chem 45 biochemistry: stoker chapter 21 enzymes & vitamins

Chapter 21

Enzymes and

Vitamins

Chapter 21

Table of Contents

Copyright © Cengage Learning. All rights reserved 2

21.1 General Characteristics of Enzymes

21.2 Enzyme Structure

21.3 Nomenclature and Classification of Enzymes

21.4 Models of Enzyme Action

21.5 Enzyme Specificity

21.6 Factors That Affect Enzyme Activity

21.7. Extremozymes

21.8 Enzyme Inhibition

21.9 Regulation of Enzyme Activity

21.10 Prescription Drugs That Inhibit Enzyme Activity

21.11 Medical Uses of Enzymes

21.12 General Characteristics of Vitamins

21.13 Water-Soluble Vitamins: Vitamin C

21.14 Water-Soluble Vitamins: The B Vitamins

21.15 Fat-Soluble Vitamins

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General Characteristics of Enzymes

Section 21.1


• Enzymes are usually proteins that act as

biological catalysts.

• Each cell in the human body contains

thousands of different enzymes.

• Enzymes cause cellular reactions to occur

millions of times faster than corresponding

uncatalyzed reactions

• An enzyme speeds a reaction by lowering

the activation energy, changing the reaction

pathway that provides a lower energy route

for conversion of substrate to product.

• As catalysts enzymes are not consumed in

the reactions

• A few enzymes are now known to be

ribonucleic acids (RNA)

Section 21.2

Enzyme Structure


Simple and Conjugated Enzymes

• Most enzymes are globular proteins; some are

simple proteins, others are conjugated proteins

• Simple enzyme: composed only of protein

(amino acid chains)

It is the 3o structure of the simple

enzymes that makes it biologically active

• Conjugated enzyme: has a non-protein part in

addition to a protein part.

1. apoenzyme protein part; inactive in itself

2. cofactor /coenzyme nonprotein organic

(coenzyme /co-substrate) or inorganic

(cofactor) moiety; the activator; loosely

bound to protein

• apoenzyme + cofactor = holoenzyme

(biologically active conjugated enzyme)

Section 21.3

Nomenclature and Classification of Enzymes


• Most commonly named with reference to their function

– type of reaction catalyzed

– identity of the substrate

• A substrate is the reactant in an enzyme-catalyzed

reaction:

– the substrate is the substance upon which the

enzyme “acts.”

– e. g., In the fermentation process, sugar is converted

to alcohol, therefore in this reaction sugar is the

substrate

Section 21.3



Three Important Aspects of the Naming Process

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

– e.g. glucose oxidase, pyruvate carboxylase, and succinate

dehydrogenase

Section 21.3



Practice Exercise

• Predict the function of the following enzymes.

a. Maltase

b. Lactate dehydrogenase

c. Fructose oxidase

d. Maleate isomerase

Section 21.3



Practice Exercise

• Predict the function of the following enzymes.

a. Maltase

b. Lactate dehydrogenase

c. Fructose oxidase

d. Maleate isomerase

Answers:

a. Hydrolysis of maltose;

b. Removal of hydrogen from lactate ion;

c. Oxidation of fructose;

d. Rearrangement (isomerization) of maleate ion

Section 21.3



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 Functional group transfer reactions

3. Hydrolases Hydrolysis reactions

4. Lyases Reactions involving addition of a group to a double bond

or removal of groups to form double bonds

5. Isomerase Isomerization reactions

6. Ligases Reactions involving bond formation coupled with ATP

hydrolysis

Section 21.3



Section 21.3



Oxidoreductase

• An oxidoreductase enzyme catalyzes an oxidation–reduction

reaction:

– oxidation and reduction reactions are always linked to one

another

– an oxidoreductase requires a coenzyme that is either oxidized

or reduced as the substrate in the reaction.

– e.g., lactate dehydrogenase is an oxidoreductase and NAD+ is

the coenzyme in this reaction.

