Glycolysis

Question 1

Describe catabolism.

Answer 1

The process of macromolecules (eg. starch, proteins, nucleic acids etc.) being hydrolysed to smaller molecules and eventually to compounds such as C02, H20 and NH3.

Question 2

Describe anabolism.

Answer 2

The opposite of catabolism essentially, the synthesis of molecules such as CO2 into larger macromolecules.

Question 3

How is starch linked?

Answer 3

Glucose molecules within starch are linked by alpha (1,4) linkages and there are also alpha (1,6) linkages which are acted on by enzymes in order to release energy.

Question 4

How is glycogen linked?

Answer 4

Similar linkage to what is seen in starch however there are more branches (alpha 1,6 linkages), which are acted on by enzymes to release energy, as glycogen is found in animals and starch is found in plants and animals typically need more energy to survive than plants so more branches leads to faster break down of the glycogen molecule.

Question 5

How is cellulose linked?

Answer 5

Linked by beta (1,4) linkages. Every other b-glucose molecule is ‘flipped’ which allows hydrogen bonds to form between cellulose chains which provides rigidity to the cellulose molecule, which enables it to provide strength to the plant cell wall.

Question 6

Describe the process of starch breakdown.

Answer 6

Starch is broken down via a series of hydrolysis reactions.

1. Alpha-amylase cleaves alpha (1,4) linkages internally (endo-hydrolase).
2. Limit dextrinase cleaves alpha (1,6) branch linkages.
3. Beta - amylase releases maltose from reducing end (exo-hydrolase).
4. Maltase hydrolyses maltose to a-glucose.

Question 7

Why is the glycolysis pathway not just a single step reaction?

Answer 7

Energy is released in smaller steps during glycolysis in order to reduce energy waste and to provide greater flexibility and versatility to the reaction.

Question 8

What is the difference between the 2 stages of glycolysis?

Answer 8

The first stage of glycolysis requires energy to go ahead however the second stage of glycolysis actually creates a net gain of ATP.

Question 9

1. Describe step 1 of glycolysis.

Answer 9

Step 1: Glucose is converted to glucose-6-phosphate. The enzyme used to catalyse the reaction is the kinase enzyme hexokinase with the use of the cofactor Mg2+. One ATP is used to phosphorylate 1 molecule of glucose. This reaction is irreversible as it is a phosphorylation reaction.

Question 10

2. What is the aldose glucose-6-phosephate converted to next?

Answer 10

Step 2: Via a reversible isomerisation reaction glucose-6-phosephate is converted to fructose-6-phosphate. This takes the molecule from an aldose to a ketose. The reaction is catalysed by the enzyme phosphoglycerate isomerase (PGI).

Question 11

3. Outline the third step in the first stage of glycolysis.

Answer 11

Step 3: Fructose-6-phosphate is phosphorylated to Fructose 1,6-bisphosphate via the enzyme phosphofructose kinase (PFK). As this is a phosphorylation reaction it cannot be reversed. Again the cofactor Mg2+ is used along with 1 molecule per molecule of fructose-6-phosphate.

Question 12

4. What two molecules is fructose-1,6-bisphosphate converted into?

Answer 12

Step 4: Fructose-1,6-bisphosphate is converted to dihydroxyacetone phosphate (DHAP), a ketose, and glyceraldehyde-3-phosphate (G3P), an aldose. This cleaving process, which takes place between carbons 3 and 4, is catalysed by aldose and is reversible.

Question 13

5. What is the final step in stage 1 of glycolysis?

Answer 13

Step 5: We already have 1 molecule of G3P however DHAP needs to be converted also to G3P from a ketose to an aldose, and this process is done so with the help of the enzyme triose phosphate isomerase. As this is an isomerism reaction, it is reversible and results in 2 G3P molecules which will be taken into stage 2 of glycolysis.

Question 14

6. How does stage 2 of glycolysis begin?

Answer 14

Step 6: Both molecules of G3P are oxidised and then phosphorylated to 1,3- bisphosphoglycerate. This reaction is catalysed by glyceraldehyde-3-phosphate dehydrogenase (GAPDH). During this process H+ is removed during oxidation and transferred to NAD+ to form NADH+H+ and an inorganic phosphate is lost. This reaction Is reversible.

Question 15

Why is G3P converted to 1,3-bisphosphate?

Answer 15

This process occurs in order to make 1,3-bisphosphate which is a high energy molecule which will lead to the process of substrate level phosphorylation.

Question 16

7. Why is step 7 in the process a very important step?

Answer 16

Step 7: 1,3-phosphoglycerate is converted to 3-phosphoglycerate and the reaction is catalysed by phosphoglycerate kinase (PK). The reaction uses the cofactor Mg2+. This reaction is important because 1 ATP is formed for every reaction and as there are 2 x 1,3-bisphosphyglycerate molecules reacting therefore 2 ATP are formed and the reaction ‘breaks even’. This means that if anymore ATP is formed then the is a net gain.

Question 17

8. What happens in step 8 of glycolysis?

Answer 17

Step 8: 3-phosphoglycerate is converted to 2-phosphglycerate via the enzyme phosphoglycerate mutase and the cofactor Mg2+.

Question 18

9. Why is step 9 important?

Answer 18

Step 9: In step 9 2-phosphoglycerate Is converted to phosphoenolpyruvate which is catalysed by enolase and involves the loss of a molecule of H2O and again the use of the Mg2+ cofactor. Phosphoenolpyruvate is a high energy molecule which is important to know when leading into step 10.

Question 19

10. What occurs in the final step of glycolysis?

Answer 19

Step 10: Phosphoenolpyruvate is converted to pyruvate with a molecule of ATP being formed (during the process of substrate level phosphorylation) each time it does so. This reaction is catalysed by pyruvate kinase using the cofactor Mg2+.

Question 20

Why is glycolysis tightly regulated?

Answer 20

So that it meets the cells energy needs and meets the cells needs for metabolites for anabolic processes.

Question 21

How is glycolysis regulated in step 3?

Answer 21

In step 3 PFK is allosterically inhibited by ATP. ATP binds at allosteric site. This creates a lower affinity of catalytic sites for fructose-3-phosphate.