20_Enzymes.pdf

Chapter 20: Enzymes & Vitamins

General Characteristics of Enzymes

Models of Enzyme Action

Enzyme Specificity

Enzymes: General Characteristics

•

Enzymes are proteins that act as a catalyst for biochemical reactions

•

The human body has 1000s of enzymes

•

Enzymes are the most effective catalysts known

•

Most enzymes are globular proteins

•

A few enzymes are now known to be ribonucleic acids (RNA)

•

Enzymes undergo all the reactions of proteins including denaturation

•

Terms:

–

Substrate

: substance upon which the enzyme “acts.”

–

Simple enzyme

: composed only of protein (amino acid chains)

–

Conjugated enzyme

: Has a nonprotein part in addition to a protein part.

• Apoenzyme: Protein part of a conjugated enzyme.

• A cofactor : Nonprotein part of a conjugated enzyme.

• A holoenzyme: Biochemically active conjugated enzyme

Nomenclature

1.Suffix

-ase

identifies it as an enzyme

–

E.g., urease, sucrase, and lipase are all enzyme designations

–

Exception: The suffix

-in

is still found in the names of some digestive

enzymes, E.g., trypsin, chymotrypsin, and pepsin

2.Type of reaction catalyzed by an enzyme is often

used as a prefix

–

E.g., Oxidase - catalyzes an oxidation reaction,

–

E.g., Hydrolase - catalyzes a hydrolysis reaction

3.Identity of substrate is often used in addition to

the type of reaction

Six Major Classes

•

Enzymes are grouped into six major classes

based on the types of reactions they catalyze

Class Reaction Catalyzed 1. Oxidoreductases Oxidation-reductions

2. Transferases Transfer of functional group from one molecule to another

3. Hydrolases Hydrolysis reactions, in which addition of a water molecule causes the bond to break

4. Lyases Reactions involving addition or removal of groups from double bonds _{that does not involve hydrolysis or oxidation}

5. Isomerase Rearrangement of functional groups, converting molecule to another _{molecule isomeric with it}

Models of Enzyme Action

•

Enzyme Active Site:

Relatively small part of an enzyme’s

structure that is actually involved in

catalysis:

– Place where substrate binds to enzyme

– Formed due to folding and bending of the protein.

– Usually a “crevice like” location in the

enzyme

Models of Enzyme Action

•

Enzyme-Substrate

Complex

: Intermediate

reaction species formed

when substrate binds with

the active site

–

Needed for the activity of

enzyme

•

Two models for substrate binding to the active site:

–

Lock and Key Model

–

Induced-Fit Model

•

Determine substrate binding:

–

H-bonding

–

Hydrophobic interactions

–

Electrostatic interactions

Enzyme Specificity

•

Absolute Specificity:

– An enzyme will catalyze a particular reaction for only one substrate (not common)

– E.g., Urease is an enzyme with absolute specificity

•

Stereochemical Specificity:

– An enzyme can distinguish between stereoisomers (chiral active site)

– L-Amino-acid oxidase - catalyzes reactions of L-amino acids but not of D-amino acids.

•

Group Specificity:

– Involves structurally similar compounds that have the same functional groups.

– E.g., Carboxypeptidase: Cleaves amino acids one at a time from the carboxyl end of the peptide chain

•

Linkage Specificity:

– Involves a particular type of bond irrespective of the structural features in the vicinity of the bond (most general)

Factors That Affect Enzyme Activity

•

Temperature

–

Higher temperature = higher

activity (there is a threshold)

–

Optimum temperature:

Temperature at which the rate

of enzyme catalyzed reaction is

maximum

–

Optimum temperature for

human enzymes is 37ºC (body

temperature)

–

Increased temperature (high

fever) leads to decreased

Factors That Affect Enzyme Activity

•

pH

–

Drastic changes in pH can

result in denaturation of

proteins

–

Optimum pH: pH at which

enzyme has maximum

activity

–

Most enzymes have optimal

activity in the pH range of 7.0

- 7.5

–

Exception: Digestive enzymes

–

Pepsin: Optimum pH = 2.0

Factors That Effect Enzyme Activity

•

Substrate Concentration

–

At a constant enzyme

concentration, the enzyme activity

increases with increased substrate

concentration.

