Sometimes students get very confused when they found that the energetic balance in his Biochemistry book does not agree with the energetic balance studied in classroom. The student can get even more confused when he/she uses some consultation books and found that values also can differ from one book to another.
The intention of this post is to clear the apparent contradictions in energetic balance of the oxidation of glucose, in some of the books more used by biomedical students.
Aerobic Glycolysis describes the oxidation of one mol of Glucose (6 carbons) up to the formation of two moles of pyruvate (3 carbons each).
This conversion involves different reactions where ATP is produced or consumed:
Glucose to Glucose-6-P - 1 ATP
Fructose-6-P to Fructose 1,6- bisphosphate - 1 ATP
Since Fructose 1,6 bisphosphate becomes two trioses phosphate, the reactions after the aldolase reaction occurs twice (one for each triose that continues the glycolysis). In one reaction, NADH.H+ is produced; in other two reactions, a Substrate Level Phosphorylation (SLP) occur.
(2) Gliceraldehide 3 (P) to (2) 1,3 bisphosphoglycerate (2) NADH.H+
(2) 1,3 bisphosphoglycerate to (2) 3 phosphoglycerate (SLP) + 2 ATP
(2) Phosphoenol pyruvate to Pyruvate (SLP) + 2 ATP
Total energetic Balance from Glucose to (2) Pyruvate
· _ 2 ATP + 2 NADH.H+ + 4 ATP = 2 ATP + 2NADH.H+
An excellent animation of glycolysis can be found here.
The NADH.H+ will be oxidized in the Respiratory Chain and during the process, will release energy enough for the synthesis of additional ATP.
Maybe it is necessary to recall now that there are two different ways of synthesizing ATP:
a) Substrate Level Phosphorylation (SLP) in which a ATP is synthesized using energy from some reactions of metabolism, (like the two reactions indicated above)
b) Oxidative Phosphorylation: Synthesis of ATP using the energy released in the Electron Transport Chain, from the oxidation in the Respiratory Chain of reduced cofactors.
So, up to this moment it has been obtained in the aerobic glycolysis 2 ATP (through SLP) and 1 NADH.H+. This NADH.H+ will release energy for the synthesis of ATP in the Respiratory Chain…. but exactly how much ATP can be synthesized from the oxidation of this NADH.H+ ?
It is a question kind of complicated, since there are different factors affecting the answer:
1.- Which “energetic yield” criteria is used?
Traditionally Biochemistry textbooks have used the criteria that with the energy released by each mol of NADH.H+ that is oxidized in the Respiratory Chain, 3 moles of ATP can be produced. More accurate calculations indicates that in fact, the energy released when 1 mol of NADH.H+ is oxidized, is just enough for the synthesis of 2.5 moles of ATP and not 3 ATP as considered before. Those calculations indicate also that the equivalent of the oxidation of 1 “mol” of FADH2 is enough for the synthesis of 1.5 moles of ATP and not 2 moles of ATP as was considered before. Anyway, different textbooks continue using the equivalents of 3 and 2 ATP for each NADH.H+ and FADH2 oxidized.
“Lehninger’s principles of Biochemistry”, Devlin’s “Textbook of Biochemistry”,” and Marks’Essential of Medical Biochemistry” base the calculations in an energetic yield of 2.5 ATP per NADH.H+ that is oxidized and 1.5 ATP for each FACH2 oxidized in the Respiratory Chain.
Harvey and Champe, in the “Lippincott Illustrated Reviews” of Biochemistry, Dawn B. Marks in “Biochemistry, Board Review Series”, Kaplan Biochemistry Lecture Notes for USMLE and “First Aid for the USMLE Step I”, use the equivalence of 3 ATP for each NADH.H+ that is oxidized and 2 ATP per FADH2. (some of them have introduced the word “approximately”, so they say that these cofactors yield approximately 3 ATP and 2 ATP respectively)
To increase the confusion, some authors, like Wilcox ( “High Yield Biochemistry”, 2nd. Edition) use sometimes an equivalence and sometimes other in the same book.
(In my opinion, the continue use of a yield of 3 ATP and 2 ATP for NADH.H+ and FADH2, is based on tradition and also because these are more intuitive values and easier to understand when talking in terms of molecules and not in term of moles. It is obvious that it is quite difficult to understand that the oxidation of one molecule of NADH.H+ releases energy for the synthesis of 2.5 molecules of ATP, since expressions that are valid when talking about moles, become absurd when are applied to molecules.)