Section 21.3



Transferase

• A transferase is an

enzyme that catalyzes

the transfer of a

functional group from

one molecule to another

• Two major subtypes:

1. kinases - catalyze

transfer of a

phosphate group

from adenosine

triphosphate (ATP)

to a substrate

Section 21.3



Transferase

• A transferase is an

enzyme that catalyzes

the transfer of a

functional group from

one molecule to another

• Two major subtypes:

2. transaminases -

catalyze transfer of

an amino group to a

substrate

Section 21.3



Hydrolase

• a hydrolase is an enzyme that catalyzes a hydrolysis reaction

• the reaction involves addition of a water molecule to a bond to cause

bond breakage

• hydrolysis reactions are central to the process of digestion:

– carbohydrases hydrolyze glycosidic bonds in oligo- and

polysaccharides

– proteases effect the breaking of peptide linkages in proteins

– lipases effect the breaking of ester linkages in triacylglycerols

Section 21.3



Lyase

• A lyase is an enzyme that catalyzes the addition or the removal of a

group in a manner that does not involve hydrolysis or oxidation

– dehydratase: effects the removal of the components of water to

form a double bond

– hydratase: effects the addition of the components of water to a

double bond

Section 21.3



Lyase

• A lyase is an enzyme that catalyzes the addition or the removal of a

group in a manner that does not involve hydrolysis or oxidation

– decarboxylase: effects the removal of carbon dioxide from a

substrate

– deaminase: effects the removal of ammonia from a substrate

Section 21.3



Isomerase

• An isomerase is an enzyme that catalyzes the isomerization

(rearrangement of atoms) of a substrate in a reaction, converting it

into a molecule isomeric with itself.

racemases – conversion of D- to L- isomer or vice versa

mutases – transfer of a functional group within a molecule

Section 21.3



Ligase

• A ligase is an enzyme that

catalyzes the formation of a bond

between two molecules involving

ATP hydrolysis to ADP:

– ATP hydrolysis is required

because such reactions are

energetically unfavorable

– synthetases – formation of

new bond between two

substrates with participation

of ATP

– carboxylases – formation of

new bond between substrate

and carbon dioxide with

participation of ATP

Section 21.3


Practice Exercise

To what main enzyme class do the enzymes that catalyze

the following chemical reactions belong?


Section 21.3


Practice Exercise

To what main enzyme class do the enzymes that catalyze

the following chemical reactions belong?


Answers:

a.Transferase

b.Lyase

Section 21.4

Models of Enzyme Action


Enzyme Active Site

• Explanations of how enzymes

function as catalysts in biochemical

systems are based on the concepts

of an enzyme active site and

enzyme-substrate complex

formation.

• The active site: relatively small part

of an enzyme’s structure that is

actually involved in catalysis:

– where substrate binds to enzyme

– formed due to folding and bending

of the protein.

– usually a “crevice like” location in

the enzyme

– some enzymes have more than one

active site

Section 21.4



Enzyme Substrate Complex

• Intermediate

reaction species

formed when

substrate binds

with the active site

• Needed for the

activity of enzyme

• Orientation and

proximity is

favorable and

reaction is fast

Section 21.4



Two Models for Substrate Binding to Enzyme

• Lock-and-Key model:

– In this model, the active site in the enzyme has a fixed, rigid

geometrical conformation

– only substrate of specific shape can bind with active site; a substrate

whose shape and chemical nature are complementary to those of the

active site can interact with the enzyme.

– fails to take into account proteins’ conformational changes to

accommodate a substrate molecule

Section 21.4




• Induced Fit Model:

– substrate contact with enzyme will change the shape of the

active site

– allows small change in space to accommodate substrate (e.g.,

how a hand fits into a glove)

– the enzyme active site, although not exactly complementary in

shape to that of the substrate, is flexible enough that it can

adapt to the shape of the substrate.

Section 21.4




Section 21.4



Forces That Determine Substrate Binding

• H-bonding

• Hydrophobic interactions

• Electrostatic interactions

Section 21.5

Enzyme Specificity


• Absolute Specificity:

– an enzyme will catalyze a particular reaction for only one

substrate

– this is most restrictive of all specificities (not common)

– e.g., catalase is an enzyme with absolute specificity for

hydrogen peroxide (H2O2)

– urease absolute specificity for urea

• Stereochemical Specificity:

– an enzyme can distinguish between stereoisomers

– chirality is inherent in an active site (amino acids are chiral

compounds)

– L-amino-acid oxidase - catalyzes reactions of L-amino acids but

not of D-amino acids.

Section 21.5

Enzyme Specificity


• 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

– considered most general of enzyme specificities

– e.g., phosphatases: hydrolyze phosphate–ester bonds in all

types of phosphate esters

Section 21.6

Factors That Affect Enzyme Activity


Enzyme Activity

• A measure of the rate at which enzyme converts

substrate to products in a biochemical reaction

• Four factors affect enzyme activity:

– Temperature

– pH

– Substrate concentration

– Enzyme concentration

Section 21.6



Temperature

• Higher temperature results in

higher kinetic energy which

causes an increase in number of

reactant collisions, therefore there

is higher activity.

