2.2 Mechanisms of Enzyme Activity [HY]

Question 1

How do enzymes affect transition states?

Answer 1

• act to provide a favorable microenvironment in terms of charge or pH, stabilize
  the transition state, or bring reactive groups nearer to one another in the active site.

Question 2

Enzyme–substrate complex

Answer 2

• in the active site
• the key catalytic activity of the enzyme
• interaction between a substrate and the active site of an enzyme also accounts for the selectivity and some regulatory mechanisms of enzymes

Question 3

Substrate

Answer 3

• molecule upon which an enzyme acts

Question 4

What stabilizes and contributes to the efficiency of the enzymes?

Answer 4

Hydrogen bonding, ionic interactions, and
transient covalent bonds within the active site

Question 5

Lock and Key Theory

Answer 5

• suggests that the enzyme’s active site (lock) is already in the appropriate conformation for the substrate (key) to bind.
• No alteration of the tertiary or quaternary structure is necessary upon binding of the substrate

Question 6

Induced Fit Model

Answer 6

• more scientifically accepted theory
• the induced fit model starts with a substrate and an enzyme active site that don’t seem to fit together
• the molecules find that the induced form, or transition state, is more comfortable for both of them
• the shape of the active site becomes truly complementary
  only after the substrate begins binding to the enzyme
• a substrate of the wrong type will not cause the appropriate conformational shift in the
  enzyme.

Question 7

Cofactors

Answer 7

• nonprotein molecules that many enzymes require
• tend to be small in size so
  they can bind to the active site of the enzyme and participate in the catalysis of the reaction, usually by carrying charge through ionization, protonation, or deprotonation.
• usually kept at low concentrations in cells, so they can be recruited only when needed
• attached in a variety of ways, ranging from weak noncovalent interactions to strong covalent ones
• generally inorganic molecules or metal ions, and are often ingested as dietary minerals

Question 8

Apoenzymes

Answer 8

• Enzymes without their cofactors
• Apo means off or away

Question 9

Holoenzymes

Answer 9

• Enzymes with cofactors

Question 10

Prosthetic groups

Answer 10

Tightly bound cofactors or coenzymes that are necessary for enzyme function

Question 11

Coenzymes

Answer 11

• small organic groups, the vast majority of which are vitamins or derivatives of vitamins such as NAD+, FAD, and coenzyme A

Question 12

B vitamins (not required)

Answer 12

B1: thiamine
B2: riboflavin
B3: niacin
B5: pantothenic acid
B6: pyridoxal phosphate
B7: biotin
B9: folic acid
B12: cyanocobalamin

Question 13

How are fat-soluble vitamins regulated?

Answer 13

By partition coefficients, which quantify the ability of a molecule to dissolve in a polar vs. nonpolar environment