Enzyme regulation zymogen

ENZYMOLOGY- For Graduate students

1. ENZYME REGULATION

2. Enzyme activity must be regulated so that the proper
levels of products are produced at all times and places
This control occurs in several ways:
- biosynthesis at the genetic level
- covalent modification after biosynthesis
- regulatory enzymes
- feedback inhibition
A common covalent enzyme modification is the addition
or removal of a phosphate group
- under high-energy conditions (high ATP and low
ADP), phosphorylation is favored
- under low-energy conditions (low ATP and high ADP),
dephosphorylation is favored
- this regulates the balance between biosynthesis and
catabolism

3. Zymogens (proenzymes) are inactive forms of enzymes
They are activated by removal of peptide sections
For example, proinsulin is converted to insulin by
removing a 33-amino acid peptide chain

4. Digestive enzymes are produced as zymogens, and
are then activated when needed
Most of them are synthesized and stored in the
pancreas, and then secreted into the small intestine,
where they are activated by removal of small peptide
sections
The digestive enzymes must be stored as zymogens
because otherwise they would damage the pancreas

5. An allosteric enzyme binds a regulator
molecule at a site other than the active site (an
allosteric site)
Regulators can be positive or negative:
- a positive regulator enhances the binding of
substrate and accelerates the rate of reaction.
- a negative regulator prevents the binding of
the substrate to the active site and slows down
the rate of reaction (non-competitive inhibition)

6. In feedback control, a product acts as a negative regulator
When product concentration is high, it binds to an
allosteric site on the first enzyme (E
1) in the sequence, and
production is stopped
When product concentration is low, it dissociates from E
1
and production is resumed
Feedback control allows products to be formed only when
needed

7. A simple enzyme consists only of protein in its active form
Other enzymes are active only when they combine with
cofactors such as metal ions or small molecules
- a cofactor that is a small organic molecule, such as a
vitamin, is called a coenzyme

8. Many enzymes require a metal ion to carry out catalysis
Metal ions in the active site are attached to one or more
amino acid side-chains
The metal ions have various functions, such as electron
exchange and substrate stabilization

9. A Zn
2+
ion in the active site of carboxypeptidase A promotes
hydrolysis of a C-terminal amino acid from a polypeptide by
interacting with the carbonyl oxygen
The Zn
2+
activates the carbonyl in a similar way as an acid catalyst

10. Coenzymes are small organic molecules that are often
required to prepare the active site for proper substrate
binding and/or participate in catalysis
Because they are not destroyed during the reaction,
coenzymes are only required in small quantities

11. Vitamins are organic molecules that are essential for
metabolism, but can not be biosynthesized; they must be
consumed in the diet
Many coenzymes come from water-soluble vitamins
Water soluble vitamins are not stored in the body, and so
should be consumed daily

12. Fat soluble vitamins are not used as coenzymes
However, they are important in vision, bone
formation, antioxidants, and blood clotting
Fat soluble vitamins are stored in the body, so
should not be consumed in excess, as they can be
toxic at high levels

13. Thiamin was the first B vitamin identified, and is part of the
coenzyme thiamin pyrophosphate (TPP)
TPP coenzyme is required by enzymes for decarboxylation of
a-keto carboxylic acids
A deficiency of thiamin results in beriberi (fatigue, weight
loss, and nerve degeneration)
Dietary sources include whole grains, milk products and
yeast

14. Riboflavin is made of the sugar alcohol ribitol and flavin
It is part of the coenzymes flavin adenine dinucleotide
(FAD) and flavin mononucleotide (FMN)
FAD and FMN are used in redox reactions involving
carbohydrates, proteins and fats
Riboflavin is needed for good vision and healthy skin,
and a deficiency can lead to cataracts and dermatitis
Dietary sources include green leafy vegetables, whole
grains, milk products, chicken, eggs and peanuts
N
N N
N
HH3C
H3C
CH2CHCHCHCH2OH
OHOHOH
O
O
D-Ribitol

15. Niacin is part of the coenzyme nicotinamide adenine
dinucleotide (NAD
+
) and NADP
+
(P = phosphate)
NAD
+
and NADP
+
are used in redox reactions involving
carbohydrates, proteins and fats
A deficiency of niacin can result in dermatitis, muscle
fatigue and loss of appetite
Dietary sources include meats, rice, and whole grains
N
OH
O
Niacin (Nicotinic Acid)