• 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 enzyme

activity

Section 21.6



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

– trypsin: optimum pH = 8.0

Section 21.6



Substrate Concentration

• At a constant enzyme

concentration, the enzyme activity

increases with increased

substrate concentration.

• Enzyme saturation: the

concentration at which it reaches

its maximum rate and all of the

active sites are full

• Turnover number: number of

substrate molecules converted to

product per second per enzyme

molecule under conditions of

optimum temperature and pH

Section 21.6



Enzyme Concentration

• Enzymes are not consumed

in the reactions they

catalyze

• At a constant substrate

concentration, enzyme

activity increases with

increase in enzyme

concentration

– the greater the enzyme

concentration, the greater

the reaction rate.

Section 21.6



Practice Exercise

• Describe the effect that each of the following changes

would have on the rate of a reaction that involves the

substrate sucrose and the intestinal enzyme sucrase.

a. Decreasing the sucrase concentration

b. Increasing the sucrose concentration

c. Lowering the temperature to 10ºC

d. Raising the pH from 6.0 to 8.0 when the optimum pH is 6.2

Section 21.6



Practice Exercise

• Describe the effect that each of the following changes

would have on the rate of a reaction that involves the

substrate sucrose and the intestinal enzyme sucrase.

a. Decreasing the sucrase concentration

b. Increasing the sucrose concentration

c. Lowering the temperature to 10ºC

d. Raising the pH from 6.0 to 8.0 when the optimum pH is 6.2

Answers:

a. Decrease rate

b. Increase rate

c. Decrease rate

d. Decrease rate

Section 21.6



Section 21.7

Extremozymes

Extremeophiles

• Organisms that thrive in extreme environments.

– Hydrothermophiles - thrive at 80o-122oC and high pressure.

– Acidophiles - optimal growth pH <3.0.

– Alkaliphiles – optimal growth pH >9.0.

– Halophiles – live in highly saline conditions (>0.2 M NaCl).

– Piezophiles – grow under high hydrostatic pressure.

– Cryophiles – grow at temps <15oC.

• A microbial enzyme that is active at conditions that would inactivate human

enzymes as well as enzymes present in most other organisms.

• Etremozymes are of high interest for industrial chemists

– enzymes are heavily used in industrial processes

– industrial processes require extremes of temp, pressure, and pH.


Extremozyme

Section 21.7

Extremozymes

Extremozyme Applications

• Biotechnology industry – production

of enzymes for industrial

applications.

• Petroleum industry – oil well drilling

operations

• Environmental scavenging and

removal of heavy metals

• Environmental clean-up using

genetically engineered

extremophiles.

• Laundry detergents used in cold

wash cycles.


Section 21.8

Enzyme Inhibition

• Enzyme Inhibitor: a substance that slows down or stops

the normal catalytic function of an enzyme by binding to

it.

• Two types of enzyme inhibitors:

– Competitive Inhibitors: compete with the substrate for

the same active site

• will have similar charge & shape

– Noncompetitive Inhibitors: do not compete with the

substrate for the same active site

• binds to the enzyme at a location other than active site


Section 21.8

Enzyme Inhibition

Reversible Competitive Inhibition • A competitive enzyme

inhibitor decreases enzyme

activity by binding to the

same active site as the

substrate.

• Binds reversibly to an

enzyme active site and the

inhibitor remains unchanged

(no reaction occurs)

• The enzyme - inhibitor

complex formation is via

weak interactions (hydrogen

bonds, etc.).

• Competitive inhibition can

be reduced by simply

increasing the concentration

of the substrate.


Section 21.8

Enzyme Inhibition

Reversible Noncompetitive Inhibition

• A noncompetitive enzyme

inhibitor decreases

enzyme activity by binding

to a site on an enzyme

other than the active site.

• Causes a change in the

structure of the enzyme

and prevents enzyme

activity.

• Increasing the

concentration of substrate

does not completely

overcome inhibition.

• Examples: heavy metal

ions Pb2+, Ag+, and Hg2+.


Section 21.8

Enzyme Inhibition

Irreversible Inhibition

• An irreversible enzyme inhibitor inactivates enzymes by

forming a strong covalent bond with the enzyme’s active

site.

– the structure is not similar to enzyme’s normal

substrate

– the inhibitor bonds strongly and increasing substrate

concentration does not reverse the inhibition process

– enzyme is permanently inactivated.

