Chapter 7 (part 1) Cofactors

Cofactors

• Cofactors are organic or inorganic molecules

that are required for the activity of a certain conjugated enzymes

• Apoenzyme = enzyme (-) cofactor • Holoenzyme = enzyme (+) cofactor • Inorganic cofactors – essential ions • Organic cofactors – coenzymes

Essential Ion Cofactors

• Activator ions – bind reversibly to enzyme and

often participate in substrate binding.

• Metal ions of metalloenzymes – cations that are

tightly bound to enzyme and participate directly in catalysis (Fe, Zn, Cu, Co).

• Metal activated enzymes – require or are

stimulated by addition of metal ions (i.e. Mg 2+ , is required by many ATP requiring enzymes)

Metal ions can function as electrophiles in active site

Zinc protease (angiotensin converting enzyme)

Coenzymes

Cosubstrates - altered in rxn and regenerated to original structure in subsequent rxn - disassociated from active site - shuttle chemical groups among different enzyme rxns.

Prosthetic groups - remains bound to enzyme - must return to original form Both cosubstrates and prosthetic groups supply reactive groups not present on amino acid side chains

Coenzymes

• Metabolite coenzymes – synthesized from

common metabolites

• Nucleoside triphosphates – (ATP) can donate

phosphates, pyrophosphates, adenosyl grroups

• S-adenosylmethionine (SAM) – donates methyl

groups

• Nucleotide sugars (uridine diphosphate glucose =

UDP-glucose) - transfer sugars in carbohydrate metabolism

Vitamin derived coenzymes

• Must be obtained from diet • Synthesized by microorganisms and

plants

• Vitamin deficiencies lead to disease

state

• Most vitamins must be enzymatically

transformed to function as a coenzyme

Vitamin Ascorbic acid (C) Niacin Riboflavin (B 2 ) Thiamin (B 1 ) Pyridoxal (B 6 ) Biotin Folate Cobalamin (B 12 ) Vitamin A Vitamin K Pantothenate (B 3 )

Vitamins

Coenzyme not a coenzyme NAD(P)+/NAD(P)H FMN & FAD Thiamin-pyrophosphate Pyridoxal phosphate Biotin Tetrahydrafolate adenosyl-and methylcobalamin Retinal Vitamin K Coenzyme A

Niacin (nicotinic acid)

O O C C OH NH 2 N NICOTINIC ACID (NIACIN) N NICOTINAMIDE • Deficiencies lead to pellagra (dermatitis,

diarrhea, dementia)

• Required in relatively high amounts compared to

other vitamins

• Not true enzyme because can be synthesized

from tryptophan in the liver

Nicotinamide Coenzymes

H C O NH 2 C O C O OH NH 2 N O PO 2 O H 2 C PO 2 O O OH CH 2 O N OH N NH 2 N N

H

N NICOTINIC ACID (NIACIN) C O NH 2 N NICOTINAMIDE

H H

N R OXIDIZED N R REDUCED OH OH(OPO 3 ) C O NH 2

reactions

NAD

+

/ NADP

+

• Serve as cofactors in oxidation/reduction • Act as co-substrates for dehydrogenases • Reduction of NAD

+ /NADP + and oxidation of NADH/NADPH occurs 2 e- at a time.

• Function in hydride ion transfer • Rxns forming NADH/NADPH are catabolic • NADH is coupled with ATP production in

mitochondria

• NADPH is an impt reducing agent in

biosynthetic reactions

• Reduced forms (NADH/NADPH) absorb light at

340 nm, oxidized forms (NAD + /NADP + ) do not

Riboflavin (B2)

• Water soluble vitamin • Severe deficiencies lead to growth retardation, reproductive problems and neural degeneration • Meat, dairy products and dark green vegetables, legumes and grains are good sources

FMN/FAD

FAD and FMN can transfer electrons one or two at a time

Quinone form Hydroquinone form semiquinone form

Thiamin

•Thiamin is the first Vitamin discovered (Vital

amine = Vitamin)

•Deficiencies lead to disease called Beriberi

(neurological disorders, heart problems, anorexia)

•Beriberi prevealent in undeveloped countries where

polished grains make up the majority of the diet.

•Associated with alcohol related disorders

(Wernickes-Korskofff syndrome – memory loss, unstable walk)

Thiamin pyrophosphate

•Serves as a cofactor in decarboxylation rxn of keto

acids

•Also functions as a prosthetic group in

transketolases (catalyze the transfer of two carbon units in carbohydrate metabolism)

Thiazolium ring is the chemically active part of TPP

Ylid = a molecule with opposite charges on adjacent atoms

HOH 2 C H 2 C OH

Pyridoxal

HC O O O HOH 2 C HOH 2 C H 2 C NH 3 O N H PYRIDOXINE CH 3 N H PYRIDOXAL CH 3 N H CH PRYIDOXAMINE 3 O O O P O O H 2 C HC O N CH 3 H PYRIDOXAL 5' PHOSPHATE O NH 3 O P O O H 2 C H 2 C O N H CH PYRIDOXAMINE 5' PHOSPHATE 3

PYRIDOXAL-PHOSPHATE

•Important in amino acid metabolism •Bound to enzyme as a Schiff base thru rxn with lysine O - H2O R C H + NH2 R 2 R C H N R 2 + H2O ALDEHYDE AMINE SCHIFF BASE • PLP functions in transamination, decarboxylation, racemization, isomerization, side-chain elimination rxns involving amino acids

PLP in transamination reaction

PLP in amino acid decarboxylation reaction