Ch. 12

Removal of electrons

oxidation

Gain of electrons

reduction

The process by which cells derive energy in the form of ATP from the controlled reaction of hydrogen with oxygen to form water

respiration

monooxygenases that are important for detoxification of many drugs and for the hydroxylation of steroids.

cytochrome P450 system

Enzymes involved in oxidation and reduction

oxidoreductases

4 groups of oxidoredectases

oxidases
dehydrogenases
hydroperoxidases
oxygenases

How do oxidases work?

Oxidases catalyze the removal of hydrogen from a substance using oxygen as a hydrogen acceptor

What do oxidases from as reaction products?

water or hydrogen peroxide

A widely distributed hemoprotein located in many tissues, which has the typical heme prosthetic group present in myoglobin, hemoglobin, and other cytochromes.

cytochrome oxidase

Cytochrome oxidase is poisoned by ______.

carbon monoxide
cyanide
hydrogen sulfide

Which type of oxidase contains copper?

cytochrome oxidase

Prosthetic groups contained in flavoprotein enzymes

FMN (flavin mononucleotide
FAD (flavin adenine dinucleotide)

How are FMN and FAD formed in the body?

By riboflavin

How are FMN and FAD bound to their apoenzyme proteins?

tightly bound, but not covalently bound

______ contain one or more metals as essential cofactors.

Metalloflavoproteins

Examples of flavoprotein enzymes

L-amino acid oxidase
Xanthine oxidase
Aldehyde dehydrogenase

An FMN-linked enzyme found in kidney with general specificity for the oxidative deamination of the naturally occurring L-amino acids

L-amino acid oxidase

Enzyme which contains molybdenum and plays an important role in the conversion of purine bases to uric acid

Xanthine oxidase

An FAD-linked enzyme present in mammalian livers, which contains molybdenum and non-heme iron and acts upon aldehydes and N-heterocyclic substrates

Aldehyde dehydrogenase

______ cannot use oxygen as a hydrogen acceptor, unlike oxidases.

dehydrogenases

2 main functions of dehydrogenases

Transfer of H from one substance to another in a coupled oxidation-reduction reaction (One substrate is oxidized at the expense of another.)
and
as components in the respiratory chain of electron transport from substrate to oxygen

Many dehydrogenases depend on _____ coenzymes.

Nicotinamide

How are dehydrogenases helpful?

Helps in oxidation reactions in the absence of O2 (anaerobic phase of glycolysis).

How is nicotinamide formed in the body?

From niacin

Catalyze oxi-redn reactions in the oxidative pathways of metabolism...
�Glycolysis
�Citric acid cycle
�Respiratory chain of mitochondria

NAD-linked dehydrogenases

Enzymes found in the reductive syntheses, such as the extramitochondrial pathway of fatty acid synthesis and steroid synthesis, and in the pentose phosphate pathway

NADP-linked dehydrogenases

Type of dehydrogenase concerned with electron transport in or to the respiratory chain.

riboflavin-linked dehydrogenases

___________ acts as carrier of electrons and transfers reducing equivalents directly to the respiratory chain.
-Succinate dehydrogenase
-Acyl CoA dehydrogenase
-Mitochondrial glycerol-3-phosphate dehydrogenase

NADH dehydrogenase

An intermediary carrier of electrons between acyl-CoA dehydrogenase and the respiratory chain

electron-transferring flavoprotein
(ETF)

Iron-containing hemoproteins in which the iron atom oscillates during oxidation and reduction

cytochromes

The only cytochrome that is NOT a dehydrogenase

cytochrome oxidase

Hydroperoxidases use ________ as substrate

hydrogen peroxide or an organic peroxide

Function of hydroperoxidases

protect the body against harmful peroxides
Peroxide accumulation can lead to generation of free radicals which can disrupt membranes and cause diseases including cancer and atherosclerosis

Where are peroxidases found?

milk
leukocytes
platelets

MECHANISM OF OXIDATION AND REDUCTION OF NICOTINAMIDE COENZYMES

�There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AH2.
�One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H-) and is transferred to the 4 position, w

catalyzes an intermediate in the oxidative decarboxylation of pyruvate and alpha-ketoglutarate)

Dihydrolypoyl dehydrogenase

Which cytochromes are classified as dehydrogenases?

Cytochromes (b, c1, c)

Function of Cytochromes b, c1, and c

They carry electrons from the flavoproteins to the cytochrome oxidase.

2 categories of HYDROPEROXIDASES

Peroxidases and Catalase

Function of peroxidases

Peroxidases reduce peroxides using various electron acceptors

Peroxidase prosthetic group

protoheme

In erythrocytes and other tissues __________, containing Se as a prosthetic group catalyzes destruction of H2O2.

glutathione peroxidase

__________ uses H2O2 as electron donor and electron acceptor.

CATALASE

_________ found in many tissues including the liver are rich in oxidases and in catalase.

Peroxisomes

Catalyze synthesis or degradation of many different types of metabolites in a 2-step process.

OXYGENASES

2 subgroups of oxygenases

(1) dioxygenases
(2) monooxygenases

How do oxygenases work?

Two step process. (1) O2 bound to the active site of the enzyme and (2) bound O2 is then reduced or transferred to the substrate.

_________ incorporate both atoms of O2 into the substrate

Dioxygenases

Mixed function oxidases, hydrolases; introduce only one atom of O2 into the substrate. The other atom of O2 is reduced to H2O and an electron donor is required for this purpose

monooxygenases

A-H + O2 + Z-H2 A-OH + H2O + Z

monooxygenase

The rate of _________ of many medicinal drugs by cytochrome P450 determines the duration of their action.

detoxification

Which co-enzyme is used by an enzyme system that reduces a substrate by transferring hydrogen's from one substrate to O2?

FAD

Xanthine oxidase is used for which reaction?

Oxidation of purine

Which enzyme catalyzes this reaction....
2H2O2 ? 2H2O2 + O2

catalase

Cytochrome P450 is classified as ____________________.

monooxygenase

To decompose H2O2 to H2O, which enzyme would you use?

Hydroperoxidase

How many molecules (or moles) of CO2 are formed in one Citric acid cycle?

2

In aerobic glycolysis, Glucose is converted to what molecule?

pyruvate

Enzyme : L-Aminoacid oxidase is used for which reaction?

Oxidative deamination of amino acids

Which enzyme catalyzes this reaction...
H2O2 + AH2 ? 2H2O + A

glutathione peroxidase

Enzymes which protect aerobic organisms against oxygen toxicity

Superoxide Dismutase

How is the superoxide free radical generated?

by the transfer of a single electron to O2

True or false: Superoxide dismutase can act as both an oxidizing as well as a reducing agent.

true

How is superoxide formed?

when reduced flavins are oxidized by O2

Superoxide can oxidize ______.

cytochrome c