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. Mg2+, is required by many ATP requiring enzymes)

Metal ions can function as electrophiles in

active site

Zinc protease (angiotensin converting enzyme)

CoenzymesCosubstrates-

- 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

• 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

Coenzymes

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

VitaminsVitamin CoenzymeAscorbic acid (C) not a coenzymeNiacin NAD(P)+/NAD(P)HRiboflavin (B2) FMN & FAD

Thiamin (B1) Thiamin-pyrophosphate

Pyridoxal (B6) Pyridoxal phosphateBiotin BiotinFolate TetrahydrafolateCobalamin (B12) adenosyl-and methylcobalaminVitamin A RetinalVitamin K Vitamin KPantothenate (B3) Coenzyme A

Niacin (nicotinic acid)

• 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

N

C

O

OH

N

C

O

NH2

NICOTINIC ACID(NIACIN)

NICOTINAMIDE

Nicotinamide Coenzymes

N

C

O

OH

N

C

O

NH2

NICOTINIC ACID(NIACIN)

NICOTINAMIDE

N

C

O

NH2

H

O

OHOH

H2C

O

PO2-

O

PO2-

O

O

OH(OPO3)OH

CH2N

NN

N

NH2

NIC

OT

INA

MID

E M

ON

OPH

OSP

HA

TE

AD

EN

OSI

NE

MO

NO

PH

OS

PHA

TE

N

C

O

NH2

H

R

N

C

O

NH2

H

R

H

OXIDIZED REDUCED

NAD+ / NADP+

• Serve as cofactors in oxidation/reduction reactions

• 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

Hydroquinoneform

Quinoneform

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

Pyridoxal

N

HOH2C

H2C

O

CH3

H

OH

N

HOH2C

HC

O

CH3

H

O

N

HOH2C

H2C

O

CH3

H

NH3

N

H2C

HC

O

CH3

H

O

OP

O

O

O

N

H2C

H2C

O

CH3

H

NH3

OP

O

O

O

PYRIDOXINE PYRIDOXAL PRYIDOXAMINE

PYRIDOXAL 5' PHOSPHATE PYRIDOXAMINE 5' PHOSPHATE

PYRIDOXAL-PHOSPHATE

CR

O

H + NH2 R2 R CH

N R2

- H2O

+ H2O

ALDEHYDE AMINE SCHIFF BASE

•Important in amino acid metabolism

•Bound to enzyme as a Schiff base thru rxn with lysine

• PLP functions in transamination, decarboxylation, racemization, isomerization, side-chain elimination rxns involving amino acids

PLP in transamination reaction

PLP in amino acid decarboxylation reaction