Introduction to Metabolic Processes

Question 1

Define catabolic pathways and give their two functions.

Answer 1

Catabolic pathways are metabolic pathways designed to disassemble complex molecules into simpler products. Two functions:
1. They make available the raw materials from which other molecules can be synthesized.
2. They provide the chemical energy required for the many activities of a cell.

Question 2

Define anabolic pathways.

Answer 2

Anabolic pathways lead to the synthesis of more complex compounds from simpler starting materials. They require energy to work and use chemical energy released by the exergonic catabolic pathways to do so.

Question 3

What does it mean to say catabolic pathways are convergent?

Answer 3

It essentially means that even though substances in the pathway begin as macromolecules having a very different structure, they are converted by the catabolic pathway to the same low-molecular-weight metabolites.

Question 4

Where does glycolysis occur in the cell, and what is its final product?

Answer 4

Glycolysis occurs in the soluble phase of the cytosol where it forms pyruvate.

Question 5

Where does the TCA cycle occur in the cell, and what is its final product?

Answer 5

The TCA cycle occurs within the mitochondria of eukaryotic cells and the cytosol of prokaryotes and leads to the final oxidation of the carbon atoms to carbon dioxide.

Question 6

What does the process of fermentation accomplish in cellular metabolism?

Answer 6

One of the products of oxidation of glyceraldehyde 3-phosphate is NADH. The formation of NADH occurs at the expense of NAD+, which is in short supply in cells. Without it, the reaction cannot continue. So, the cell regenerates NAD+ through fermentation, which transfers electrons from NADH to pyruvate.

Question 7

Define allosteric modulation.

Answer 7

Allosteric modulation is a mechanism by which the activity of an enzyme is either inhibited or stimulated by a compound that binds to a site, called the allosteric site. This site is distinct from the active site of an enzyme.

Question 8

Define feedback inhibition.

Answer 8

Feedback inhibition is a type of allosteric modulation that will inactivate the beginning enzyme of a reaction if it receives a signal that enough product has been made on the ending side.

Question 9

Describe the process of photosynthesis.

Answer 9

Photosynthesis is a process that converts carbon dioxide and water into oxygen and carbohydrates. Essentially, it is an oxidation reaction of water since electrons are stripped away from a low energy state to a high energy state.

Question 10

Describe the process of respiration.

Answer 10

Respiration is a process that converts oxygen and carbohydrates into carbon dioxide and water. It is an energetically favorable reaction.

Question 11

What is the change in G of the breakdown of glucose?

Answer 11

The breakdown of glucose into carbon dioxide and water is -686 kcal/mol.

Question 12

Describe the function and structure of NAD+ and NADH.

Answer 12

Cells use NADH as an electron source/storage molecule for reduction/oxidation reactions. NAD+ is composed of two ribose groups, one adenine group, and one nicotinamide group that acts as an electron acceptor.

Question 13

Which hormones favor the synthesis of new materials from food (favor the anabolic pathway)?

Answer 13

Insulin
Sex steroids
Growth hormone and thyroxine (favors protein synthesis)

Question 14

Which hormones favor the breakdown of food into smaller molecules (favor the catabolic pathway)?

Answer 14

Glucagon
Glucocorticoids
Epinephrine
Growth hormone and thyroxine (favors carbohydrate breakdown)

Question 15

What are the three broad phases of cell metabolism?

Answer 15

1. The breakdown of large macromolecules to simple subunits which occurs outside the cells.
2. The breakdown of simple sugars to acetyl-CoA (a small energy harvest)
3. The final oxidation of acetyl-CoA to water, carbon dioxide in mitochondria (a large energy harvest)

Question 16

What are the three uses of monomers for a cell?

Answer 16

1. To build new macromolecules of the same class.
2. To convert into other compounds.
3. To degrade them to extract a measure of their free energy.

Question 17

What is the purpose of the first three steps of glycolysis?

Answer 17

To generate a molecule that can easily be split into two parts.

Question 18

What is the purpose of feedback inhibition?

Answer 18

Feedback inhibition regulates glycolysis and is necessary for determining which foods should be broken down in the catabolic pathway vs what should be used for the synthesis of materials in the anabolic pathway.

Question 19

How does allosteric inhibition work?

Answer 19

In allosteric inhibition, an allosteric inhibitor (which is a product of the pathway itself) binds to the allosteric site of an enzyme, distorting its figure and inhibiting its ability to function. This decreases the enzyme’s affinity for substrates.

Question 20

How does allosteric activation work?

Answer 20

If the active site of an enzyme is naturally distorted and less than efficient at its job, an allosteric activator can bind at the allosteric site (which is different from the active site) and improve the function of the enzyme by improving the fit of the substrate into the active site.

Question 21

What are the three enzymes of glycolysis that exhibit allosteric enzyme regulation?

Answer 21

Pyruvate kinase, glyceraldehyde 3-P dehydrogenase, and phosphofructokinase.

Question 22

Describe allosteric enzyme regulation as it relates to phosphofructokinase.

Answer 22

Phosphofructokinase has both positive and negative allosteric regulators. This enzyme catalyzes fructose 6-P + ATP into fructose 1,6-bisphosphate. ATP and citrate (citric acid) act as feedback inhibitors, but ADP acts as a positive allosteric activator.

Question 23

How are glycolysis and glucose synthesis (gluconeogenesis) regulated relative to each other?

Answer 23

Gluconeogenesis is the “opposite” of glycolysis in that it synthesizes glycogen rather than breaks down glucose. The glycolytic pathway involves three thermodynamically irreversible reactions (with large negative changes in G). This means different enzymes are used in these key reactions, allowing independent regulation of the pathways. High levels of ATP feedback inhibit glycolysis; likewise, high levels of AMP inhibit gluconeogenesis. Thus, production vs consumption needs match.