TORRES, Alessandra Isobelle D.
1NUR5
HW-NUR8103-Lecture- Carbohydrates Metabolism I
Glycolysis
After digestion of carbohydrates, glucose enters the cell membrane and into the cytoplasm,
glycolysis takes place where glucose-6-phosphate (a 6C-molecule) is split into two 3C
molecules of gly-3-P, producing 2 pyruvate molecules. The two pyruvates are then converted to
two acetyl-CoA and enter the mitochondria where the citric acid occurs.
1. What reactions are involved in the so called, energy-investing reactions? What chemical
compound is needed to initiate the reaction?
The reactions involved in energy-investing reactions are isomerization, cleavage, and
phosphorylation. The chemical compounds needed to initiate the reactions mentioned above
are the following: for phosphorylation the enzyme, hexokinase (step 1), and
phosphofructokinase (step 3), for isomerization, the enzyme, phosphoglucose isomerase (step
2), and for cleavage, the enzyme aldolase (step 4) is used.
2. Cite the steps in the glycolysis pathway where these energy-investing reactions occur.
Energy investing reactions are reactions wherein adenosine triphosphate is used. These
reactions occur in steps 1 to 5. It is called an energy investing reaction because there is an
investment of two ATP molecules. The first step involves glucose being phosphorylated, a
process wherein a phosphate group has been added to the molecule derived from ATP, by
hexokinase. The second step is involved in the isomerization of Glucose 6-phosphate by the
enzyme, phosphoglucose isomerase. The third step involves the isomerization of Fructose
6-phosphate, by the enzyme phosphofructokinase to form fructose 1,6-bisphosphate. During
this step, another ATP molecule is used. Step 4 involves the cleavage of FBP to yield two
3-carbon molecules., glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate
(DHAP), by the enzyme aldolase. The last step concerns the conversion of GAP and DHAP by
the enzyme triosephosphate isomerase, into Glyceraldehyde-3-phosphate dehydrogenase
(GAPDH).
3. The conversion from glu-6-P to fru-6-P involves isomerization, why?
The open-chain form of glu-6-P has an aldehyde group at carbon 1, while the
open-chain form of fru-6-P has a ketone group at carbon 2, this makes them isomers. The
conversion of glu-6-P to fru-6-P involves isomerization as it converts an aldose into a ketose.
This conversion is what allows the glycolysis pathway to continue as Fru-6-P can enter the next
stage.
4. What stages in the glycolysis pathway are considered energy harvesting reactions? What are
the products of these reactions that will prove its energy yield?
Energy harvesting reactions are steps in the glycolysis pathway that results in the
formation of four ATP, two NADH, and two pyruvate molecules. This occurs in step 6 to step 10
of the glycolysis pathway. In step 6, the GAPDH is oxidized and NAD+ is reduced to NADH +
H+ from NAD. Oxidation is coupled to the phosphorylation of the C1 carbon, producing
1,3-bisphosphoglycerate. In step 7, 1,3 bisphosphoglycerate is converted to 3-phosphoglycerate
by phosphoglycerate kinase, which makes 1,3 bisphosphoglycerate lose a phosphate group.
This phosphate group is then transferred to a molecule of ADP, yielding 2 ATP, as there is two
1,3 bisphosphoglycerate. In step 8, the phosphate group of 3-phosphoglycerate is transferred to
the 2nd carbon to form 2-phosphoglycerate. In step 9, the conversion of 2-phosphoglycerate to
phosphoenolpyruvate is done through dehydration by the enzyme, enolase. In the final step,
phosphoenolpyruvate is converted into pyruvate through transferring a phosphate group by
pyruvate kinase. The phosphate group attached to carbon 2 of the phosphoenolpyruvate is
transferred to a molecule of ADP, thereby yielding ATP, as there are two molecules of
phosphoenolpyruvate 2 ATP molecules are yielded.
5. The conversion from gly3-P to 1,3-diP uses NAD+ and a dehydrogenase enzyme. What is the
role of NAD+? What reactions therefore are involved in these conversions?
The role of NAD+ in this reaction is an oxidizing coenzyme that pulls a hydrogen atom of
GAP, converting NAD to NADH. NAD+ oxidizes the aldehyde to a carboxylic acid by
glyceraldehyde 3-phosphate dehydrogenase. This dehydrogenase enzyme also joins the
carboxylic acid and orthophosphate, thereby forming 1,3-Bisphosphoglycerate.
6. The conversion from 1,3-di-P-glycerate to 3-phosphoglyceric acid uses ADP. Why?
The conversion from 1,3-di-P-glycerate to 3-phosphoglyceric acid uses ADP in order to
yield energy, ATP, as it has a high phosphoryl-transfer, which means that a phosphate group
will transfer to ADP forming energy ATP. It is hydrolyzed to a carboxylic acid and the energy
released is used to generate ATP from ADP.
7. In a reaction where the enzyme involved is phosphoglycerate mutase, what happens to the
substrate, 3-phosphoglycerate?
