2. dehydrogenases:
there are a large number of enzymes in this class.
They perform two main functions:
1. transfer of hydrogen from one substrate to
another in a coupled oxidation- reduction
reaction. These dehydrogenases often utilize
common coenzymes (act as hydrogen carriers)
e.g. NAD+ and sine the reactions are reversible
this enable the reducing equivalents (H+) to be
freely transferred within the cell . this type of
reactions enable oxidative processes to occur in
the absence of oxygen such as during the
anaerobic phase of glycolysis.
2. as components in respiratory chain of electron
transport .
Note: dehydrogenases cannot use oxygen as a
hydrogen acceptor.
Dehydrogenases require a supportive co
enzymes to complete their reactions, these co
enzymes could be either nicotin amide
coenzymes or riboflavins co enzymes.
a. dehydrogenases using nicotin amide
coenzymes:
these dehydrogenases use nicotin amide adenine
dinucleotide (NAD+) or nicotinamide adenine
dinucleotide phosphate (NADP+ ) or both of
them.
These co enzymes are synthesized from the
vitamin B3 (niacin).
These co enzymes are reduced by a suitable
electron acceptor.
These co enzymes may freely and reversibly
dissociate from their respective apoenzymes.
Generally, NAD-linked dehydrogenases catalyze
oxidoreduction reactions in the oxidative
pathways of metabolism particularly in
glycolysis, in citric acid cycle and in
respiratory chain of mitochondria.
While, NADP-linked dehydrogenases are found
mainly in reductive syntheses , like extra
mitochondrial pathway of fatty acid synthesis,
steroid synthesis and also in the pentose
phosphate pathway.
b. dehydrogenases using flavin
coenzymes:
the flavin groups associated with these
dehydrogenases are similar to FMN and FAD
occurring in oxidases . they are generally more
tightly bound to their apoenzymes than are the
nicotinamide coenzymes. Most of the riboflavin
– linked dehydrogenases are concerned with
electron transport in the respiratory chain.
For example:
The cytochromes
The cytochromes are iron containing
hemoproteins in which the iron atom oscillates
between Fe+3 and Fe+2 during oxidation and
reduction . they are classified as
dehydrogenases (except for cytochrome
oxidase and cytochrome p450).
These cytochromes are important for the
transportation of electrons in the respiratory
chain ,e.g cytochrome b, c1 and c.
3. hydroperoxidase:
two types of enzymes found in animals and plant
fall into this category : peroxidases and
catalases.
a. Peroxidases:
Mainly found in leucocytes, platelets, and other
tissues.
In the reactions catalyzed by peroxidase ,
Hydrogen peroxide is reduced with the help of
different substances such as ascorbate
(vitamin C) and quinones .
The overall reaction is complex but can
summarized as in the following:
e.g. glutathione peroxidase , containing selenium
as prosthetic group.
This enzyme catalyses the destruction of H2O2
and lipid hydroperoxides through the
conversion of reduced glutathione to its
oxidized form , thus protecting membrane
lipids and hemoglobin against oxidation by
peroxides.
b. Catalases:
Is a hemo protein containing 4 heme groups.
In addition to possessing peroxidase activity, it is
also able to use one molecule of H2O2 as a
substrate electron donor and another molecule
of H2O2 as an oxidant or electron acceptor.
Catalase is found in blood, bone marrow,
mucous membranes, kidneys and liver.
It functions to destroy the H2O2 formed by the
action of oxidase
Note: peroxisomes are found in many tissues
including the liver.
They are rich in oxidases and in catalase. Thus .
the enzymes that producing H2O2 are grouped
with the enzyme that destroy it. however many
other biochemichal reactions can produce
H2O2 like xanthine oxidase which must be
considered as additional sources of H2O2.
4. Oxygenases
These are enzymes that are concerned with the
synthesis or degradation of different types of
metabolites, they catalyze the incorporation of
oxygen into a substrate molecules.
Oxygenases may be divided into two subgroups ,
dioxygenases and monooxygenases.
a. dioxygenases act by incorporating both atoms
of molecular oxygen into the substrate.
A + O2 → AO2
For example:L-tryptophan dioxygenase
(tryptophan pyrolase) and carotenase.
b. monooxygenases they act by incorporating
only one atom of molecular oxygen into the
substrate and the other oxygen atom is reduced
to water, an additional electron donor or
cosubstrate (Z) being necessary for this
purpose :
AH + O2 + ZH2 → AOH + H2O +
Z
Cytochromes P450 are monoxygenases
important for the detoxification of many drugs
and hydroxylation of steroids.
Cytochrome p450(CYP) are a family of heme
containing monooxygenases (more than 50
enzymes) , these enzymes are mainly located
in the endoplasmic reticulum in the liver and
intestine but are also found in the
mitochondria in some tissues.
The Cytochromes P450 of the endoplasmic
reticulum of the liver have an important role in
detoxification of variety of xenobiotics.
Mitochondrial Cytochromes P450 systems are
found in steroidogenic tissues such as adrenal
cortex , testis, ovary and placenta and are
concerned with the biosynthesis of steroid
hormones from cholesterol.