Protein molecules are chains of amino acids that play many important roles in living systems. As far as is known, proteins have existed as long as life on earth has existed and are an essential ingredient in all cells. For most organisms, including humans, proteins must be ingested and digested in order to obtain essential nutrients that cannot be synthesized by the organism itself.
- Main Article: Ribosome
In both eukaryotic and prokaryotic cells, proteins are manufactured in ribosomes, which are made up of two sub-units of specially folded strands of RNA as well as some small proteins. A eukaryotic ribosome (e.g., that of a human) is slightly larger than the prokaryotic ribosome (e.g., that of a bacterial cell), but the two perform essentially the same essential function. Ribosomes connect amino acids to each other via peptide bonds in order to form strings called polypeptides.
- Main Article: Protein structure
The sequence of amino acids is determined by the genetic code on DNA; the simple sequence of amino acids alone is referred to as a protein's primary structure. Different residual groups on the amino acids can interact with water or each other, sometimes linking to each other, forming a certain three-dimensional structure, such as an alpha helix or a beta sheet; localized three-dimensional structure is referred to as a protein's secondary structure. A protein's tertiary structure is the entire three-dimensional conformation of a given peptide. Quaternary structure exists in proteins that are made up of multiple peptides, and it is the entire three-dimensional structure of every peptide and the links between them.
Proteins are used within organisms for an incredibly vast array of functions which include but are not limited to structure, sending and receiving messages, while enzymes are proteins which catalyze chemical reactions. This process is also called a metabolic pathway. Nearly every bodily process in humans involves proteins in some fashion. Without such proteins, life on earth as it is now could not exist.
Some proteins provide structure. For example, some connective tissue like cartilage is made of collagen and elastin, two structural proteins. Nails and hair are made of a protein called keratin. Proteins such as fibrin and tubulin help cells retain their shape and help divide DNA strands evenly to daughter cells during mitosis and meiosis.
Muscle fibers are largely made up of the proteins actin and myosin, the latter of which being a motor protein which pulls against actin in order to contract muscles; it is the action and interaction of these proteins which allow humans to walk, swallow and have a heartbeat.
Proteins that send messages from cell to another cell are called hormones. For example, the protein insulin is released from the pancreas when blood glucose levels are high, and it signals other cells in the body to take up the sugar from the blood. (In Type I diabetes, insufficient insulin is made, and cells do not know to take up glucose, resulting in high blood sugar and a lack of glucose in the cells.)
- Main Article: Enzyme
It can be most easily seen in digestion, which in fact begins enzymatic action in saliva, that enzymes are proteins that catalyze chemical reactions.. An enzyme called amylase begins to digest the starch in the mouth. Various other enzymes, peptidases and lipases, digest proteins and fats, respectively, which were obtained in the diet.
In the diet
Protein is an important part of the human diet. Proteins are made from amino acids (see "Synthesis" section, above), yet humans cannot make all of their own amino acids. Amino acids that cannot be produced within the body are called essential, meaning they must be taken up in the diet, whereas amino acids that can be produced in the body are nonessential.
During digestion, proteins from food are broken down in the stomach by the enzyme pepsin, and they are further broken down in the small intestine by a family of enzymes called peptidases. Individual amino acids travel through the bloodstream to the cells, where they can be utilized to form proteins.
"Amino acids in excess of those needed for biosynthesis cannot be stored, in contrast with fatty acids and glucose, nor are they excreted. Rather, surplus amino acids are used as metabolic fuel. The α-amino group is removed, and the resulting carbon skeleton is converted into a major metabolic intermediate. Most of the amino groups of surplus amino acids are converted into urea through the urea cycle, whereas their carbon skeletons are transformed into acetyl CoA, acetoacetyl CoA, pyruvate, or one of the intermediates of the citric acid cycle."
As a record of evolution
Many proteins are highly conserved among species, meaning humans, for example, share many of the same proteins with not only chimpanzees and other primates, but also birds, fish, reptiles, and even plants and bacteria. In contrasting the differences between the same proteins in different species, it is possible to determine which species came from common ancestors, tracing evolution.
- Stryer, Lubert; Berg, Jeremy Mark; Tymoczko, John L. (2002). “23. Protein Turnover and Amino Acid Catabolism”, Biochemistry. San Francisco: W.H. Freeman. ISBN 0-7167-3051-0.