Coenzyme Complex nonprotein - PowerPoint Presentation

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Coenzyme

Complex

nonprotein

organic cofactor required for activity of some enzymes.

Most vitamins are precursors of coenzymes.

Nucleotide coenzyme

:

Coenzyme that resembles (e.g., FMN) or contains a nucleotide as a component (e.g., FAD, NAD, NADP, CoA).

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the special contribution of coenzymes to the catalytic activity of many enzymes

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Adenine nucleotides are

components

of many enzyme cofactors

For example, removal of the adenine nucleotide (3-phosphoadenosine

diphosphate

) from

acetoacetyl-CoA

, the coenzyme A derivative of

acetoacetate

, reduces its reactivity as a substrate for -

ketoacyl-CoA

transferase

(an enzyme of lipid metabolism) by a factor of 10

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Ubiquinone

(also called coenzyme Q) and

plastoquinone

(Fig)

are

isoprenoids

that function as lipophilic electron carriers in the oxidation-reduction reactions that drive ATP synthesis in mitochondria and chloroplasts, respectively. Both ubiquinone and

plastoquinone

can accept either one or two electrons and either one or two protons

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Flavin

nucleotides

are tightly bound in

flavoproteins

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FAD and FMN, the

flavin

nucleotides, serve as tightly bound prosthetic groups of

flavoproteins

. They can accept either one or two electrons.

Flavoproteins

also serve as light receptors in

cryptochromes

and

photolyases

.

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Pyridine nucleotide coenzymes

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A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme protein is called a

prosthetic group.

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Coenzymes act as transient carriers of specific functional groups. Most are derived from vitamins, organic nutrients required in small amounts in the diet

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Many enzymes require

nonprotein

coenzymes or cofactors for their catalytic function.

Coenzymes

in the

metabolic pathways

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Cellular oxidation of glucose to carbon dioxide Requires specialized electron carriers

Cells convert glucose to CO

2

in a series of controlled reactions, some of which are oxidations.

Electrons removed in these oxidation steps are transferred to coenzymes specialized for carrying electrons, such as NAD and FAD

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NAD, NADP, FMN, and FAD are

water-soluble coenzymes

that undergo reversible oxidation and reduction in many of the

electron transfer

reactions of metabolism.

The nucleotides

NAD and NADP

move readily from one enzyme to another;

the

flavin

nucleotides

FMN and FAD are usually very tightly bound to the enzymes, called

flavoproteins

, for which they serve as prosthetic groups.

Lipid-soluble

quinones

such as

ubiquinone

and

plastoquinone

act as

electron carriers

and proton donors in the

nonaqueous

environment of membranes.

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In many organisms, a central energy- conserving process is the stepwise oxidation of glucose to CO

2

, in which some of the energy of oxidation is conserved in ATP as electrons are passed to O

2

.

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NAD and NADP are the freely diffusible coenzymes of many

dehydrogenases

. Both NAD and NADP accept two electrons and one proton. NAD and NADP are bound to

dehydrogenases

in a widely conserved structural motif called the

Rossmann

fold.

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FAD and FMN, the

flavin

nucleotides, serve as tightly bound prosthetic groups of

Flavoproteins

, can accept either one or two electrons.

Flavoproteins

also serve as light receptors in

cryptochromes

and

photolyases

.

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Rapidly dividing cells, such as those of bone marrow, skin, and intestinal mucosa, use the

pentoses

to make RNA, DNA, and such coenzymes as ATP, NADH, FADH

2

, and coenzyme A.

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Coenzyme A (Fig. 16–3) has a reactive

thiol

(OSH) group that is critical to the role of

CoA

as an

acyl

carrier

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Pyridoxal

phosphate participates in the transfer of amino groups to -

ketoglutarate

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the electron-carrying cofactor and the activating groups

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Coenzyme

s are small organic molecules, often derivatives of vitamins

They may or may not be modified (e.g., oxidized or reduced) in the reaction.

Those that are altered are also termed

co- substrates

.

Metal ions are often required for enzyme

reaerions

and referred to as a

cofactor

.

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Intracellular range of concentration

0.01 -1.0

meq

L

-1

Unchanged at the end of the reaction

the coenzyme is chemically altered during the reaction.

Coenzymes may be covalently bound to the enzyme or free to associate and dissociate from the protein.

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The enzyme without its cofactor is called the

apoenzyme

and

haloenzyme

when the cofactor is bound.

Prosthetic group

bound either very rightly or covalently to an enzyme

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The functional portion

of the coenzyme

nicotinamide

ring

the

flavin

ring

the

sulfhydryl

group

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