Proteins are ubiquitous in all living organisms and they fulfill many vital roles to support cell function. Essentially all proteins in living organisms are composed of the same 20 common amino acids. The number of amino acids that make up each protein varies considerably, and the molecular weight of proteins will likewise vary from a few thousand to more than 1 million Daltons. Proteins are synthesized in the cell cytoplasm on ribosomes that use mRNA as a template to direct the order of amino acids that are attached to the growing polypeptide chain. The mRNA template is derived from genes that contain DNA and are present in the chromosomes in the cell nucleus. The genes contain the master code for all proteins that can be produced in the cell. Each protein has three to four levels of structural organization, which define the unique properties of each protein regarding its structure and function in the cell. Some proteins share similar structure and function across or within plant and animal kingdoms since they are thought to be derived from the same primordial genes. Over time, other proteins have evolved in structure and function and may be unique to the organism from which they are derived. The millions of proteins that have been identified to date are catalogued in searchable databases that have been organized into more than 7500 families according to their relatedness in structure and function.
After production on the ribosome, some proteins are further processed through covalent attachment of carbohydrates to the protein or formation of complexes with lipids. Some proteins must be removed after production as they are no longer needed or are defective following production. The cell maintains an active surveillance system by which such proteins are removed from the cytoplasm and are degraded back to amino acids and small peptides.
Animals consume bacterial, plant, and animal proteins to obtain amino acids used in the production of protein macromolecules that sustain life. Most of the proteins consumed would be considered foreign to the organism and, if they were freely absorbed intact from the GI tract, might elicit immune defense reactions detrimental to the organism. To prevent this, the GI tract serves as a largely impermeable barrier for absorption of intact proteins into the circulation. Proteases are released into the GI tract and are also present in the vicinity of intestinal epithelial cells that effectively degrade ingested proteins into small peptides and amino acids that can be absorbed into the systemic circulation. Humans must ingest protein because they cannot synthesize 9 of the 20 common amino acids found in nature and must therefore obtain them from dietary sources. For millennia, the vast majority of the millions of proteins produced by plants and animals have been safely consumed by humans as food.
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