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

N

N N

NHH3C

H3C

CH2 CH CH CH CH2 OH

OHOHOH

O

O

D-Ribitol

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

N

OH

O

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

HO CH2 C CH C N CH2 CH2 C OH

O

H

OH OCH3

CH3

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

O CHOH

CH2OH

OHHO

O

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

CH3 CH3

CH3 CH3H3C CH3

H3C

CH3 CH3

CH2OHH3C CH3

CH3

Beta-carotene

Retinol (vitamin A)

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

O

CH3

HO

H3C

CH3

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

CH3

CH3

O

O CH3 CH3

Vitamin K1 (phylloquinone)

CH3

CH3

O

O CH3 CH3

Vitamin K2 (menaquinone)