–

Substrate saturation

: the

concentration at which it reaches

its maximum rate and all of the

active sites are full

Factors That Effect Enzyme Activity

•

Enzyme Concentration

:

–

Enzymes are not consumed

in the reactions they

catalyze

–

At a constant substrate

concentration, enzyme

activity increases with

increase in enzyme

concentration

Practice: Enzyme Activity

•

Pepsin is an important gastric enzyme in the

stomach. It is responsible hydrolyzing

protiens.

–

Pepsin is not functional outside of the stomach.

Why?

Allosteric Enzymes

•

Allosteric Enzymes

have

two kinds of binding

sites, substrate and

regulator

–

Negative regulators

inhibit enzyme activity

–

Positive regulators

Feedback Control

•

Feedback control

is a process in which the activation

or inhibition of the first reaction in a reaction

Vitamins: General Characteristics

• Must be obtained from dietary sources

• Human body can’t synthesize in enough amounts

• Enough vitamin can be obtained from balanced diet

• Essential for proper functioning of the body

• Needed in micro and milligram quantities

• Supplemental vitamins may be needed after illness

• Many enzymes contain vitamins as part of their structures - conjugated enzymes

• Two Classes

– Water Soluble and Fat Soluble

• Synthetic and natural vitamins are same

Water-Soluble Vitamins

•

Vitamin C

•

Humans, monkeys, apes and guinea pigs need Vitamin C

•

Co-substrate in the formation of structural protein collagen

•

Involved in metabolism of certain amino acids

•

100 mg/day saturates all body tissues - Excess vitamin is excreted

•

RDA (mg/day):

–

Great Britain: 30

Water-Soluble Vitamins

•

Vitamin B:

–

Thiamin (vitamin B

)

–

Riboflavin (vitamin B

)

–

Niacin (nicotinic acid, nicotinamide, vitamin B

)

–

Vitamin B

(pyridoxine, pyridoxal, pyridoxamine)

–

Folate (folic acid)

–

Vitamin B

(cobalamin)

–

Pantothenic acid (vitamin B

)

–

Biotin

•

Must be chemically modified before functional

•

Exhibit structural diversity

Fat-Soluble Vitamins:

Vitamin A (retinoids)

•

Vision:

In the eye- vitamin A combines with opsin protein

to form the visual pigment rhodopsin which further

converts light energy into nerve impulses that are sent to

the brain.

•

Regulating Cell Differentiation

- process in which immature

cells change to specialized cells with function.

Examples: Differentiation of bone marrow cells, white blood cells, and red

blood cells.

•

Maintenance of the healthy of epithelial tissues via

epithelial tissue differentiation.

• Lack of vitamin A causes such surfaces to become drier and harder than normal.

•

Reproduction and Growth:

In men, vitamin A participates

in sperm development. In women, normal fetal

Fat-Soluble Vitamins:

Vitamin D

•

Two forms active in the body: Vitamin D

2

and D

3

•

Sunshine Vitamin: Synthesized in the skin by UV

light from sun

•

It controls correct ratio of Ca and P for bone

mineralization (hardening)

Fat-Soluble Vitamins:

Vitamin E

•

Four forms of Vitamin Es:

α

-,

β

-,

γ

- and

δ

-Vitamin E

•

Alpha-tocopherol is the most active biological

active form of Vitamin E

•

Peanut oils, green and leafy vegetables and whole

grain products are the sources of vitamin E

Fat-Soluble Vitamins: Vitamin K

•

Two major forms; K

1

and K

2

•

K

1

found in dark green, leafy vegetables

•

K

2

is synthesized by bacteria that grow in

colon

•

Dietary need supply: ~1/2 synthesized by

bacteria and 1/2 obtained from diet