Regulation of Enzyme Activity
• 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
Zymogens
• 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
Digestive Enzymes
• 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
Allosteric Enzymes
• 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)
Feedback Control
• 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 (E1) in the sequence, and production is
stopped
• When product concentration is low, it dissociates from E1 and
production is resumed
• Feedback control allows products to be formed only when needed
Enzyme Cofactors
• 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
Metal Ions as Cofactors
• 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
A Zinc Carboxypeptidase
• A Zn2+ 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 Zn2+ activates the carbonyl in a similar way as an acid catalyst
Functions of Coenzymes
• 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
Water Soluble Vitamins
• 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
Fat Soluble Vitamins
• 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
Thiamin (Vitamin B1)
• 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
-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
Riboflavin (Vitamin B2)
• 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
O
H3C
N
H3C
N
N
N
H
O
D-Ribitol
CH2 CH CH CH CH2 OH
OH OH OH
Niacin (Vitamin B3)
• 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
O
OH
N
Niacin (Nicotinic Acid)
Pantothenic Acid (Vitamin B5)
• 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
CH3 OH O
HO CH2 C
CH3
O
CH C N CH2
H
CH2 C OH
Pyridoxine (Vitamin B6)
• Pyridoxine and pyridoxal are two forms of vitamin B6
• 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
Cobalamin (Vitamin B12)
• Cobalamin consists of four
pyrrole rings with a Co2+
• It is a coenzyme involved in
the transfer of methyl
groups, acetyl choline
synthesis and red blood cell
production
• A deficiency in vitamin B12
can lead to pernicious
anemia and nerve damage
• Dietary sources include
beef, chicken, fish and milk
products (strict vegans
should take B12
supplements)
Ascorbic Acid (Vitamin C)
• 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
CH2OH
O
HO
O
CHOH
OH
Folic Acid (Folate)
• 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
Vitamin A
• 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
H3C
CH3
CH3
Beta-carotene
CH3
CH3
CH3
H3C
H3C
CH3
CH3
CH3
CH3
CH2OH
Retinol (vitamin A)
CH3
H3C
CH3
Vitamin D
• Vitamin D (D3) 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)
Vitamin E
• Vitamin E (-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
CH3
HO
CH3
H3C
O
CH3
CH3
CH3
CH3
CH3
Vitamin K
• Vitamin K1 (in plants) has a saturated side chain
• Vitamin K2 (in animals) has a long unsaturated side chain
• Vitamin K2 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
O
O
CH3
CH3
CH3
CH3
O
CH3
CH3
Vitamin K1 (phylloquinone)
O
CH3
CH3
Vitamin K2 (menaquinone)