ENZYME REGULATION

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 (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 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

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)

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

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

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 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

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

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 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 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 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

NHH3C

H3C

CH2 CH CH CH CH2 OH

OHOHOH

O

O

D-Ribitol

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)

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

HO CH2 C CH C N CH2 CH2 C OH

O

H

OH OCH3

CH3

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 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 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

CH2OH

OHHO

O

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 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

CH3 CH3

CH3 CH3H3C CH3

H3C

CH3 CH3

CH2OHH3C CH3

CH3

Beta-carotene

Retinol (vitamin A)

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 (-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

H3C

CH3

CH3

CH3

CH3 CH3 CH3

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

CH3

CH3

O

O CH3 CH3

Vitamin K1 (phylloquinone)

CH3

CH3

O

O CH3 CH3

Vitamin K2 (menaquinone)