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 coenzymeNiacinNAD(P)+/NAD(P)HRiboflavin (B2) FMN & FADThiamin (B1) Thiamin-pyrophosphatePyridoxal (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

CH2 N

NN

N

NH2

NIC

OTI

NA

MID

E M

ON

OPH

OSP

HA

TEA

DEN

OSI

NE

MO

NO

PHO

SPH

ATE

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

timeHydroquinone

formQuinone

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

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

+ H2OALDEHYDE 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