Protein Processing Digestion Absorption and Metabolism

Protein digestion begins when the food reaches the stomach and stimulates the release of hydrochloric acid (HCl) by the parietal cells located in the gastric mucosa of the GI (gastrointestinal) tract. Hydrochloric acid provides for a very acidic environment, which helps the protein digestion process in two ways: (1) through an acid-catalyzed hydrolysis reaction of breaking peptide bonds (the chemical process of breaking peptide bonds is referred to as a hydrolysis reaction because water is used to break the bonds); and (2) through conversion of the gastric enzyme pepsinogen (an inactive precursor) to pepsin (the active form). Pepsinogen is stored and secreted by the "chief cells" that line the stomach wall. Once converted into the active form, pepsin attacks the peptide bonds that link amino acids together, breaking the long polypeptide chain into shorter segments of amino acids known as dipeptides and tripeptides. These protein fragments are then further broken down in the duodenum of the small intestines. The brush border enzymes, which work on the surface of epithelial cells of the small intestines, hydrolyze the protein fragments into amino acids.

The cells of the small intestine actively absorb the amino acids through a process that requires energy. The amino acids travel through the hepatic portal vein to the liver, where the nutrients are processed into glucose or fat (or released into the bloodstream). The tissues in the body take up the amino acids rapidly for glucose production, growth and maintenance, and other vital cellular functioning. For the most part, the body does not store protein, as the metabolism of amino acids occurs within a few hours.

Amino acids are metabolized in the liver into useful forms that are used as building blocks of protein in tissues. The body may utilize the amino acids for either anabolic or catabolic reactions. Anabolism refers to the chemical process through which digested and absorbed products are used to effectively build or repair bodily tissues, or to restore vital substances broken down through metabolism. Catabolism, on the other hand, is the process that results in the release of energy through the breakdown of nutrients, stored materials, and cellular substances. Anabolic and catabolic reactions work hand-in-hand, and the energy produced in catabolic processes is used to fuel essential anabolic processes. The vital biochemical reaction of glycolysis (in which glucose is oxidized to produce carbon dioxide, water, and cellular energy) in the form of adenosine triphosphate, or ATP, is a prime example of a catabolic reaction. The energy released, as ATP, from such a reaction is used to fuel important anabolic processes, such as protein synthesis.

The metabolism of amino acids can be understood from the dynamic catabolic and anabolic processes. In the process referred to as deamination, the nitrogen-containing amino group (NH2) is cleaved from the amino acid unit. In this reaction, which requires vitamin B6 as a cofactor, the amino group is transferred to an acceptor keto-acid, which can form a new amino acid. Through this process, the body is able to make the nonessential amino acids not provided by one's diet. The keto-acid intermediate can also be used to synthesize glucose to ultimately yield energy for the body, and the cleaved nitrogen-containing group is transformed into urea, a waste product, and excreted as urine.

gastric: related to the stomach mucosa: moist exchange surface within the body gastrointestinal: related to the stomach and intestines environment: surroundings enzyme: protein responsible for carrying out reactions in a cell intestines: the two long tubes that carry out the bulk of the processes of digestion epithelial cell: sheet of cells lining organs throughout the body hydrolyze: to break apart through reaction with water energy: technically, the ability to perform work; the content of a substance that allows it to be useful as a fuel nutrient: dietary substance necessary for health glucose: a simple sugar; the most commonly used fuel in cells fat: type of food molecule rich in carbon and hydrogen, with high energy content anabolic: promoting building up catabolism: breakdown of complex molecules glycolysis: cellular reaction that begins the breakdown of sugars deamination: removal of an NH2 group from a molecule keto-acid: an acid compound containing the reactive CO group

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