Many enzymes exist as inactive forms known as zymogens or Proenzymes • proenzymes are synthesized as inactive precursors that are subsequently activated by cleavage of one or a few specific peptide bonds. • a energy source (ATP) is not needed for cleavage. contrast with reversible regulation by phosphorylation, even proteins located outside cells can be activated by this means. • Proteolytic activation, in contrast with allosteric control and reversible covalent modification, occurs just once in the life of an enzyme molecule i.e. the process is irreversible.

1. Proteolytic ActivationProteolytic Activation
Mechanisms and Examples

2. Zymogens
• Many enzymes exist as inactive forms known as
zymogens or Proenzymes
• proenzymes are synthesized as inactive precursors that
are subsequently activated by cleavage of one or a few
specific peptide bonds.
• a energy source (ATP) is not needed for cleavage.
contrast with reversible regulation by phosphorylation,
even proteins located outside cells can be activated by
this means.
• Proteolytic activation, in contrast with allosteric control
and reversible covalent modification, occurs just once in
the life of an enzyme molecule i.e. the process is
irreversible.
• Many enzymes exist as inactive forms known as
zymogens or Proenzymes
• proenzymes are synthesized as inactive precursors that
are subsequently activated by cleavage of one or a few
specific peptide bonds.
• a energy source (ATP) is not needed for cleavage.
contrast with reversible regulation by phosphorylation,
even proteins located outside cells can be activated by
this means.
• Proteolytic activation, in contrast with allosteric control
and reversible covalent modification, occurs just once in
the life of an enzyme molecule i.e. the process is
irreversible.

3. Chymotripsin
• Chymotrypsin is a digestive enzyme that hydrolyzes proteins
in the small intestine.
• Its inactive precursor, chymotrypsinogen, is synthesized in the
pancreas. The zymogens are synthesized in the acinar cells of
the pancreas and stored inside membrane-bounded granules.
• Chymotrypsinogen, a single polypeptide chain consisting of
245 aa residues, is devoid of enzymatic activity. It is converted
into a fully active enzyme when the peptide bond joining
Arg15 and Ile 16 is cleaved by trypsin to form π -
Chymotrypsin
• π - Chymotrypsin subsequently undergoes autolysis to
specifically excise two dipeptides, Ser 14–Arg 15 and Thr
147–Asn 148, thereby yielding the equally active enzyme α-
chymotrypsin
• Chymotrypsin is a digestive enzyme that hydrolyzes proteins
in the small intestine.
• Its inactive precursor, chymotrypsinogen, is synthesized in the
pancreas. The zymogens are synthesized in the acinar cells of
the pancreas and stored inside membrane-bounded granules.
• Chymotrypsinogen, a single polypeptide chain consisting of
245 aa residues, is devoid of enzymatic activity. It is converted
into a fully active enzyme when the peptide bond joining
Arg15 and Ile 16 is cleaved by trypsin to form π -
Chymotrypsin
• π - Chymotrypsin subsequently undergoes autolysis to
specifically excise two dipeptides, Ser 14–Arg 15 and Thr
147–Asn 148, thereby yielding the equally active enzyme α-
chymotrypsin

5. Conformational change in Chymotripsin
newly formed N-terminal Ile16 turns inward and forms an ionic bond with
Asp194 in the interior of the chymotrypsin. Protonation of this amino
group stabilizes the active form of chymotrypsin.
This electrostatic interaction triggers following conformational changes.
• Met 192 moves up which was deeply buried,
• residues 187 and 193 become more extended.
• substrate-specificity site for aromatic and bulky nonpolar groups forms.
• NH groups that forms hydrogen bond with substrate oxygen and stabilize
tetrahedral intermediate is not appropriately located in chymotrypsinogen,
and so the oxyanion hole is incomplete in the zymogen.
newly formed N-terminal Ile16 turns inward and forms an ionic bond with
Asp194 in the interior of the chymotrypsin. Protonation of this amino
group stabilizes the active form of chymotrypsin.
This electrostatic interaction triggers following conformational changes.
• Met 192 moves up which was deeply buried,
• residues 187 and 193 become more extended.
• substrate-specificity site for aromatic and bulky nonpolar groups forms.
• NH groups that forms hydrogen bond with substrate oxygen and stabilize
tetrahedral intermediate is not appropriately located in chymotrypsinogen,
and so the oxyanion hole is incomplete in the zymogen.

