Digestion and Catabolism of Proteins

Dietary proteins undergo initial breakdown in the stomach by pepsinogen, an inactive proenzyme that gets converted to active pepsin in the presence of HCl.

HCl, secreted by parietal cells, plays a critical role in protein digestion by denaturing the proteins, making them more susceptible to enzymatic attack.

Endopeptidases cleave internal peptide bonds in proteins, while exopeptidases break peptide bonds at the ends, generating smaller polypeptides and individual amino acids.

The digestion of proteins starts in the stomach with pepsin, continues with pancreatic enzymes, and culminates in the small intestine with the release of di- and tripeptides and individual amino acids for absorption.

Unlike fats and carbohydrates, the body cannot store excess amino acids. These excess amino acids are promptly broken down.

Amino acids serve as fundamental building blocks for various crucial molecules, including hormones, neurotransmitters, and signaling molecules.

The collection of amino acids readily available in the body is referred to as the amino acid pool. This pool is constantly replenished by dietary intake and protein breakdown, and utilized for biosynthesis and energy production.

Catabolism of amino acids involves breaking down their structure for energy production and to synthesize other important molecules.

Pancreatic enzymes are initially produced as inactive precursors called zymogens. Their activation is triggered by a cascade of events initiated by enteropeptidase.

Enteropeptidase, an enzyme present in the small intestine, activates trypsinogen to trypsin. Trypsin then activates other pancreatic zymogens, creating a chain reaction.

Deficiencies in pancreatic enzyme secretion, due to conditions like pancreatitis or cystic fibrosis, lead to impaired digestion of fat and protein. This results in steatorrhea, the presence of excess fat in the stool.

Celiac disease is an autoimmune disorder where the small intestine is damaged by gluten, a protein found in wheat, barley, and rye. This damage impairs nutrient absorption.

Aminopeptidase, an enzyme on the surface of intestinal cells, breaks down oligopeptides into smaller peptides and free amino acids.

Most free amino acids are transported into intestinal cells using a sodium-dependent secondary active transport mechanism. Di- and tripeptides are taken up by a proton-linked transporter (PepT1).

Branched-chain amino acids (BCAAs) are not metabolized by the liver and are instead sent to muscle tissue for use as fuel.

Defects in amino acid transport systems in the small intestine and kidneys can lead to conditions like cystinuria, where specific amino acids cannot be absorbed properly.

A chemical reaction that involves the transfer of an amino group from an amino acid to a keto acid, producing a new amino acid and a new keto acid.

An enzyme that catalyzes the transfer of an amino group from one molecule to another.

The process of removing an amino group from an amino acid, releasing ammonia and forming a keto acid.

An enzyme that catalyzes the oxidative deamination of glutamate, releasing ammonia and forming alpha-ketoglutarate.

This amino acid is unique because it is the only one readily undergoing oxidative deamination.

A process where a molecule gains an amino group from a donor molecule.

The enzyme responsible for transferring an amino group from alanine to α-ketoglutarate, forming pyruvate and glutamate. It is found mainly in the liver but also in other tissues like the kidney, muscles, and heart.

The enzyme responsible for transferring an amino group from aspartate to α-ketoglutarate, forming oxaloacetate and glutamate. It is found in the heart, skeletal muscles, liver, kidney, and red blood cells.

The amino acid pool is the total amount of free amino acids available in the body. These amino acids are sourced from dietary proteins (exogenous) and the breakdown of body proteins (endogenous).

The amino acid pool is depleted when amino acids are used for various purposes such as protein synthesis, energy production, and the formation of other biomolecules.

Protein turnover refers to the continuous process of protein synthesis and degradation. It ensures a constant supply of proteins and allows the body to remove damaged or unnecessary proteins.

The first step in amino acid catabolism involves removing the amino group, forming ammonia and a corresponding alpha-keto acid. The carbon skeleton is then used for energy production or biosynthesis.

Transamination is a critical biochemical reaction that transfers an amino group from an amino acid to an alpha-keto acid. This process forms a new amino acid and an alpha-keto acid.

The carbon skeletons of amino acids are broken down into intermediates that can be used in various metabolic pathways, leading to the production of energy, glucose, fatty acids, and ketone bodies.

The process of removing the nitrogen from amino acids is crucial for energy production. The nitrogen is then either incorporated into new molecules or excreted as urea.

The presence of the alpha-amino group protects amino acids from being readily broken down for energy, providing a reserve of building blocks.