16. Pantothenic acid is part of coenzyme A
Coenzyme A is involved in energy production,
conversion of lipids and amino acids to glucose and
synthesis of cholesterol and steroid hormones
A deficiency of pantothenic acid can result in
fatigue, retarded growth, cramps, and anemia
Dietary sources include salmon, meat, eggs, whole
grains, and vegetables
HOCH
2CCHCNCH
2CH
2COH
O
H
OHOCH3
CH3

17. Pyridoxine and pyridoxal are two forms of vitamin B
6
They are converted to the coenzyme pyridoxal
phosphate (PLP)
PLP is involved in the transamination of amino acids
and the decarboxylation of carboxylic acids
A deficiency of pyridoxine may lead to dermatitis,
fatigue and anemia
Dietary sources include fish, meat, nuts, whole grains
and spinach

18. Cobalamin consists of
four pyrrole rings with a
Co
2+
It is a coenzyme involved
in the transfer of methyl
groups, acetyl choline
synthesis and red blood
cell production
A deficiency in vitamin B
12

can lead to pernicious
anemia and nerve damage
Dietary sources include
beef, chicken, fish and
milk products (strict
vegans should take B
12

supplements)

19. Ascorbic acid is a very polar hydroxy ester that is a weak
acid
It is involved in the synthesis of hydroxyproline and
hydroxylysine, two modified amino acids that are required
for collagen synthesis
A deficiency of vitamin C can lead to slow-healing wounds,
weakened connective tissue, bleeding gums and anemia
Dietary sources include berries, citrus fruits, tomatoes, bell
peppers, broccoli and cabbage
O
CHOH
CH
2OH
OHHO
O

20. Folic acid (folate) consists of pyrimidine, p-aminobenzoic
acid (PABA) and glutamate
It forms the coenzyme THF used in the synthesis of
nucleic acids
A deficiency can lead to abnormal red blood cells, anemia,
poor growth, hair loss and depression
Dietary sources include green leafy vegetables, beans,
meat, seafood, yeast, asparagus and whole grains
Some derivatives of folic acid, such as methotrexate, are
inhibitors of the enzyme that converts folic acid to THF
- these are used as anti-cancer drugs, especially for
leukemias

21. Vitamin A can exist as an alcohol (retinol), an aldehyde
(retinal) or a carboxylic acid (retinoic acid)
In the retina of the eye, retinol undergoes cis-trans
isomeration as part of photoreception
Vitamin A is also involved in synthesis of RNA and
glycoproteins
A deficiency in vitamin A can lead to night blindness,
depressed immune response and growth inhibition
Dietary sources include yellow and green fruits and
vegetables
Beta-carotenes are converted to vitamin A in the liver
H
3C CH
3
CH
3
CH
3 CH
3
CH
3 CH
3
H
3C CH
3
H
3C
CH
3 CH
3
CH2OH
H
3C CH
3
CH3
Beta-carotene
Retinol (vitamin A)

22. Vitamin D (D
3
) is synthesized from 7-dehydrocholesterol
in skin exposed to sunlight
It regulates the absorption of phosphorus and calcium
during bone growth
A deficiency in vitamin D can result in weakened bones
Dietary sources include cod liver oil, egg yolk, and
vitamin D enriched foods (such as milk)

23. Vitamin E (a-tocopherol) acts as an antioxidant in
cells
Not much is know about its mechanism, but it may
prevent the oxidation of unsaturated fatty acids
A deficiency of vitamin E can lead to anemia
Dietary sources include meat, nuts, vegetable oils,
whole grains, and vegetables
Synthetic vitamin E is a mixture of the alpha and
beta forms (enantiomers)
- only the alpha form can be utilized by our cells


O
CH3
HO
H
3C
CH
3
CH
3
CH
3
CH
3 CH
3 CH
3

24. Vitamin K
1 (in plants) has a saturated side chain
Vitamin K
2 (in animals) has a long unsaturated side chain
Vitamin K
2 is needed for the synthesis of zymogens for
blood clotting
A deficiency of vitamin K can lead to extended bleeding
from small cuts and increased bruising
Dietary sources include meat, spinach and cauliflower
CH
3
CH
3
O
O CH
3 CH
3


Vitamin K
1 (phylloquinone)
CH
3
CH3
O
O CH
3 CH
3

Vitamin K2 (menaquinone)