Heavy metal poisoning is an example of
noncompetitive inhibitor of an enzyme.
What type of bonds are affected
by heavy metal ions?
Irreversible Inhibition
Covalent bonds make a permanent change to
enzyme’s active site shape.
e.g. organophosphate insecticides & nerve gases.
Enzyme is permanently inhibited.
Sarin nerve gas
Foreign vs.Normal Inhibition
• Foreign: reversible & irreversible
– Caused by the influence of agents foreign
to the normal cells
• Normal: allosteric enzymes
– Regulation by “normal” cell components
Allosteric regulation of Enzyme Activity
Allosteric enzymes have 2 or more protein chains
(o4 structure)
And 2 kinds of binding sites:
1. Substrate binding site = active site
2. Regulator binding site - molecule binding here
alters structure of active site
Reversible
and
rapid in
action
Regulators bind to a specific site on the enzyme
called the allosteric site
positive regulator
negative regulator
Feedback control is a common mechanism for
allosteric activity
enzyme 1
enzyme 2
enzyme 3
A ----> B ----> C ----> D
Product D is a negative regulator of enzyme 1, so...
Feedback & Feedforward control:
Activation or inhibition of the 1st reaction
in a reaction sequence is controlled by
a product of the reaction sequence.
Feedback animation
In positive feedforward, earlier reactants in a
metabolic reaction sequence
feed-forward positively on later steps
If A is accumulating,
speed up down stream products to use “A” up.
The enzyme phosphofructokinase (PFK)
is regulated by:
• High concentrations of ATP, which inhibit PFK
• High concentrations of ADP, which stimulate PFK
• High concentrations of AMP, which stimulate PFK
• High concentrations of citrate, which inhibit PFK
ATP and citrate are allosteric negative regulators
ADP and AMP are allosteric positive regulators.
DNA Digestion lab animation
Restriction Fragment Lengths Polymorphism
Early genetic splicing experiment
Polymerase Chain Reaction
Another mechanism of regulating enzyme activity is to
keep dangerous enzymes in compartments where
access to their substrates is limited.
e.g. proteolysis of cell proteins by enzymes is
controlled by keeping these enzymes within lysosomes
Or, take a proteolytic enzyme
(dangerous because they break peptide bonds)
and keep it in an inactive form until needed.
The inactive form of a proteolytic enzyme
is called a proenzyme or
a zymogen
Digestive and blood clotting enzymes
are kept this way until needed,
then they are activated!
Covalent modification: enzyme activity controlled by
other enzymes or chemicals
Enzyme exists in 2 states:
modified (e.g. phosphorylated) = “activated”
unmodified (e.g. unphosphorylated) = “inactivated”
Here the proteolytic enzyme zymogen form
is activated by HCl acid in the stomach:
e.g. pepsinogen is converted to pepsin when needed.
Conversion of zymogen to proteolytic enzyme often involves
removal of a polypeptide chain segment from the zymogen
structure.
Bacterial infection (pathogens) account
for many severe diseases world-wide.
Examples:
Tuberculosis and Leprosy
Staph and Strep
Syphilis
Bubonic plague
Cholera and typhoid fever
Gastroenteritis
Urinary tract and Wound infections
Many sexually transmitted diseases
Much research on fighting bacterial infections
revolves around inactivating bacterial enzymes.
Earliest “antibiotics” were the Sulfa drugs
Antibiotics kill or stop the growth of bacteria.
Sulfanilamide is a structural mimic of
PABA (p-aminobenzoic acid) which is
needed by bacteria to make a
coenzyme called Folic acid.
Humans get Folic acid from diet, don’t
use PABA.
Structures of selected antibiotic
sulfa drugs in use today
PABA
Penicillins
Discovered accidentally by Fleming in 1928
Developed ten years later into penicillin products
Inhibit enzyme: transpeptidase used to strengthen
polysaccharide strands in gram + bacterial cell walls.
Structures of selected penicillins in use today
The selective binding of penicillin to
the active site of transpeptidase.
Disorders and diseases frequently “leak” organ
enzymes into blood and urine.
Doctors utilize this for diagnoses of various ailments.