Metabolism
Cellular Biochemistry
and Metabolism 1
(CLS 331)
Dr. Samah Kotb
Lecturer of Biochemistry
2015
Metabolism
Metabolism is the study of the chemical reactions that occur
inside living cells. Updates over 2000 such reactions have been
documented. Although this is a large number, there is a high
degree of organization and order that govern these reactions.
Metabolic reactions do not occur at random inside cells but take
the shape of chains or series known as Metabolic Pathways.
 Some pathways are short made of 2 or 3 reactions or long
made of 20, 30 or 40 reactions.
Metabolism
• A metabolic pathway can be defined as a specific
sequence of enzymatically catalyzed reactions in
which the product of one reaction serves as the
substrate of the next reaction in the chain. The
following is an example of a metabolic pathway
made of six reactions:-
Shapes of Metabolic Pathways
A. Linear
B. Branched
C. Cyclic
Types of Pathways
There are 2 types of metabolic pathways that occur
inside cells:-
1) Catabolic Pathways
These are made of reactions through which energy rich
nutrient molecules are broken down by chemical
reactions into simple end products. As a result of
catabolic pathways energy is produced and released to
the cell.
Types of Pathways
• 1) Catabolic Pathways
Types of Pathways
2) Anabolic Pathways:
These are pathways made of reactions that result in
the synthesis of biomolecules using basic unit
components.
These
biomolecules
are
either
biologically or structurally important to the cells.
Anabolic reactions & pathways require an input of
energy to take place.
• 2) Anabolic Pathways:
•
Link between Catabolism & Anabolism:
Stages of catabolism
Stages of catabolism
1. Stage1: Digestion:- Hundreds of proteins and many kinds of
polysaccharides & lipids are broken down chemically or
enzymatically into their building block units.
2. Stage 2: Intracellular:- The building block units arrive inside
cells from the blood. They are degraded via Catabolic Pathways
into a common product namely acetyl-CoA.
3. Stage 3: Intracellular:- Acetyl-CoA is completely degraded
into CO2 and H2O via reactions of the TCA cycle.
The energy of Catabolism
• The energy of catabolism is used by cells to drive
various activities that require expenditure of energy.
Regulation
of Cellular Metabolism
Cells use 3 mechanisms to regulate (speed up or slow
down) the rate of metabolic activity:-
1) Allosteric enzymes.
2) Availability of enzymes.
3) Hormonal regulation.
Metabolism
and biochemical pathways
• 1) Allosteric Regulation:
This is done by the use of allosteric enzymes which are usually key
enzymes of metabolic pathway.
 Structure of allosteric enzymes:Enzymes are globular proteins made of peptide chains
that have folded on each other in a specific manner that
results in the formation of a globular macro protein with
an active site at the periphery of the protein molecule.
Regulation
of Cellular Metabolism
• 1) Allosteric
Regulation:
Allosteric regulation mechanism
Allosteric modulators are of 2 types:i. An allosteric stimulator module of the enzyme.
ii. An allosteric inhibitor molecule of the enzyme.
Allosteric regulation mechanism
• As the pathway occurs at a fast rate there will be
accumulation of the end product G. When the levels
of G reach a certain high level within the cell it will
act as an allosteric inhibitor of E1 by binding to the
allosteric site of the enzyme. Thus the catalysis of
the reaction A  B will be switched off thereby
slowing down the whole pathway. Note that E1 is
the key enzyme of the overall pathway.
Allosteric regulation mechanism
 With time the levels of G will start dropping as it
leaves the cell. Thus the allosteric inhibition of E1 is
removed and the enzyme restores its catalytic
activity.
 In other cases of allosteric inhibition the end product
is not involved. Instead a side product of the pathway
acts as the allosteric modulator. In many cases this
side product is the ATP/ADP level inside the cell.
Allosteric regulation mechanism
Regulation of Cellular Metabolism
2) Availability of enzymes
Upon consumption of a meal rich in carbohydrate and as digestion in
the GIT occurs, large amounts of glucose will be absorbed into the
blood. As a result there will be an influx of glucose into cells. Cells
react by synthesizing larger than usual amounts of glucose degradative
enzymes in order to increase the rate of its catabolism. The levels of
amino acids (a.a.) catabolic enzymes or fatty acids (f.a.) catabolic
enzymes stays normal. If the meal was rich in proteins, a.a. catabolic
enzymes will be synthesized in large amounts.
Regulation of Cellular Metabolism
3) Hormonal Regulation:
•Hormones are organic type molecule synthesized and secreted by
special type tissues known as endocrine glands.
• The hormones pour from the gland directly in to the blood and are
transported in the blood until they reach what is known as target
tissue cells.
• The hormone is recognized by its target cells; by way of receptors
present the outer surface of the cell membrane. As a result of the
binding between the hormone and its receptor a series of chemical
reactions inside the cell occurs that result in either an increase or a
decrease in the rate of a certain biochemical pathway.
Regulation of Cellular Metabolism
3) Hormonal Regulation
Regulation of Cellular Metabolism
3) Example for the Hormonal Regulation
Adrenaline is a hormone synthesized and secreted by the
adrenal medullary cells. Adrenaline leaves the gland and is
transported in the blood until it reaches its target cells which
are liver and muscle cells. As the hormone binds to its target
cells large amounts of an intracellular secondary messenger
(Cyclic AMP) are synthesized. This will cause an increase in
the catabolic pathway responsible for breaking down
Glycogen into glucose (G).
Regulation of Cellular Metabolism
• The extra glucose leaves muscle and liver & is
distributed to the various tissues. Adrenaline is
released from the adrenal medulla under conditions
of emotional or physical stress. The body under
such conditions requires extra catabolism of glucose
for extra energy production.
Regulation of Cellular Metabolism
Other Hormones include:Insulin synthesized and secreted by the β cells of the
pancreas. Its target cells include muscle & adipose
tissue.
It acts to lower plasma glucose levels by:
1. Increase the rate of glycolysis.
2. Increase the rate of glycogenesis.
3. Increase the rate of entry of glucose into cells.