Understanding Proteolytic Cleavage: The Activation of Chymotrypsin
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Introduction
Proteolytic cleavage is a vital biochemical process that underpins the functionality of many digestive enzymes in our body. In this article, we'll focus on chymotrypsin, a critical digestive enzyme that exemplifies the importance of proteolytic activation. We'll delve into how chymotrypsin is synthesized, its inactive form as chymotrypsinogen, and the intricate mechanism of its activation through proteolytic cleavage.
What is Chymotrypsin?
Chymotrypsin is a serine protease, specifically known for breaking peptide bonds at the carboxyl side of certain amino acids with bulky, hydrophobic aromatic side chains. This specificity is crucial for the proper digestion of proteins we consume.
Zymogen Form of Chymotrypsin
Before discussing its activation, it's essential to address that chymotrypsin is initially produced as an inactive zymogen known as chymotrypsinogen. This zymogen consists of a single polypeptide chain made up of 245 amino acids. However, chymotrypsinogen lacks a properly formed active site; thus, it cannot carry out its intended digestive functions.
The Role of the Pancreas in Chymotrypsin Production
Chymotrypsinogen is produced in the pancreas within specialized cells called acinar cells. These acinar cells synthesize zymogens and store them in membrane-bound granules.
Exocytosis of Zymogens
When stimulated by hormones or action potentials, these granules undergo exocytosis, releasing zymogens into the pancreatic duct, which subsequently empties into the duodenum (the first part of the small intestine).
Activation of Chymotrypsinogen through Proteolytic Cleavage
The transition from inactive chymotrypsinogen to active chymotrypsin is a classic example of proteolytic activation.
Role of Trypsin
The activation process is initiated by trypsin, another active digestive enzyme. Trypsin cleaves a specific peptide bond in chymotrypsinogen to form pi chymotrypsin. Here's how the activation occurs:
- Cleavage: Trypsin cleaves the bond between arginine (15) and isoleucine (16) in chymotrypsinogen, producing pi chymotrypsin.
- Formation of Pi Chymotrypsin: This newly formed pi chymotrypsin is still not the fully functional enzyme. Its next step involves interacting with other pi chymotrypsin molecules.
- Further Cleavage: Pi chymotrypsin cleaves several other peptide bonds within its structure, removing dipeptides and resulting in three active chains connected by disulfide bonds.
Formation of Alpha Chymotrypsin
The culmination of these interactions results in the formation of alpha chymotrypsin, the fully active form of the enzyme.
- Disulfide Bond Formation: The three chains are held together by disulfide bonds, allowing the active site and the oxyanion hole to form correctly.
- Active Site Configuration: The localized conformational changes lead to the stabilization of the tetrahedral intermediate, essential for its proteolytic function.
Importance of Proteolytic Activation
The process of proteolytic cleavage is crucial as it transforms inactive enzymes into their active forms, ready to carry out their roles in digestion. This method of activation is not limited to chymotrypsin alone; several digestive enzymes require such activation to function effectively.
Trypsin as the Master Activator
Remarkably, trypsin itself is also activated through proteolytic cleavage. An enzyme known as enterokinase (or enterpeptidase) activates trypsin from its zymogen form, trypsinogen. Once activated, trypsin can:
- Activate itself and other zymogens, amplifying the digestive process.
- Activate various digestive enzymes, including proelastase and procarboxypeptidase.
Conclusion
In summary, the proteolytic cleavage of chymotrypsinogen is an exemplary process illustrating how digestive enzymes are activated in our body. Understanding this process is crucial for appreciating how our body digests proteins efficiently. Chymotrypsin is just one of the many digestive enzymes, and recognizing the role of each allows for a deeper insight into our digestive system's functionality. With trypsin serving as a master activator, the cascade of activation illustrates the intricate nature of our enzymatic processes that ensure the proper breakdown of dietary proteins.
