L3-Regulation of Enzyme Activity

Question 1

LO1: List three characteristics of regulatory enzymes

Answer 1

1. catalyze irreversible (non-equilibrium) reactions
2. catalyze the rate-limiting (slowest) step in a pathway
3. catalyze the committed step in a pathway (first step that is committed solely to the formation of a particular product

Question 2

LO2: List the 6 main types of enzyme regulation

Answer 2

1. changes in abundance of enzyme
2. alteration in availability of essential cofactors or pool size of substrate
3. use of isozymes
4. use of allosteric effectors
5. covalent modification
6. use of larger catalytic units

Question 3

LO2: How do changes in abundance of enzyme occur?

Answer 3

Induction/repression: change in transcription levels of enzymes
Degradation: change in rate to change level of enzymes

Question 4

LO2: How is the availability of cofactors/substrate altered?

Answer 4

• many enzymes require certain cofactors to be active
• Km of many enzymes will be in the range of concentration of their particular substrates in a particular cell/tissue, so a change in the concentration of the substrate itself will dramatically effect enzyme rate

Question 5

LO2: How do isozymes work?

Answer 5

definition: two or more different enzymes that catalyze the same reaction, but have different kinetic/regulatory properties and usually have distinct tissue/cellular specification

Question 6

LO2/LO4: How do allosteric effectors work?

Answer 6

• can have an activating or inhibiting effect
• binding occurs at site other than active site (allosteric means other site)
• activators increase Vmax or decrease Km or both
• inhibitors decrease Vmax or increase Km or both

Question 7

LO2/LO3/LO5: How does covalent modification work?

Answer 7

Can be reversible or irreversible depending on the type, includes phosphorylation and proteolysis

Question 8

LO2: Describe catalytic units and the three main types

Answer 8

• enzymes combine so that product of one reaction can be used as substrate for another reaction without first mixing with the surroundings (increases efficiency of subsequent reactions)
  1. macromolecular complexes (PDH complex)
  2. multifunctional proteins (CAD protein synthesizes four different reactions/fatty acid synthetase in lipogenesis catalyzes seven different reactions)
  3. metabolon: enzymes in a pathway are located within close proximity to each other in cell, but not organized in a multienzyme complex

Question 9

LO2/LO3/LO5: Name the classes of enzymes that participate in phosphorylation/dephosphorylation

Answer 9

Kinases catalyze transfer of phosphate from ATP to a hydroxyl group to form a phosphate ester linkage and create a phosphorylated enzyme

Phosphatases use water to hydrolyze the phsophate ester and return enzyme to its dephosphorylated form

Question 10

LO2/LO3/LO5: Name the classes of enzymes that participate in proteolysis

Answer 10

-proteases

Question 11

LO2/LO3/LO5: What are the consequences of phosphorylation/dephosphorylation?

Answer 11

• typically alter an enzyme’s Km and Vmax, and/or alter an enzyme’s response to allosteric modifications
• result in very rapid changes in pathways
• in catabolic pathways, phosphorylation tends to activate enzymes (glyocgen phosphorylase)
• in anabolic pathways, phosphorylation tends to inhibit enzymes (glycogen synthase)

Question 12

LO5: What are the consequences/characteristics of proteolysis? (5)

Answer 12

1. it is a type of irreversible covalent modification
2. the cleavage of peptide bonds only occurs at a limited number of sites
3. conformational restraints can be removed via proteolysis
4. a new function is gained due to the conformational change
5. cascade systems are often activated as a result of limited proteolysis (blood clotting, complement activation, proteolytic enzymes in the gut)