondrial membrane is impermeable to NADH.
14. the G3P shuttle is the main shuttle mechanism in which cytosolic NAD+ is regenerated by G3P dehydrogenase, which transfers the electrons from NADH2 to DHAP, forming G3P. G3P then diffuses across the mitochondrial membrane and donates its electrons to FAD, forming FADH2 and reforming DHAP, which can then diffuse back and be reused in the reaction.
15. the malate-aspartate shuttle is the other way that NAD+ is regenerated: NADH transfers its electrons to oxaloacetate, reducing it to malate, which is then transported across the mitochondrial membrane via a translocase. in the membrane, the malate reduces NAD to NADH2, which can then be used in the electron transport chain. the oxaloacetate that is formed is transaminated to aspartate, which is transported back across the mitochondrial membrane via another translocase, and transaminated back into oxaloacetate in the cytosol.
16. anaerobic glycolysis occurs when the the oxidative capacity of a cell is limited. in this reaction, the electrons from NADH2 are donated to pyruvate, creating lactic acid, catalyzed by the enzyme lactate dehydrogenase.
17. the G3P shuttle produces 1.5 moles of ATP per NADH2, whereas the malate-asparate shuttle produces 2.5 moles of ATP per NADH2.
18. in anaerobic glycolysis, pyruvate is reduced to lactate and thus cannot produce any more ATP besides the two moles (per glucose) that is produced during glycolysis. in aerobic glycolysis, in addition to these two ATP's, the pyruvate is oxidized and used in the mitochondria to produce 25 ATP (per glucose), and the NADH produced can add another 3 (if using the G3P pathway) or 5 (if using the malate aspartate pathway) ATP molecules.
19. lactic acid is produced from the reduction of pyruvate in anaerobic glycolysis via lactate dehydrogenase. at the normal intracellular pH of 7.35, lactic acid dissociates into lactate and H+, which is then transferred out of the cell and eventually into the blood, where it can influence body pH.
20. tissues that have a low ATP demand and low O2 availability. also, blood cells have undergo anaerobic glycolysis because aerobic metabolism might interfere with the loading and unloading of O2 from hemoglobin. also, in other tissues with mitochondria and O2 availability, anaerobic glycolysis is used when the cell's energetic demands are not met by aerobic processes.
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