Control of
Metabolism
Chapter 4
Topics
1. Overview of metabolic control at various
level
2. Enzyme reactions and cofactors
3. Regulation of enzyme activities
4.1: Overview of metabolic
control at various levels
 Environment around bacteria is constantly
changing.
 So they adapt to limitations
 They developed complex mechanism for
regulation of catabolic and anabolic
pathways
 Bacteria exert control over their
metabolism at every possible stage.
 Starting at the level of the gene that
encodes for a protein and ending with
alteration or modifications in the protein
after it is produced.
Points for regulation of various metabolic processes.
Conditions Affecting Enzyme
Formation in Bacteria
• Bacterial cells can change patterns of enzymes to
adapt to their specific environment.
• The concentration of an enzyme in a bacterial cell
depends on the presence of the substrate for the
enzyme.
• Constitutive enzymes are always produced by cells
independently of the composition of the medium in
which the cells are grown.
• Example: The enzymes that operate during glycolysis
and the TCA cycle. They are present at around the
same concentration in cells at all times.
• Inducible enzymes are produced "turned on" in cells
in response to a particular substrate.
• They are produced only when needed.
• In the process of induction, the substrate, or a
compound structurally similar to the substrate,
evokes formation of the enzyme and is called an
inducer.
• Repressible enzymes synthesis are regulated or
"turned off" by the presence of (for example) the
end product of a pathway that the enzyme normally
participates in. In this case, the end product is called
a co-repressor of the enzyme.
4.2: Enzyme reactions and
cofactors
 Regulatory enzymes are usually the
enzymes that are the rate-limiting, or
committed step, in a pathway, meaning
that after this step a particular reaction
pathway will go to completion.
In bacterial cells, enzymatic reactions may be
regulated by two unrelated modes:
1. Control or regulation of enzyme activity (feedback
inhibition or end product inhibition), which mainly
operates to regulate biosynthetic pathways; and
2. Control or regulation of enzyme synthesis,
including end-product repression, which functions
in the regulation of biosynthetic pathways, and
enzyme induction and catabolite repression, which
regulate mainly degradative pathways.
Properties of Enzymes
• Biological catalysts
• Not consumed during a chemical reaction
• Speed up reactions from 1000 - 1020, with
a mean increase in rate of 100,000
• Exhibit stereospecificity  act on a single
stereoisomer of a substrate
• Exhibit reaction specificity  no waste or
side reactions
Enzymes provide a surface on which reactions take place
• Active site: the area on the enzyme surface where the
enzyme forms a loose association with the substrate
• Substrate: the substance on which the enzyme acts
• Enzyme-substrate complex: formed when the substrate
molecule collides with the active site of its enzyme
• Endoenzymes (intracellular)/exoenzymes (extracellular)
• Activation energy: the minimum energy required to
start a chemical reaction
• Transition state – the intermediate stage in a reaction in
which the old bonds break and new bonds are formed
Nomenclature
• Typically add “-ase” to name of substrate
e.g. lactase breaks down lactose (dissacharide of
glucose and galactose)
Classifications
IUBMB classifies enzymes based upon the class of organic
chemical reaction catalyzed:
1. Oxidoreductase - catalyze redox reactions; dehydrogenases,
oxidases, peroxidases, reductases
2. Transferases - catalyze group transfer reactions; often
require coenzymes
3. Hydrolases - catalyze hydrolysis reactions
4. Lyases - lysis of substrate; produce contains double bond
5. Isomerases - catalyze structural changes; isomerization
6. Ligases - ligation or joining of two substrates with input of
energy, usually from ATP hydrolysis; often called synthetases
or synthases
Energy Requirements of a
Chemical Reaction
Figure 5.2
Enzyme Components
• Apoenzyme: Protein
• Holoenzyme: Apoenzyme plus cofactor
• Cofactor: Nonprotein component (e.g.
magnesium, zinc)
• Coenzyme: Organic cofactor (Eg: NADH, FADH)
Many enzymes can catalyze a reaction only if
coenzymes, or cofactors are present.
Components of a Holoenzyme
Figure 5.3
The Parts of an Enzyme
Coenzymes
Many are derived from vitamins. Eg:
1. Niacin
NAD (Nicotinamide adenine dinucleotide)
2. Riboflavin
FAD (Flavin adenine dinucleotide)
3. Pantothenic Acid
CoEnzyme A
Mechanism
1.
2.
3.
4.
