Exam 3 Long Questions

Question 1

Describe the activation cascade for digestive proteases and why this process is important.

Answer 1

First, digestive proteases and peptidases are synthesized as inactive precursor forms called proenzymes or zymogens. Enteropeptidase cleaves trypsinogen to form active trypsin; this starts an activatio cascade. As more proenzymes are converted into active enzymes, the active enzymes are available to digest proteins and activate other proteoenzymes. This is important because you don’t want activated proteases degrading all your proteins.

Question 2

What are the three principal ways metabolic processes are regulated? Give one detailed example of one of these from glycolysis or gluconeogenesis.

Answer 2

1. The amount of enzymes
2. The activity of enzymes: GPase is activated by phosphorylation of a specific serine residue when glucose levels are low.
3. The accessibility of substrates

Question 3

Describe the three functions of carbohydrates and give examples of each.

Answer 3

1. Fuel: monosaccharides like glucose, fructose, and galactose have entry points into ATP-generating metabolic pathways.
2. Structure: Cellulose with its beta-linkages yields a straight chain capable of interacting with other cellulose molecules to form strong fibrils.
3. Signaling/Recognition: Glycosylated proteins can serve as distinguishing marks within the cell and at the cell surface. ABO blood groups are defined by characteristic carbohydrate patterns.

Question 4

Describe an example of how carbohydrates are used in signaling or recognition.

Answer 4

Carbohydrate patterns on the ABO blood groups differentiate the different types of blood because of the components of the carbohydrate chain.

Question 5

Compare and Contrast the plant complex carbohydrates: amylose, amylopectin, and cellulose (organism, monomer units, polymer form, branching, degradation by amylase?)

Answer 5

a. Glycogen: animals, Monomer: glucose, Polymer form: α-1, 4-glycosidic bonds with α-1, 6-glycosidic bonds for every 10 units, Branching: YES, Degradation by amylase? YES
b. Amylose: plants, glucose, α-1, 4-glycosidic bonds, Branching: NO, Degradation by amylase? YES
c. Amylopectin: plants. Glucose. Polymer form: α-1, 4-glycosidic bonds with α-1, 6- glycosidic bonds for every 30 units, Branching: YES, Degradation by amylase? YES
d. Cellulose: plants, glucose, β-1, 4-glycosidic bonds, Branching: NO, Degradation by amylase? NO

Question 6

Describe the molecular basis of lactose intolerance.

Answer 6

There is a lack of the enzyme lactase that is necessary to break lactose into galactose and glucose. Gut bacteria metabolise lactose which generates methane and hydrogen.

Question 7

Describe the two fermentation pathways and how they contribute to sustaining glycolysis.

Answer 7

1. Ethanol Fermentation: Decarboxylation of pyruvate is catalyzed by pyruvate decarboxylase. Reduction of acetylaldehyde to ethanol by NADH is done in a reaction catalyzed by alcohol dehydrogenase. Contributes because acetylaldehyde accepts the electrons in this fermentation and regenerates NAD.
2. Lactic acid fermentation: Pyruvate accepts the electrons from NADH to form lactate in a reaction catalyzed by lactate dehydrogenase. Regeneration of NAD in the reduction of pyruvate to lactate.
   Contribution: Regeneration of NAD sustains glycolysis under anaerobic conditions

Question 8

Describe the molecular basis of how glycolysis helps pancreatic beta-cells sense blood glucose levels.

Answer 8

Glucose enters beta-cells by GLUT2, which only allows entry when glucose is plentiful. Glycolysis and cellular respiration activate to increase the ATP/ADP ratio which triggers the closing of a potassium channel. The altered cellular ionic environment then triggers the opening of a calcium channel so that the calcium flows into the cell. The influx of calcium causes insulin-containing vesicles to fuse to the membrane and release insulin into the blood

Question 9

Explain the specific control mechanism for the activity of the bifunctional regulatory enzyme PFK2/FBPase2 in response to blood glucose levels.

Answer 9

1. LOW Blood Glucose levels: Gluconeogenesis must be stimulated and glycolysis inhibited. The bifunctional enzyme is phosphorylated to inactivate PFK2 activity and activate FBPase activity.
2. HIGH Blood Glucose Levels: Glycolysis must be stimulated and gluconeogenesis must be inhibited. The bifunctional enzyme is dephosphorylated to inactivate FBPase2 and activate PFK2.

Question 10

What is aerobic glycolysis and how does it contribute to cancer tumor growth?

Answer 10

Aerobic glycolysis is the metabolism of glucose to lactate even in the presence of oxygen. Acidification protects the tumor from immune responses. The cancer cells grow more rapidly than the blood vessels that supply them, making the area hypoxic.

Question 11

Describe an example of how glycolysis regulation is more complex in liver cells vs. muscle cells. Why is this necessary?

Answer 11

Hexokinase regulation:
1) Muscle: First step of glycolysis is inhibited by its product G6P. If PFK is inhibited, its substrate (F6P) accumulates which results in accumulation of G6P. So PFK inhibition leads to hexokinase inhibition.
2) Liver: Has same control as muscle cells, but hexokinase is not the primary enzyme for glucose phosphorylation in the liver. Glucokinase phosphorylates glucose in the liver and has a lower affinity for glucose than hexokinase so it can only bind what glucose is in excess of hexokinase.
This is necessary because it ensures that hexokinase in muscle and brain cells get first dibs on glucose when it is limiting.

Question 12

Describe the steps that occur for the generation of free glucose during gluconeogenesis and how these serve as a control point.

Answer 12

In the liver, G6P is transported into the lumen of the ER. G6Pase is an integral membrane on the inner surface of the ER which catalyses the synthesis of glucose and Pi from G6P. using a Calcium binding protein.