– e.g., chemical warfare agents (nerve gases) and

organophosphate insecticides


Section 21.8

Enzyme Inhibition


Section 21.9

Regulation of Enzyme Activity

• Enzyme activity is often regulated by the cell to conserve

energy. If the cell runs out of chemical energy, it will die

• Cellular processes continually produces large amounts

of an enzyme and plentiful amounts of products if the

processes are not regulated.

• General mechanisms involved in regulation:

– Proteolytic enzymes and zymogens

– Covalent modification of enzymes

– Feedback control regulation of enzyme activity by

various substances produced within a cell

• The enzymes regulated are allosteric enzymes


Section 21.9


Properties of Allosteric Enzymes

• All allosteric enzymes have

quaternary structure:

• Have at least two binding sites:

1. active site - where the substrate

binds lock-and-key

2. allosteric site (meaning “another

site”) - where the regulator

binds; distorts active site

• some regulators speed up

enzyme action (positive

allosterism); activators

• some regulators slow

enzyme action (negative

allosterism); inhibitors


Section 21.9



Section 21.9


Feedback Control

• A process in which activation or inhibition of the first

reaction in a reaction sequence is controlled by a

product of the reaction sequence.

• Regulators of a particular allosteric enzyme may be:

– products of entirely different pathways of reaction

within the cell

– compounds produced outside the cell (hormones)


A B C D Enzyme 1 Enzyme 2 Enzyme 3

Feedback Control

Enzyme 1 inhibited by product D

Section 21.9


Proteolytic Enzymes and Zymogens

• Mechanism of regulation

by production of enzymes

in an inactive forms

(zymogens).

• Zymogens, also known

as pro-enzymes, are

“turned on” at the

appropriate time and

place

– example: proteolytic

enzymes: hydrolyze

peptide bonds in proteins


Section 21.9


Covalent Modification of Enzymes

• A process in which enzyme activity is altered by

covalently modifying the structure of the enzyme

– Involves adding or removing a group from an enzyme

• Most common covalent modification - addition and

removal of phosphate group:

– phosphate group is often derived from an ATP

molecule.

– addition of the phosphate (phosphorylation) catalyzed

by a kinase enzyme

– removal of the phosphate group (dephosphorylation)

catalyzed by a phosphatase enzyme.

– phosphate group is added to (or removed from) the R

group of a serine, tyrosine, or threonine amino acid

residue in the enzyme regulated.


Section 21.9



• Many common prescription drugs exert their mode of

action by inhibiting enzymes

• Examples:

– Angiotensin Converting Enzyme (ACE) inhibitors

• Management of blood pressure and other heart

conditions

– Sulfa drugs – antibiotics (antimetabolites)

– Penicillins – antibiotics

• Antibiotic: a substance that kills bacteria or inhibits its

growth

Section 21.9



ACE Inhibitors

• Angiotensin II is an octapeptide

hormone that increases blood pressure

via constriction of blood vessels.

• ACE converts Angiotensin I to

angiotensin II in the blood.

• ACE inhibitors block ACE reaction and

thus reduce blood pressure.

– Lisinopril is an example of a ACE

inhibitor

Angiotensin I

Angiotensin II

ACE

Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu

His-Leu +

Asp-Arg-Val-Tyr-Ile-His-Pro-Phe

ACE

inhibitors

block this

reaction

Section 21.9



Sulfa Drugs

• Derivatives of sulfanilamide

• Sulfa drugs exhibit antimetabolite activities

– sulfanilamide is structurally similar to

PABA (p-aminobenzoic acid) which

bacteria need to produce coenzyme folic

acid

– sulfanilamide is a competitive inhibitor of

enzymes responsible for converting

PABA to folic acid in bacteria

– folic acid deficiency retards bacterial

growth and that eventually kills them

– sulfa drugs don’t affect humans because

we get folic acid pre-formed from food

Section 21.9



Penicillins

• Bacteria have one structural feature not found in

animal cells – a cell wall.

• The bacterial cell wall precursor is a polymer of a

repeating disaccharide unit with attached polypeptide

side chains that end with a D-alanyl-D-alanine unit.

• Transpeptidase catalyzes the formation of peptide

cross links between polysaccharide strands in

bacterial cell walls

• Penicillin acts by complexing with the enzyme

transpeptidase that is responsible for cell wall

synthesis

• Selectively inhibits transpeptidase by covalent

modification of serine residue

• The structural similarity between the penicillins and

D-alanyl-D-alanine allows the antibiotic to act as

inhibitory substrates for the transpeptidase enzyme.

