A Road Map for Cellular Respiration Cytosol Mitochondrion High-energy electrons carried by NADH High-energy electrons carried mainly by NADH Glycolysis Glucose 2 Pyruvic acid Krebs Cycle Electron Transport
Glycolysis: stage 1 The three steps of stage 1 begin with the phosphorylation of glucose by hexokinase Energy used, none extracted
Phosphoryl transfer reaction. Kinases transfer phosphate from ATP to an acceptor. Hexokinase has a more general specificity in that it can transfer phosphate to other sugars such as mannose. Δ G°’= -4.0 kcal mol-1
Glucose phosphorylation: step 1 Glucose is a relatively stable molecule and is not easily broken down. The phosphoylated sugar is less stable. ATP serves as both source of phosphate and energy needed to add phosphate group to the molecule.
Step 2: Isomerization glucose 6-phosphate fructose 6-phophate aldose to ketose isomerization reversible, G°´= 1.7 kJ/mole 6 carbon ring 5 carbon ring Enzyme: phosphoglucoisomerase
The conversion of an aldose to a ketose . Phosphoglucose Isomerase Δ G°’= .40 kcal mol-1
Formation of fructose-6-phosphate: step 2 by phosphoglucose isomerase The enzyme opens the ring, catalyzes the isomerization, and promotes the closure of the five member ring.
Phosphofructokinase The 2 nd investment of an ATP in glycolysis. Bis means two phosphate groups on two different carbon atoms. Di means two phosphate groups linked together on the same carbon atom. PFK is an important allosteric enzyme regulating the rate of glucose catabolism and plays a role in integrating metabolism.
Glycolysis: stage 2 Two 3-carbon fragments are produced from one 6-carbon sugar No energy used or extracted
Step 4: Cleavage to two triose phosphates Reverse aldol condensation ; converts a 6 carbon atom sugar to 2 molecules, each containing 3 carbon atoms. Enzyme: aldolase
Step 6: Formation of 1,3-Bisphosphoglycerate Done in two steps glyceraldehyde 3-phosphate 1,3 bisphosphoglycerate Enzyme: glyceraldehyde-3-phosphate dehydrogenase addition of phosphate, oxidation, production of NADH, formation of high energy compound
The fate of glyceraldehyde 3-phosphate Stage 3: The energy yielding phase. Glyceraldehyde 3-phosphate DH Δ G°’ = 1.5 kcal mol -1 1,3-BPG has a high phosphoryl-transfer potential. It is a mixed anhydride. An aldehyde is oxidized to carboxylic acid and inorganic phosphate is transferred to form acyl-phosphate. NAD + is reduced to NADH. Notice, under anaerobic conditions NAD + must be re-supplied.
7: Phosphoglycerate Kinase Substrate-level phosphorylation Δ G°’ = -4.5 kcal mol -1 ATP is produced from P i and ADP at the expense of carbon oxidation from the glyceraldehyde 3-phosphate DH reaction. Remember: 2 molecules of ATP are produced per glucose. At this point 2ATPs were invested and 2ATPs are produced.
Step 9: Removal of Water leads to formation of double bond (enol) little energy change in this reaction, ΔG = + 1.7 kJ/mole because the energy is locked into enolphosphate. Phosphate group attached by unstable bond, therefore high energy Enzyme: enolase
Generation of second very high energy compound by a dehydration reaction Enolase Δ G°’ = .4 kcal mol -1 Dehydration reaction the energy is locked into the high energy unfavorable enol configuration by phosphoric acid ester
An enol phosphate is formed: step 9 Dehydration elevates the transfer potential of the phosphoryl group, which traps the molecule in an unstable enol form Enol: molecule with hydroxyl group next to double bond
Step 10: Formation of Pyruvate & ATP Enzyme: pyruvate kinase phosphoenolpyruvate pyruvate second substrate level phosphorylation yielding ATP highly exergonic reaction, irreversible , ΔG = -31.4 kJ/mole.
Maintaining Redox Balance NAD + must be regenerated for glycolysis to proceed Glycolysis is similar in all cells, the fate of pyruvate is variable
Diverse fates of pyruvate To citric acid cycle
Under anaerobic conditions pyruvate is converted to lactate. Exercising muscle is an example. The NAD + that is consumed in the glyceraldehyde 3-phosphate reaction is produced in the lactate DH reaction. The redox balance is maintained. The activities of glyceraldehyde 3-phosphate DH and Lactate DH are linked metabolically. What happens to the lactate after a run?
In anaerobic yeast, pyruvate->ethanol Pyruvate is decarboxylated. Acetaldehyde is reduced.