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Carboxypeptidase – Function, Types and Significance

Carboxypeptidase is a digestive enzyme that breaks down proteins by cleaving amino acids from the C-terminal end of a polypeptide chain. It plays a key role in protein digestion in the small intestine.

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Things worth knowing about "Carboxypeptidase"

Carboxypeptidase is a digestive enzyme that breaks down proteins by cleaving amino acids from the C-terminal end of a polypeptide chain. It plays a key role in protein digestion in the small intestine.

What is Carboxypeptidase?

Carboxypeptidase refers to a group of enzymes classified as exopeptidases. They cleave individual amino acids from the C-terminal end (the carboxyl end) of a polypeptide chain. This process is a critical step in the breakdown of dietary proteins within the human digestive system.

Types of Carboxypeptidase

Several types of carboxypeptidases exist, differing in their substrate specificity and biological origin:

Carboxypeptidase A: Produced by the pancreas, it preferentially cleaves amino acids with aromatic or aliphatic side chains, such as phenylalanine, tyrosine, and tryptophan.
Carboxypeptidase B: Also of pancreatic origin, it preferentially cleaves basic amino acids such as arginine and lysine.
Carboxypeptidase C and D: These types are found predominantly in plants and other non-mammalian organisms.

Mechanism of Action

Carboxypeptidases function as zinc metalloproteinases, meaning a zinc ion (Zn²⁺) at the active site of the enzyme is essential for catalytic activity. The zinc ion activates a water molecule, which then hydrolyzes the peptide bond at the C-terminal end of the substrate. This mechanism allows the sequential release of individual amino acids from the polypeptide chain.

Production and Activation

Carboxypeptidases A and B are synthesized in the pancreas as inactive precursors called zymogens (procarboxypeptidase A and B). This inactivation protects the pancreatic tissue from the self-digesting effects of the enzymes. Once secreted into the duodenum (the first part of the small intestine), the zymogens are activated by the enzyme trypsin, converting them into their active forms.

Function in the Digestive System

During protein digestion, carboxypeptidases work in close coordination with other digestive enzymes:

Endopeptidases such as pepsin (in the stomach) and trypsin and chymotrypsin (in the small intestine) first cleave long protein chains into shorter peptides.
Carboxypeptidases then act on these shorter peptides, releasing individual amino acids from their C-terminal ends.
The released amino acids are subsequently absorbed through the intestinal mucosa into the bloodstream and used by the body for the synthesis of proteins and other metabolic processes.

Clinical Significance

Carboxypeptidases are not only important for digestion but also play roles in various other biological processes:

Blood pressure regulation: Angiotensin-converting enzyme (ACE), which belongs to the carboxypeptidase family, is involved in converting angiotensin I to angiotensin II, thereby regulating blood pressure.
Wound healing and fibrinolysis: Carboxypeptidase B (also known as thrombin-activatable fibrinolysis inhibitor, TAFI) plays a role in the regulation of blood coagulation.
Pancreatic disease: In pancreatitis (inflammation of the pancreas), premature activation of carboxypeptidase zymogens within the pancreas can lead to self-digestion of the organ.
Cancer research: Carboxypeptidases are being investigated as potential target molecules in antibody-directed enzyme prodrug therapy (ADEPT).

Laboratory Diagnostics

Measuring carboxypeptidase activity in the blood or stool can be diagnostically relevant in certain clinical contexts, such as suspected exocrine pancreatic insufficiency or chronic pancreatitis. Reduced levels may indicate impaired enzyme production by the pancreas.

References

Stryer, L., Berg, J. M., Tymoczko, J. L. (2015). Biochemistry. 8th edition. W. H. Freeman and Company.
Whitcomb, D. C. & Lowe, M. E. (2007). Human Pancreatic Digestive Enzymes. Digestive Diseases and Sciences, 52(1), 1–17. PubMed PMID: 17211729.
World Health Organization (WHO). Proteins and Amino Acids. Available at: https://www.who.int

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