Biochem Lecture 12

Question 1

Why is it important to be able to control enzyme reactions?

Answer 1

Every process has a way of being controlled. This allows the ability to start and stop processes and to accelerate or slow them down.
Control helps the cell to be more efficient by only doing things when they are needed.
Control is necessary for homeostasis. This is achieved through maintain a constant cell environment despite changing external conditions.

Question 2

What are multi-enzyme pathways?

Answer 2

Most metabolic pathways in the body require multiple steps. This means that the product of one process is the substrate for another process.

Question 3

What is the rate limiting step in a pathway?

Answer 3

It is always the slowest step in the process.

Question 4

How are complex and multi-enzyme processes controlled?

Answer 4

Multi-step processes are usually controlled by making changes to the rate limiting step only. This affects the entire pathway. Rate limiting enzymes catalyze an irreversible step.

Question 5

What are the four basic categories of enzyme regulation?

Answer 5

1. Compartmentalization
2. Substrate availability
3. Enzyme quantity
4. Enzyme activity

Question 6

What are the two ways that compartmentalization can control an enzyme’s activity?

Answer 6

Separation in space and in time.

Organelles create ideal environments for a reaction to happen, so the reaction won’t happen elsewhere.

Question 7

In relation to substrate concentration, when will changes have significant influences on the rate, and when will they have minimal impact on the rate?

Answer 7

When substrate concentration is near the Km value, any change in [S] will have a significant effect on rate.
When [S] is near Vmax, changes cannot have a significant impact on rate.

Question 8

What is the absolute quantity of an enzyme?

Answer 8

It results from the net balance between the rate of synthesis and degradation of the enzyme itself.

Question 9

What are inducers in regard to enzyme concentration?

Answer 9

Inducers promote the synthesis of enzymes.

Question 10

Is enzyme quantity a fast or slow way to regulate the rate of an enzymatic process?

Answer 10

It is a slow way because it takes time to build new enzymes and to degrade them.

Question 11

What are the passive means of enzyme regulation?

Answer 11

Compartmentalization and substrate availability.

Question 12

What are the active means of enzyme regulation?

Answer 12

Enzyme quantity and enzyme activity.

Question 13

What is product inhibition?

Answer 13

The product of the enzymatic reaction inhibits the function of the enzyme itself. Products often are similar to their precursors and can sometimes bind to the active site of the enzyme and prevent it from doing its job.

Question 14

What is allosteric regulation?

Answer 14

A different effector or modulator molecule binds to a different site on the enzyme, causing a conformational change that affects the enzyme’s function.

Question 15

What are the 5 characteristics shared by allosteric enzymes?

Answer 15

1. Effector binds to a regulatory site and not to the active site.
2. Effector binds reversibly
3. No need for structural similarity between substrate and effector
4. Allosteric enzymes have quaternary structure
5. Sigmoidal curve of enzymatic activity

Question 16

What does oligomeric mean?

Answer 16

Two or more subunits.

Question 17

What is the equivalent Km value for allosteric enzymes?

Answer 17

It is the K0.5 value because allosteric enzymes don’t follow michaelis-menten kinetics which have a hyperbolic curve.

Question 18

How are allosteric effectors classified?

Answer 18

Homotropic=same chemical nature as true substrate
Heterotropic=different chemical

Activators=have positive effect and increase enzyme activity
Inhibitors=neg effect and decrease activity

Question 19

What is feedback inhibition?

Answer 19

Downstream product of a multi-enzyme pathway inhibits an upstream enzyme.

Question 20

What is feedforward activation?

Answer 20

Upstream substrate of a multi-enzyme pathway activates a downstream enzyme.

Question 21

What is the difference between feedback inhibition and product inhibition?

Answer 21

Feedback control happens through a regulatory site on the enzyme and product inhibition has structural similarities to the original substrate and binds to the active site to inhibit the enzyme activity.

Question 22

Many enzymes have a T and an R conformation. One is active and the other is not. How do activators and inhibitors interact with these conformations, and how do these interactions influence the reaction rate chart?

Answer 22

Activators bind to the R configuration and stabilize it. This is a left shift on the graph.
Inhibitors stabilize the T conformation and cause a right shift on the graph.

Question 23

What is cooperativity?

Answer 23

The binding of one substrate to a subunit has an effect on neighboring subunits so they are more likely to bind the substrate. This is called positive cooperativity.

Question 24

What are the most common types of enzyme regulation?

Answer 24

Allosteric and covalent regulation.

Question 25

What is covalent regulation?

Answer 25

Addition or removal of specific chemical groups via covalent bonding that have an immediate effect on the enzyme.
Examples of reversible modifications are:
phosphorylation
adenylation
uridylylation
ribosylation
methylation

Question 26

What are the two protein enzymes involved in phosphorylation?

Answer 26

Protein kinase: transfers a phosphorus group from ATP to the enzyme
Protein phosphatase: catalyzes hydrolysis of the phosphate group off the enzyme

Question 27

What amino acids are phosphorylated?

Answer 27

The phosphorus group is added to the hydroxyl group of serine threonine and tyrosine.

Question 28

What is an example of irreversible covalent modification of an enzyme?

Answer 28

Limited proteolysis. Many proteins are built as an inactive precursor called a proenzyme or zymogen. In order to become active, part of the protein must be cleaved. This causes an irreversible conformational change in the protein.