• Since animal cells do not have cell walls, there are

no such enzymes to be affected and penicillin has no

effect on animal cells.

Section 21.9



Enzyme Kinetics: Michaelis – Menten

Kinetics of Enzyme Action

k1 k3

E + S ↔ ES ↔ E + P

k2 k4

Michaelis- Menten Equation::

υ = (vmax) (S)

Km + (S)

Vmax is the turnover number

When υ = ½ vmax:

Km = (S)

Section 21.9



Enzyme Kinetics: Lineweaver –

Burke Plots

• an alternative linear

transformation of the M-M

equation

• estimation of the value of Km is

inconvenient from Michaelis

Equation plot and several more

convenient forms of the

equation have been developed.

• The reciprocal of the equation, a

linear form called the

Lineweaver – Burke plot is

used.

• 1/υ = Km + (S) / vmax (S) =

Km / vmax (S) + (S) / vmax (S) =

Km / vmax x 1 / (S) + 1 /

vmax (eqn for st. line)

Section 21.9



Enzyme Kinetics:

Section 21.9



Enzyme Kinetics:

Competitive inhibitor: Noncompetitive inhibitor: Uncompetitive inhibitor:

- binds free E - binds free E & ES complex - binds ES complex

- reversible - reversible; irriversible - irriversible

-Vmax the same - Vmax decreases - Vmax decreases

- Km increases - Km constant - Km decreases

Section 21.10

Prescription Drugs That Inhibit Enzyme Activity


– Different cells in the body produce enzymes for the same type of

reactions.

– Enzymes that catalyze the same reactions but vary slightly in structure

are called isoenzymes.

– For example, there are five isoenzymes for lactate dehydrogenase

(LDH), an enzyme that converts lactic acid to pyruvic acid.

Isoenzyme LDH1 LDH2 LDH3 LDH4 LDH5

Subunits H4 H3M H2M2 HM3 M4

Abundant in Heart Heart Kidneys Spleen Liver, skeletal muscle

kidneys kidneys, brain

brain, rbc

Clinical Applications of Enzymes

Section 21.10



• Enzymes produced in certain organ/tissues if found in blood serum

may indicate certain damage to that organ/tissue


Serum Enzymes used in diagnosis of tissue damage

Organ Condition Diagnostic Enzymes

Heart Myocardial infarction Lactate dehydrogenase (LDH1

) ; Creatine

kinase (CK2

) ; Glutamic oxaloacetic

transaminase (GOT)

Liver Cirrhosis, carcinoma, Glutamic pyruvic transaminase (GPT) ;

Hepatitis Lactate dehydrogenase (LDH5

) ;

Alkaline phosphatase (ALP) ; GOT

Bone Rickets, carcinoma Alkaline phosphatase (ALP)

Pancreas Pancreatic diseases Amylase ; Cholinesterase ; Lipase (LPS)

Prostate Carcinoma Acid phosphatase (ACP)

Section 21.10




Section 21.2

Enzyme Structure


Coenzymes / Cofactors

• the water-soluble vitamins, which include all B-vitamins and Vitamin C,

act as coenzymes or coenzyme precursors

• cofactors are bound to the enzyme for it to maintain the correct

configuration at the active site

• provide additional chemically reactive functional group

Section 21.2

Enzyme Structure



Section 21.2

Enzyme Structure



Cofactors

=============================================================

Metal Ion Enzymes

-------------------------------------------------------------------------------------------------------------------------

Ca 2+ Thromboplastin

Cu2+ Tyrosinase, cytochrome oxidase

Fe2+ ; Fe3+ Cytochrome oxidase, catalase, dehydrogenase

Mg2+ Pyruvate kinase

Mn2+ Arginase, pyruvate carboxylase, phosphatase, succinic dehydrogenase,

glycosyl transferases, cholinesterase

K+ Pyruvate kinase

Zn2+ Carbonic anhydrase, carboxypeptidase, lactic dehydrogenase, alcohol

dehydrogenase

========================================================================

Section 21.12

General Characteristics of Vitamins


• Vitamin: An organic compound essential for proper functioning of the body

• Must be obtained from dietary sources because human body can’t synthesize them in enough amounts

• Needed in micro and milligram quantities

– 1 gram of vitamin B is sufficient for 500,000 people

• Enough vitamin can be obtained from balanced diet

• Supplemental vitamins may be needed after illness

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

enzymes

• Two classes of vitamins

– Water-Soluble and Fat-Soluble

• Synthetic and natural vitamins have the same function

– 13 Known vitamins

Section 21.12



Section 21.12



Vitamin C

• Humans, monkeys, apes and guinea pigs need dietary vitamins

• Co-substrate in the formation of structural protein collagen

- collagen also contains hydroxylysine and hydroxylproline.