in our introductions who proteolytic cleavage we mentioned that digestive enzymes are examples of enzymes found
inside our body which are activated proteolytically and so this is what I'd like to focus on in this lecture and I'd
like to begin by focusing on a specific digestive enzyme found inside our body known as chymotrypsin now actually we
already spoke about chymotrypsin in detail when we discuss proteases and we said that chymotrypsin is actually an
example of a serine protease that breaks peptide bonds on the carboxyl side of specific amino acids those amino acids
that contain bulky hydrophobic aromatic side chains now chymotrypsin is initially synthesized in its zymogen
form in the inactive form and the zymogen form of chymotrypsin is known as chymotrypsinogen and chymotrypsinogen is
a single polypeptide chain that consists of 245 individual amino acids now the the chymotrypsinogen is not fully
functional in fact it's not functional at all and that's because the active side and the oxyanion hole of this
particular zymogen is not yet formed it's not in the proper conformation to be able to actually fit this substrate
molecule so what has to happen is this chymotrypsinogen has to actually be activated proteolytically and we'll see
how that happens in just a moment first let's actually discuss where the chymotrypsinogen is formed so if we
study the pancreas of our body in the pancreas we're going to find these special cells exocrine cells known as
acinar cells and it's the acinar cells of the pancreas which are responsible for forming this time--as trypsinogen as
well as other digestive zymogens and all these imagens are essentially stored in membrane bound organelles
membrane-bound granules shown in green and so all these granules that contain the zymogens basically accumulate on the
apex side of these as in ourselves and when the cell is stimulated by some type of hormone or some type of action
potential these granules basically exit the cell via exocytosis and they release all these zymogens
into the duct and then the dog basically empties out into larger duct which eventually empties out into the
pancreatic duct and it's the pancreatic duct that connects directly to the initial portion of the small intestine
we call the duodenum and once these imagens are inside the intestine they only begin to cleave those proteins when
the zymogens are activated into their fully functional form so the question is how exactly is chymotrypsinogen actually
activated proteolytically well as it turns out interestingly enough it's actually another active digestive enzyme
known as trypsin and other proteins that is responsible for activating chymotrypsinogen into its active form
chymotrypsin so let's take a look at how that actually takes place by looking at the following diagram so in Part A we
basically have that in active zymogen the chymotrypsinogen are noticed it consists of 145 individual amino acids
so this is not functional because it's active side does not have the correct orientation and the oxyanion hole that
is used to basically stabilize the tetrahedral intermediate is not formed and so what must happen is to actually
activate the enzyme activity of this molecule trypsin and active form another digestive enzyme basically must clean at
a single peptide bond this inactive chymotrypsinogen and so what it does is it Cleaves the peptide bond between the
amino acid is I solution so the bond holding these two amino acids is cleaved by trypsin and this activates this
zymogen to form something we call pi chymotrypsin now pi chymotrypsin is not yet the fully functional enzyme what pi
chymotrypsin does is it goes on to other pi chymotrypsin molecules and in cleaves those molecules at several sites and
what it does is it ultimately removes Todai peptides from this molecule so it removes a dipeptide from this region to
basically remove two amino acids that's why we go from 15 to 213 and we also cleave in this section and we remove
untie peptide and so that's why we have two amino acids missing in this section and so once we form these three
individual chains these three individual chains are held together by disulfide bonds and now the active site takes the
proper confirmation and the oxyanion hole that is used to tow to us stabilize that tetrahedral intermediate it takes
on that perfect form so that once the active site is formed it can actually fit that substrate intermediate and once
the reaction takes place the tetrahedral intermediate can be stabilized by that fully formed oxyanion hole so once again
we see that trypsin Cleaves the peptide between the peptide bond between the arginine 15 and the isoleucine 60
producing this act of pi chymotrypsin now this act of pi chymotrypsin goes on reacts with another pi chymotrypsin and
that removes two dipeptides to produce a total of three individual chains and these three chains which are held
chymotrypsin molecule we call alpha chymotrypsin now what's so different between the act of alpha chymotrypsin
and the inactive chymotrypsinogen well as it turns out the active side and the oxyanion hole are not formed correctly
in this zymogen form and what that proteolytic cleavage does is it allows for a localized conformational change to
basically take place within this region and as a result of that localized conformational change that basically
creates the proper conformation of the active side and also creates that oxyanion hole that is needed to
stabilize the tetrahedral intermediate that is formed in that proteolytic reaction that kind of trypsin actually
carries out so we see that proteolytic activation of chymotrypsinogen causes a local conformational change that allows
the active side and the oxyanion hole to actually form so we conclude that by proteolytically cleaving this inactive
chymotrypsinogen so the entire structure of this chymotrypsin doesn't actually change too much but because of a small
localized change in this section of that enzyme that creates a perfect active site that can fit the substrate molecule
and also creates the oxy and whole that will be used by the chymotrypsin to basically stabilize and decrease that
transition state that is formed in that proteolytic reaction that is carry out that is carried out by the digestive
enzyme chymotrypsin now this is only one of the many different types of digestive enzymes that exist inside our body and
the reason we have these different digestive enzymes is because each digestive enzyme has a slightly
the different peptide bonds that are found within the proteins that we actually ingest and the interesting
thing about the trypsin molecule that we discussed earlier trypsin doesn't only activate the chymotrypsinogen it also
activates many others i'm engines and so in a way we can imagine that trypsin is actually the master activator which is
responsible for actually activating the majority of the zymogens found inside our body now the question is what
activates the trypsin itself well the cells of our body basically produce a special type of enzyme known
as an Terra peptidase so it's the intera peptidase that is produced by our body that actually activates trypsin from
trypsinogen remember trypsinogen is the zymogen the inactive form of trypsin and when antara peptidase basically
proteolytically cleaved a bond in trypsin the entire structure of the trypsinogen or not trypsin trypsinogen
the entire structure of the trypsinogen changes and that creates the proper conformation of the active site that now
allows that trypsin to basically carry out its activity and what trips and does is it activates not only for other
different zymogen but it also activates itself trips and once activated basically goes on to nearby trypsinogen
molecules and activates them to produce trypsin so this is an amplification effect and trypsin can also go on to
activate Pro elastase in two last days chymotrypsinogen ensue chymotrypsin which we spoke about just a moment ago
pro lipase which activates lipase and lipase is used to basically break down the lipids that we ingest into our body
and finally pro carboxy peptidase into carboxy peptidase so we see that trypsin the master activator that
proteolytically activates the majority of the adjust of enzymes including itself and trips in itself is activated
by antara peptidase and it's the collective activity the collective action of all these enzymes found inside
our small intestine and the stomach that allows us to actually break down all different types of peptide bonds and all
different types of proteins that we ingest into our body so we see that enzymes can be controlled not only
allosteric ly and not only via covalent modification methods but also via proteolytic activation by using the