The metabolism of proteins starts in the stomach, where various enzymes mediate the breakdown of proteins into their constituent amino acids that are then transported into the bloodstream for circulation to the liver and cells throughout the body.
Proteins are hydrolyzed by a variety of enzymes in cells. Most of the time, the amino acids are recycled into the synthesis of new proteins or are used as precursors in the synthesis of other important biological molecules, such as hormones, nucleotides, or neurotransmitters. However, if there are excess amino acids, or if the body is in a state of starvation, some amino acids will be shunted into the pathways of glucose catabolism. When the body is starved of carbohydrates and fats it will result in the break down of protein tissue which can result in high levels of amino acids being removed from the kidneys.
Digestion of proteins begins in the stomach, as shown in the figure below, where hydrochloric acid (HCl) and the enzyme pepsin begin the process of breaking down proteins into their constituent amino acids. As the chyme enters the small intestine, it mixes with bicarbonate and digestive enzymes. The bicarbonate neutralizes the acidic HCl, and the digestive enzymes break down the proteins into smaller polypeptides and amino acids. Enteropeptidase, an enzyme located in the wall of the small intestine, activates trypsin, which in turn activates chymotrypsin. These enzymes liberate the individual amino acids that are conveyed across the intestinal wall into the cell. The amino acids are eventually transported into the bloodstream for dispersal to the liver and cells throughout the body to create new proteins.
If there are excess amino acids, or if the body is in a state of starvation, some amino acids will be shunted into the pathways of glucose catabolism. Each amino acid must have its amino group removed (deamination) prior to the carbon chain’s entry into these pathways. When the amino group is removed from an amino acid, it is converted into ammonia through the urea cycle. The remaining atoms of the amino acid result in a keto acid: a carbon chain with one ketone and one carboxylic acid group. In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the principal waste product in mammals produced from the nitrogen originating in amino acids; it leaves the body in urine. The keto acid can then enter the citric acid cycle.
When deaminated, amino acids can enter the pathways of glucose metabolism as pyruvate, acetyl CoA, or several components of the citric acid cycle. For example, deaminated asparagine and aspartate are converted into oxaloacetate and enter glucose catabolism in the citric acid cycle. Deaminated amino acids can also be converted into another intermediate molecule before entering the pathways. Several amino acids can enter glucose catabolism at multiple locations, as shown below.
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• Protein metabolism starts in the stomach, where various enzymes break down proteins into amino acids.
• These amino acids are then transported into the bloodstream for circulation to the liver and cells throughout the body to create new proteins.
• When in excess, the amino acids are processed and stored as glucose or ketones. The liberated nitrogen waste is converted to urea in the urea acid cycle and eliminated in the urine.
• During starvation, amino acids can be used as an energy source and processed through the Krebs cycle.
Metabolism: the sum of the chemical reactions that take place within each cell of a living organism that provide energy for vital processes and for synthesizing new organic material
Enzyme: a substance produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction.
Amino acids: Organic compounds that are the building blocks of proteins and contain amine and carboxyl functional groups, as well as a side chain specific to each amino acid.
Hydrolysis: A chemical reaction in which a molecule of water breaks a chemical bond.
Catabolism: A series of metabolic pathways that break molecules down into smaller forms, which can be oxidized to release energy or be used as reactants in other reactions.
Pepsin: the chief digestive enzyme in the stomach, which breaks down proteins into polypeptides
Chyme: the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food.
Bicarbonate: acts as a buffer that resists changes in pH when acid or alkali is added to it.
Polypeptides: Short chains of amino acids connected by peptide bonds.
Enteropeptidase: An enzyme produced in the digestive system that activates trypsin, which subsequently activates digestive enzymes.
Trypsin: An enzyme found in the digestive system that hydrolyzes and activates digestive enzymes, including chymotrypsin.
Chymotrypsin: An enzyme found in the digestive system that breaks down proteins and polypeptides through proteolysis.
Deamination: The removal of an amino group from a molecule.
Urea cycle: A biochemical pathway that produces urea from ammonia for excretion.
Keto acid: Organic compounds that contain a carboxylic acid group and a ketone group, of which pyruvate is one example; alpha-keto acids are important intermediates in metabolism.
Citric acid cycle (Krebs cycle): A biochemical pathway used by aerobic organisms to release stored energy found in carbohydrates, fats, and proteins into ATP and carbon dioxide; this occurs through the oxidation of acetyl-CoA.
Pyruvate: The conjugate base of pyruvic acid, the simplest alpha-keto acid, which contains a carboxylic acid and a ketone group; an important intermediate in metabolism.
Acetyl-CoA: A molecule that is involved in protein, carbohydrate and lipid metabolism by delivering an acetyl group to the citric acid cycle, which will be oxidized for energy production.