- hydroxylation of lysine and proline in collagen formation are

catalyzed by enzymes that require ascorbic acid (Vit. C) and

iron.

- in Vit. C deficiency, hydroxylation is impaired, and the triple helix of

the collagen is not assembled properly.

- persons deprived of Vit. C develops scurvy, a disease whose

symptoms include skin lesions, fragile blood vessels, loose

teeth, and bleeding gums

• Involved in metabolism of certain amino acids

Section 21.14

Water-Soluble Vitamins: The B Vitamins


• Major function: B Vitamins are

components of many coenzymes

• Serve as temporary carriers of

atoms or functional groups in

redox and group transfer

reactions associated with

metabolism

• The preferred and alternative

names for the B vitamins

– Thiamin (vitamin B1)

– Riboflavin (vitamin B2)

– Niacin (nicotinic acid,

nicotinamide, vitamin B3)

– Pantothenic acid (vitamin

B5)

– Vitamin B6 (pyridoxine,

pyridoxal, pyridoxamine)

– Folate (folic acid)

– Vitamin B12 (cobalamin)

– Biotin

•

Section 21.14

Water-Soluble Vitamins: The B Vitamins


Section 21.15

Fat-Soluble Vitamins

Vitamins A, D, E, K

• Involved in plasma membrane processes

• More hydrocarbon like with fewer functional groups

• Occur in the lipid fractions of their sources

• Their molecules have double bonds or phenol rings, so oxidizing

agents readily attack them

• Destroyed by prolonged exposures to air or to the organic peroxides

that develop in fats and oils turning rancid.

• Because the fat-soluble vitamins are easily oxidized, they destroy

oxidizing agents (which are involved in the development of coronary

heart disease, genetic mutations, and cancer)


Section 21.15


Vitamin A

• a primary alcohol of molecular

formula C20H30O; occur only in

the animal world, where the

best sources are cod-liver oil

and other fish-liver oils, animal

liver and dairy products

• provitamin A is found in the

plant world in the form of

carotenes. Provitamins have no

vitamin activity; however, after

ingestion in the diet, -carotene

is cleaved at the central

carbon-carbon double bond to

give 2 molecules of Vit. A.


Section 21.15


Functions of Vitamin A

• 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: a process in which immature cells

change to specialized cells with function.

– example: differentiation of bone marrow cells white blood cells

and red blood cells.

• Maintenance of the health of epithelial tissues via epithelial tissue

differentiation.

– lack of vitamin A causes skin surface to become drier and

harder than normal.

• Reproduction and Growth: in men, vitamin A participates in sperm

development. In women, normal fetal development during

pregnancy requires vitamin A.


Section 21.15


Vitamin D - Sunshine Vitamin

• The antirachitic vitamin

• Necessary for the normal

calcification of bone tissue

• It controls correct ratio of Ca and

P for bone mineralization

(hardening)

• Two forms active in the body:

Vitamin D2 and D3

• Pigment in the skin, 7-

dehydrocholesterol, is a

provitamin D; when irradiated by

the sun becomes converted to

Vit. D3

• humans exposed to sunlight year-

round do not require dietary Vit. D


Section 21.15


Vitamin E - Antisterility vitamin

• Alpha-tocopherol is the most active

biological active form of Vitamin E

• tocopherol Greek, promoter of childbirth

• functions in the body as an antioxidant in

that it inhibits the oxidation of unsat’d fatty

acids by O2

• Primary function: Antioxidant – protects

against oxidation of other compounds


Section 21.15


Vitamin K - Antihemorrhagic vitamin

• Vit K is synthesized by

bacteria that grow in colon

• Active in the formation of

proteins involved in

regulating blood clotting

• Deficiency may occur

during the first few days

after birth, because

newborns lack the intestinal

bacteria that produce Vit. K

and because they have no

store of Vit. K (it does not

cross the placenta)

• Deficiency may also occur

following antibiotic therapy

that sterilizes the gut


chem 45 biochemistry: stoker chapter 21 enzymes & vitamins

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