6. • The activation of trypsinogen : two-
stage process.
• Enteropeptidase, a single-pass
transmembrane serine protease that is
located in the duodenal mucosa,
specifically hydrolyzes trypsinogen’s
Lys 15-Ile 16 peptide bond at its N
terminal end, forming an hexapeptide.
Trypsin
• The activation of trypsinogen : two-
stage process.
• Enteropeptidase, a single-pass
transmembrane serine protease that is
located in the duodenal mucosa,
specifically hydrolyzes trypsinogen’s
Lys 15-Ile 16 peptide bond at its N
terminal end, forming an hexapeptide.
• Formed small amount of trypsin cleaves trypsinogen to form its
active form. I.e. tripsinogen activation is autocatalytic

7. Trypsin activates other proteases

8. Fibrinogen
• made up of three globular units connected by two rods. - 340-kd
• six chains: 2 each of Aα, Bβ, and ϒ. The rod regions are triple-stranded a-
helical coiled coils, a recurring motif in proteins.

9. Fibrinogen
B peptideA peptide
Thrombin
(Aα - Bβ – ϒ)2
cleaves four arginine-glycine
bonds in central globular region
Fibrin monomer
B peptideA peptide
18 residues,
(A α)2
20 residues
(B β)2
(αβϒ)2
fibrinopeptides

10. Thrombin
Gla Kringle Kringle Serine Protease
Gla Kringle Kringle Serine Protease
Arg 274 – Thr 275Factor Xa + Va
factor Xa is enhanced by binding to
activated Factor V (Va), termed the
prothrombinase complex. Prothrombin is
produced in the liver and is post-
translationally modified in a vitamin K-
dependent reaction that converts ten
glutamic acids on prothrombin to gamma-
carboxyglutamic acid (Gla). In the presence
of calcium, the Gla residues promote the
binding of prothrombin to phospholipid
bilayers .
Serine Protease
Arg 323 – Ile 324
factor Xa is enhanced by binding to
activated Factor V (Va), termed the
prothrombinase complex. Prothrombin is
produced in the liver and is post-
translationally modified in a vitamin K-
dependent reaction that converts ten
glutamic acids on prothrombin to gamma-
carboxyglutamic acid (Gla). In the presence
of calcium, the Gla residues promote the
binding of prothrombin to phospholipid
bilayers .

11. Other examples
• Some protein hormones are synthesized as inactive precursors. For example,
insulin is derived from proinsulin by proteolytic removal of a peptide.
• The fibrous protein collagen, the major constituent of skin and bone, is
derived from procollagen, a soluble precursor.
• Many developmental processes are controlled by the activation of
zymogens. For example, in the metamorphosis of a tadpole into a frog, large
amounts of collagen are resorbed from the tail in the course of a few days.
Likewise, much collagen is broken down in a mammalian uterus after
delivery. The conversion of procollagenase into collagenase, the active
protease, is precisely timed in these remodeling processes.
• Programmed cell death, or apoptosis, is mediated by proteolytic enzymes
called caspases, which are synthesized in precursor form as procaspases.
• Some protein hormones are synthesized as inactive precursors. For example,
insulin is derived from proinsulin by proteolytic removal of a peptide.
• The fibrous protein collagen, the major constituent of skin and bone, is
derived from procollagen, a soluble precursor.
• Many developmental processes are controlled by the activation of
zymogens. For example, in the metamorphosis of a tadpole into a frog, large
amounts of collagen are resorbed from the tail in the course of a few days.
Likewise, much collagen is broken down in a mammalian uterus after
delivery. The conversion of procollagenase into collagenase, the active
protease, is precisely timed in these remodeling processes.
• Programmed cell death, or apoptosis, is mediated by proteolytic enzymes
called caspases, which are synthesized in precursor form as procaspases.

12. Thank YouThank You