Substrate binding
Formation enzyme substrate complex
Production formation and dissociation
Enzyme recovery
The Mechanism of Enzymatic
Action
Figure 5.4a
Each substrate binds to an active site, producing an
enzyme-substrate complex. The enzyme helps a
chemical reaction occur, and one or more products are
formed
Factors Influencing Enzyme
Activity
1.
2.
3.
4.
5.
Temperature
pH
Substrate concentration
Enzyme concentration
Inhibitors
Temperature
• Enzymes are affected by heat
• Most enzymes have an optimum
temperature, near normal body
temperature at which they catalyze a
reaction most rapidly
• The rate at which an enzyme catalyzes a
reaction increases with temperature up to
the optimum T
Effect of Temperature on Enzyme
Activity
Figure 5.5a
pH
• Enzymes are affected by extremes of pH
• Even small pH changes can alter the
electrical charges on various chemical
groups in enzyme molecules, thereby
altering the enzyme’s ability to bind its
substrate and catalyze a reaction
• Enzymes catalyze a reaction most rapidly
at an optimum pH, near neutral,
Effect of pH on Enzyme Activity
Figure 5.5b
Substrate concentration
• Increasing the [substrate] increases the rate of
reaction (enzyme activity).
• enzyme saturation limits reaction rates. An
enzyme is saturated when the active sites of all
the molecules are occupied most of the time.
• At the saturation point, the reaction will not
speed up, no matter how much additional
substrate is added. The graph of the reaction
rate will plateau.
Effect of Substrate Concentration on
Enzyme Activity
Figure 5.5c
Enzyme concentration
• The higher the concentration of an
enzyme the greater should be the initial
reaction rate
• This will last as long as substrate present
Enzyme Inhibition
• Competitive inhibitor: A molecule similar in structure to a
substrate can bind to an enzyme’s active site and compete
with substrate
• Noncompetitive inhibitors: attach to the enzyme at an
allosteric site, which is a site other than the active site
• distort the tertiary protein structure and alter the shape of
the active site
• Feedback inhibition: regulates the rate of many metabolic
pathways when an end product of a pathway accumulates
and binds to and inactivates the first enzyme in the
metabolic pathway
Enzyme Inhibitors: Competitive
Inhibition
Figure 5.7a–b
Competitive inhibition of enzymes
Enzyme Inhibitors:
Noncompetitive Inhibition
Figure 5.7a, c
Noncompetitive (allosteric) inhibition of enzymes
4.3: Regulation of enzyme activities
Mechanisms
1. Enzyme modification
2. Enzyme denaturation
3. Isoenzyme regulation
4. Allosteric binding
5. Feedback inhibition
Enzymes modification
Can either activate it or inhibit it by altering the conformation
of the enzyme or by serving as a functional group in the active
site
Enzyme denaturation
Enzyme denaturation (cont’)
Isoenzyme regulation
• Enzymes that catalyze the same reaction
(catalytically and structurally similar) but
are encoded by different genes
•Glycogen phosphorylase-synthesize in
liver, brain and muscle-involves in
degradation of glycogen
•Isoenzymes = isoforms
Allosteric regulation of enzyme
activity
• Allosteric enzyme: an oligomer whose biological
activity is affected by other substances binding
to it
• these substances change the enzyme’s activity
by altering the conformation(s) of its
4°structure
• Allosteric regulators generally act by
increasing or decreasing the enzyme’s affinity
for the substrate
• The key to allosteric behavior is the existence of
multiple forms for the 4°structure of the enzyme
• allosteric effector: a substance that modifies
the 4° structure of an allosteric enzyme
• homotropic effects: allosteric interactions that
occur when several identical molecules are
bound to the protein.
• heterotropic effects: allosteric interactions
that occur when different substances are
bound to the protein.
• A change in conformational structure at one location of
a multisubunit protein that causes a conformational
change at another location on the protein
• Effectors – i) serves as stimulants to enzyme (+ve
effectors)
= increase catalytic activity
– ii) inhibitors (-ve effectors) to enzyme
= reduce/inhibit catalytic activity
- Act by reversible, noncovalent binding to a site on the
enzyme
Homotropic allosterism
Heterotrophic allosterism
Enzyme inhibition: Feedback
inhibition
• Product (usually ultimate product) of a pathway
controls the rate of synthesis through inhibition
of an early step (usually the first step)
• to conserve material and energy by preventing
accumulation of intermediates
Enzyme Inhibitors: Feedback
Inhibition
Figure 5.8
End of chapter