Proteins are complex organic compounds whose basic structure is a primary chain of amino acids covalently bonded via peptide bonds and folded into a certain shape. This tertiary fold dictates the function of the protein within the organism. They can take on many forms and perform distinct functions including (but certainly not limited to) structural, enzymatic, information storage, intracellular signaling, and transportation roles. In terms of structure, proteins are important in the construction and integrity of different body tissues including connective and muscular, as well as endocrine/exocrine glands.
Proteins are synthesized by cytoplasmic structures composed of catalytic rRNA and proteins called ribosomes. A few peptides are synthesized in non-ribosomal reactions, such as glutathione. Ribosomes translate information coded in mRNA (transcribed from genomic DNA, specific regions known as genes) after editing and manipulation by nuclear and cytoplasmic spliceosomes. The nascent protein is elongated in the ribosome through addition of different amino-acyl moieties from the tRNA carrier. The start site and sequence of addition is specified by codons (sets of three nucleotides) in the mRNA molecule. The formation of the protein chain requires the codon, a molecule of transfer RNA (tRNA) charged with a particular amino acid, and the tRNA's anti-codon. A molecule of water is expelled after each peptide bond is formed, hence the name "condensation reaction".
After the protein has been generated, it is assisted by other proteins termed chaperones that aid in the formation of the proper conformation. A ribosomal protein is always synthesized in one direction so, invariably, proteins also exhibit the physical property known as stereoisomerism. All polypetides possess free N-terminal(Amine; NH2) and C-terminal (Carboxylic Acid; COOH) ends. The protein may then be further edited post-translationally by certain enzymes, by for example a piece being cut out and the two halves re-ligated. Examples of other modifications include, isoprenylation, palmitoylation, myristoylation, methylation, acetylation, N-terminal proteolysis, and disulfide bond linkage. These modifications often have some bearing on the function and localization of the protein. After these processes occur, the protein can be transported to other sub-cellular organelles/locations such as the plasma membrane, golgi apparatus, endoplasmic reticulum, and vesicles where they perform their pre-programmed function. Membrane proteins are unfolded by a signal recognition particle, and then inserted into the lipid bilayer of a vesicle. Such vesicular proteins are usually shuttled to the plasma membrane, or reserved cytoplasmically for secretion at a later time.
Adult humans need a minimum of 1 gram of protein for every kilogram of body weight per day to keep from slowly breaking down their own tissues. Failure to receive enough protein can cause stunted growth, loss of muscle mass, decreased immunity, weakening of the heart and respiratory system, and eventually death.
There are two types of the standard 20 amino acids embedded in the genetic code: essential and non-essential.
There are 13 amino acids the body can manufacture on its own. Essential amino acids are those which cannot be made by the body and can only be obtained from the diet. There are 9 amino acids the human body cannot produce on its own.
Protein is also classified as complete or incomplete.
Complete protein foods supply all of the essential amino acids the body needs to build new proteins. Incomplete protein foods do not supply all the essential amino acids.
Complete protein food sources tend to come from meat and other animal products, fish, eggs, and milk products. Incomplete proteins come from plant sources, fruits, vegetables, grains, and nuts. Plant proteins can be combined to include all of the essential amino acids and form a complete protein, such as eating rice and beans together. Protein is only one component of a healthy diet.