Enzyme Catalysis

Question 1

What are the characteristics of an enzyme?

Answer 1

Most are proteins
Increase rate of reaction
Do not affect delta G
Do not affect reaction equilibrium

Question 2

What are the enzymes characteristics?

Answer 2

Activity-the higher the activity the better the enzyme
Unit=amount of substrate (in micromoles) converted to product per unit time
Specific activity=units/mg protein
Specificity
Regulation: optimum temperature, pH

Question 3

What is the pH optimum for some enzymes?

Answer 3

Pepsin(stomach) active at pH 2
Trypsin (intestine) active at pH 7
Alkaline phosphatage active at pH 9

Question 4

What are the different classes of enzymes?

Answer 4

Oxidoreductases-electron transfer, H+, NAD+
Transferases -Transfer o C, N or P containing group
Hydrolases-cleavage of bonds by addition of water
Lyases-cleavage of C-C, C-S and certain C-N bonds
Isomerases-intramolecular group transfer
Ligases-ligation of 2 substrates at the expense of ATP hydrolysis

Question 5

What are the different mechanisms of activity?

Answer 5

Catalysis by proximity
Acid base catalysis
Catalysis by strain
Covalent catalysis

Question 6

What are the characteristics of catalysis by proximity?

Answer 6

Active sit creates local high concentration to increase the chances of 2 substrates reacting
Active site creates proper orientation of substrates

Question 7

What are the characteristics of acid-base catalysis?

Answer 7

An ionizable functional group may provide or accept proton as par of catalysis

Question 8

What are the characteristics of catalysis by strain?

Answer 8

Substrate binds in a manner which strain the bond to be broken
Causes a conformation change

Question 9

What are the characteristics of covalent catalysis?

Answer 9

A covalent enzyme-substrate complex may be formed
The modified enzyme becomes a substrate for a subsequent reaction
Reaction of enzyme-substrate to product may be energetically more favorable

Question 10

What a re the characteristics of cofactors, coenzymes and prosthetic groups?

Answer 10

Small non protein molecules and metal ions
Derived from vitamins
Coenzymes are loosely bound and act as shuttles
Prosthetic groups are tightly bound and covalent
Cofactors are inorganic substances

Question 11

What are the characteristics of prosthetic groups?

Answer 11

Tightly and stably incorporated into protein
Metal ions are the most common type ("metalloenzymes")
Examples are:
Thiamine pyrophosphate (thiamine, B1)
Riboflavin (B2)
Pyridoxal phosphate (pyridoxine, B6)
Biotin
FAD (flavin adenine dinucleotide)
FMN (Flavin mononucleotide)
Lipoic acid

Question 12

What are the characteristics of cofactors?

Answer 12

Bind only transiently to substrate or enzyme
Are required for activity
Mg2+ required for enzymes involving ATP
Can be metalloenzymes or metal-activated enzymes

Question 13

What are the characteristics of metalloenzymes?

Answer 13

Metal is tightly bound as a prosthetic group
No need of excess metal ions in solution
Examples are catalase and superoxide dismutase

Question 14

What are the characteristics of metal activated enzymes cofactors?

Answer 14

Metal is loosely bound
Need 2-10 times excess metal ions in solution
Examples are DNase, RNNase, ATPase

Question 15

What are the characteristics of coenzymes?

Answer 15

Serve as shuttles or transfer agents
Act more like a substrate and product of an enzyme catalyzed reaction
Examples are FADH2, NADH, CoA

Question 16

What are isozymes?

Answer 16

Distinct enzymes with different sequences which catalyze the same reaction
They have regulatory differences, kinetic differences, express differentially in different tissues
Isozymes have the same equilibrium constant Keq