Besides this factor that affects all the calculations that involve the production of ATP from the oxidation of NADH.H+ or reduced flavoproteins, there is other factor that is specifically related to the oxidation of NADH.H+ produced in the cytosol; the kind of shuttle that is used for transporting the cytosolic NADH.H+ to the mitochondria, where this reduced cofactor will be oxidized in the Respiratory Chain.
2.- Which shuttle is used for transporting NADH.H+ produced in the cytosol to the mitochondria?
The internal mitochondria membrane is not permeable to NAD+ or NADH.H+ (the mitochondria has their own pool of these nucleotides).
There are two different mechanisms for transporting the reduction equivalents of cytosolic NADH.H+ through the internal membrane of the mitochondria:
a) the malate aspartate shuttle.
b) The glicerolphosphate shuttle.
a) Malate-Aspartate Shuttle:
The reduction equivalents of the cytosolic NADH.H+ are transferred to oxalacetate to form malate, in a reaction catalyzed by a cytosolic malate dehydrogenase:
Cytosol: Oxalacetate + NADH.H+ —-à Malate + NAD+
Malate can pass through the mitochondria membranes and enter the matrix. Once in the matrix the malate is dehydrogenated by a mitochondrial malate dehydrogenase:
Mitochondria: Malate + NAD+ ——à oxalacetate + NADH.H+
The oxalacetate is transaminated to aspartate to go out of the mitochondria and once in the cytosol, the aspartate is transaminated to oxalacetate beginning a new cycle.
Since this malate-aspartate shuttle regenerates NADH.H+ inside the mitochondria, the energy yielding of the cytoplasmatic NADH.H+ is the same as if it was generated directly in the mitochondria (2.5 or 3 ATP, depending on the equivalence followed)
b) The glycerol phosphate shuttle
With this shuttle, the reduction equivalents of the cytosolic NADH.H+ are transferred to dihydroxyacetone phosphate to form glycerol 3-phosphate, in a reaction catalyzed by a cytosolic glycerol 3-phosphate dehydrogenase, that oxidizes the cytosolic NADH.H+ :
Cytosol: dihydroxyacetone (P)+ NADH.H+ —-à glycerol 3 (P) + NAD+
The glycerol 3 (P) is dehydrogenated by a mitochondrial glycerol 3 (P) dehydrogenase located on the outer surface of the inner membrane of the mitochondria, that transfer the reduction equivalents to FAD in the inner membrane:
Inner membrane: glycerol 3 (P) + FAD –à dihydroxiacetone (P) + FADH2
Observe that the reduction equivalents have been transferred to FAD and not to NAD. It means that FADH2 is the cofactor that will be oxidized in the Respiratory Chain, and so, the use of this shuttle will yield less ATP: 1.5 or 2 ATP, depending on the equivalence that is followed.
So, the answer to the original question, “ATP yield in Aerobic Glycolysis: 5, 6, 7 or 8 ATP/glucose?”, can be:
a) 2 ATP from SLP + 3 ATP if it is considered that the reduction equivalents of the cytoplasmatic NADH.H+ are transported through the glycerol phosphate shuttle, and that for each FADH2 oxidized in the respiratory chain, the energy released can yield 1.5 ATP
b) 2 ATP from SLP + 4 ATP if it is considered that the reduction equivalents of the cytoplasmatic NADH.H+ are transported through the glycerol phosphate shuttle, and that for each FADH2 oxidized in the respiratory chain, the energy released can yield 2 ATP
c) 2 ATP from SLP + 5 ATP if it is considered that the reduction equivalents of the cytoplasmatic NADH.H+ are transported through the malate aspartate shuttle and that for each NADH.H+ oxidized in the respiratory chain, the energy released can yield 2 .5 ATP
d) 2 ATP from SLP + 6 ATP if it is considered that the reduction equivalents of the cytoplasmatic NADH.H+ are transported through the malate aspartate shuttle and that for each NADH.H+ oxidized in the respiratory chain, the energy released can yield 6 ATP
In summary, the answer to the original question…“ATP yield in Aerobic Glycolysis: 5, 6, 7 or 8 ATP/glucose?”, can be:
“ATP yield in Aerobic Glycolysis: 5, 6, 7 or 8 ATP/glucose!”.
So, the bottom line is that for answering a question involving energetic balance of a metabolic pathway, it is important to know:
- which equivalence is used (which one use your professor? Which one is used in the kind of exam you are taking? Most of the Biochemistry review books for USMLE use the equivalence of 3 ATP per NADH.H+ and 2 ATP per FADH2).
- In the case of NADH.H+ generated in the cytosol (NADH.H+ from glycolysis is not the only NADH.H+ generated in cytosol!), which shuttle is used to move the reduction equivalents from the cytosol to the mitochondria.