SLIDE BÀI GIẢNG HÓA SINH vitamin

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Reading material
• Principles of Biochemistry with a Human
Focus by Garrett and Grisham, First
Edition, 2002, pages 453-468
• Handbook of NonPrescriptions Drugs,
11th edition, Chapter entitled “Nutritional
Products” by Loyd V. Allen, Jr.


Vitamins
• a group of organic compounds needed in small
quantities in the diet for normal activity of
tissues
• between 14 – 20 substances have been identified
as vitamins
• many vitamins act as cofactors, coenzymes or
prosthetic groups for enzymes
• most vitamins are derived from diet
• no calories are derived from vitamins


Vitamins
• first vitamin discovered was thiamine or
B1
• the term vitamin is derived from the fact
that the substances are needed for life
(vita) and because thiamine happened to
be an amine the term was coined as such

• however, not all vitamins are amines or
nitrogen containing compounds


Vitamins
• vitamin requirements are usually
expressed as RDA’s (recommended
dietary allowances)
• guidelines are provided by 2
organizations:
• the Food and Nutrition Board of the National
Academy of Sciences- National Research Council
• the Food and Drug Administration (FDA)


RDAs
• applications of RDAs include:
• evaluating the adequacy of the national food
supply
• establishing standards for menu planning
• establishing nutritional policy for public
institutions/organizations and hospitals
• evaluating diets in food consumption studies
• establishing labeling regulations
• setting guidelines for food product formulation
• developing materials for nutritional education


RDAs
• RDAs have limitations:

• they are too complex for direct consumer use
• they do not state ideal or optimal levels of intake
• the allowances for some categories are based on
limited data
• the data on some nutrients in foods is limited
• they do not evaluate nutritional status
• they do not apply to seriously ill or malnourished
patients


Vitamin deficiencies
• primary food deficiency
• crop failure
• food storage loss
• food preparation loss

• diminished food intake
•
•
•
•

poverty
anorexia
food fadism
chronic diseases


Vitamin deficiencies
• diminished absorption

• absorption defect
• parasites
• malignancies

• increased requirements
•
•
•
•

rapid growth
increased physical activity
pregnancy
hyperthyroidism

• increased loss
• drug therapy
• diuresis
• lactation


Vitamin loss
Loss is seen mainly in storage or food preparation

• Vitamin A: sensitive to oxygen and light
• Vitamin D: usually little loss
• Vitamin E: sensitive to oxidation especially
when heated or with alkali
• Vitamin K: sensitive to acids, alkali, light and
oxidizing agents

• Vitamin C: very sensitive to oxidation,
especially when heated in contact with metals
• Vitamin B complex: water solubility results in
loss in cooking water
• Riboflavin is sensitive to light


Vitamins
• Vitamins are typically divided into 2
groups:
– The fat soluble vitamins
• A, D, E, and K

– The water soluble vitamins
• The B vitamins (B1, B2, B3, B6, B7, B12 and
pantothenic acid)
• Ascorbic acid (vitamin C)


Bogus vitamins
•
•
•
•
•
•
•

Vitamin B4
Vitamin B10

Vitamin B11
Vitamin B15
Vitamin B13
Vitamin B17
Vitamin B19

adenine
identical with folic acid
“

“

“

“

pangamic acid
orotic acid
laetrile
wormser’s secret formula


Cofactors
• provide “chemical teeth” for enzymes
• sometimes referred to as coenzymes
• enzymes: proteins with catalytic activity
– simple enzymes: large protein (polypeptide) that
catalyzes a reaction. The enzyme gets all the “tools”
(chemical teeth) it needs from the amino acids.
However, there are only 20 different amino acids

– conjugated enzymes : apoenzyme + cofactor =
holoenzyme


EXAMPLE:Proteases: enzymes that cleave
peptide bonds

H

O

R'

H

N

N
N
R

H

H

H2O

R'

O

N

N
OH

O

H

protease
R

+

H2N
O

Enzymes perform catalytic reactions such as hydrolysis; the
side chains of amino acids participate in the reactions


example of a simple enzyme
Usually electron-rich
side chains are involved
in the catalysis
CH2OH

H
N

CH2
N

Aliphatic chains are
normally involved in
hydrophobic interactions

CH2-COOH

all these tools come from amino acids
in the protein active site

A serine protease enzyme such as chymotrypsin


HYDROLYTIC CATALYSIS
O
R'

N

NH
H

HN
COO-

N

O

ASP

N
H

R

HIS

COOH

ASP

SER

O-

HIS

SER

H2O

R'
R' NH2
O

N

HN
COO-

ASP

O

HIS

SER

NH
R

O-

HN

N
COOH

ASP

O

HIS

SER

R

Example of a conjugated enzyme
cofactor needed for reaction

Zinc protease such
as ACE

H

Zn+2

O

R'

OH
N

N
N
R

H

O

PRODUCTS + ENZYME

Cofactors
• all water-soluble vitamins with the exception of
vitamin C are converted/activated to cofactors
• only vitamin K of the fat-soluble vitamins is
converted to a cofactor
• not all vitamins are cofactors; i.e., lipoic acid is
not a vitamin
• cofactors may also act as carriers of specific
functional groups such as methyl groups and
acyl groups


The water soluble
vitamins


Pantothenic acid (vitamin B5)
H

CH3 OH
HO

CH2 C
CH3

CH C

N

CH2 CH2 COOH

O

First recognized in 1933 as a growth factor for yeast (Roger
J. Williams)


Pantothenic acid
• a yellow viscous oil (free acid)
• stable to moist heat (not to dry heat) and
to oxidizing and reducing agents
• hydrolyzed in acid or alkaline medium
• sources (numerous): liver, kidney, eggs,
lean beef, milk, molasses, cabbage,
cauliflower, broccoli, peanuts, sweet
potatoes, kale (derive its name from
everywhere)


Pantothenic acid
• serves in its activated form as the cofactor for
coenzyme A (CoA) and the acyl carrier protein (ACP)
• first phosphorylated by ATP to 4’phosphopantothenate
• next is the formation of 4’-phosphopantetheine by
addition of cysteine and decarboxylation
• adenylation by ATP forms dephospho-CoA
• phosphorylation to the 3’-OH of the ribose generates
CoA (coenzyme A)

H

O

N
S
O

Acetyl CoA

OH

H

H

N

N

N

O
O

SH

P

O-

O

O
NH2

O
O

P

N

O-

N

O
N
H2C

O
H

H

H

H
OPO3

Coenzyme A

OH

N

CH3


Coenzyme A
• performs a vital role by transporting acetyl
groups from one substrate to another
• the key to this action is the reactive thioester
bond in the acetyl form of CoA
• the thioester bond is stable enough that it can
survive inside the cell, but unstable enough that
acetyl-CoA can readily transfer the acetyl
group to another molecule


Example of an acetylation reaction

CH3

CH3

H3C

H3C

N

N
H3C

OH

H3C

choline

O

acetyl CoA

CoA

O

CH3

acetylcholine

Acetylcholine is an important neurotransmitter in
the autonomic nervous system (cholinergic) and in the brain