3-phosphoglycerate is catalyzed by phosphoglycerate mutase, which is an enzyme that
transfers a functional group from one position on a molecule to another. In this reaction, an
additional phosphate group is added to the 2′ position of the 3-phosphoglycerate.
Phosphoglycerate mutase removes the phosphate from the 3’ position leaving just the 2′
phosphate, thereby producing 2-phosphoglycerate.
8. The formation of pyruvic acid (a keto-acid) involves what reaction? Account for the total ATP
produced from glycolysis.
The formation of pyruvic acid involves substrate-level phosphorylation through the use of
the enzyme, pyruvate kinase. In this reaction, the phosphate group attached to the 2′ carbon of
the PEP is transferred to a molecule of ADP, yielding ATP. There are 2 ATP molecules
produced in glycolysis.
Citric Acid Cycle
9. What does amphibolic pathway mean? What three molecules produced during the citric acid
cycle are indirect or direct source of high-energy compounds?
An amphibolic pathway is a metabolic pathway that is able to participate in both anabolic
and catabolic pathways. In short, an amphibolic pathway requires energy to grow and build as
well as energy to break down. The citric acid cycle produces one GTP/ATP and reduced forms
of NADH and FADH2. NADH and FADH₂ are indirect sources of energy produced while GTP is
a direct source of energy.
10. Which steps of aerobic metabolism of pyruvate through the citric acid cycle are control
points?
Among the 8 steps, steps 3 and 4 are the control points. The third step involves the
enzyme, isocitrate dehydrogenase, which oxidizes isocitrate, and as a result releases a
molecule of carbon dioxide, leaving behind a five-carbon molecule—α-ketoglutarate. This step is
important as it regulates the speed of the citric acid cycle. In the fourth step the five-carbon
molecule, α-ketoglutarate, is oxidized, reducing NAD+ to NADH. This reduction then causes the
release of a molecule of carbon dioxide in the process.
11-17. Two moles of pyruvic acids produced per molecule of glucose from glycolysis in
the cytoplasm, enters mitochondria, successively.
11. In the initial reaction, considering the products in the conversion of pyruvic acid to
acetyl-CoA, aside from the condensation reaction of pyruvic acid with CoA-SH, what is the other
reaction involved?
The other reaction involved is an oxidative decarboxylation wherein the isomer or citrate
is oxidatively decarboxylated which results in α- ketoglutarate (five-carbon compound), which
then is oxidatively decarboxylated to yield succinate (four-carbon compound).
12. What reactions between acetyl-CoA and oxaloacetic acetic acid are involved in the
formation of citric acid?
Acetyl CoA (two-carbon molecule) condenses with oxaloacetate (4 carbon molecule) to
form citrate (6 carbon molecule).
13. In the 2nd and 3rd steps, what are the reactions involving the formation of isocitric acid?
In the 2nd step, citrate undergoes isomerization to become isocitrate. In the third step, it
undergoes oxidation to form alpha-ketoglutarate.
14. Compare the number of carbons between isocitric and a-ketoglutaric acid.
An isocitric acid has 6 carbons while a-ketoglutaric acid has 5 carbons.
15. What are the reactions involved in the formation of succinyl-CoA? Compare the structure of
a-ketoglutaric acid and succinyl CoA.
Succinyl-CoA is formed through the decarboxylation and oxidation of the ketone to a
carboxyl group, as well as a substitution of CoA at the carboxyl group. The structures of succinyl
CoA and -ketoglutaric acid differ in the number of carbons present as Succinyl-CoA has 4
carbons while α- ketoglutaric acid has 5 carbons.
16. In the 8th step, identify the change of functional group of the reactant and the product.
In the 8th step, malate is converted into oxaloacetate. Malate undergoes oxidation as its
alcohol group becomes a ketone group in oxaloacetate.
17. Account the sum of ATP produced from glycolysis and citric acid cycle.
The sum of ATP produced from glycolysis and the citric acid cycle is 36, as 34 comes
from glycolysis and 2 comes from the citric acid cycle.
Electron Transport Chain
18-20. The inner membrane has specialized protein channels Coenzymes where H+ and
electrons pass through and translocated, from the inner to the intermembrane space.
18. Show the reaction, what happens to NADH from the citric acid cycle as it moves to
coenzymes and delivers H+ and electrons.
Reaction: ​NAD+​ + ​H+​ 2e- → ​NADH
As NADH moves through the citric acid cycle, it loses electrons as it is reduced by
enzymes, isocitrate dehydrogenase, oxaloacetate, and α-ketoglutarate dehydrogenase, and
then forming NADH.
19. As H+ ions remain in the intermembrane space, electrons are continuously passed on
through a series of co-enzymes. Which acts as the final acceptor of electrons?
Oxygen is what acts as the final electron acceptor.
20. As H+ increases in the intermembrane space, the proton gradient also increases. What is
the effect of this increase in proton gradient to H+ ions?
An increase in the proton gradient would indicate an increase in the H+ ions and this
would result in large electrical membrane potential, as well an increase in ATP productions, as
the H+ ions in the intermembrane space would serve as the energy source for the production of
ATP.