14 Proteins
Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective; they may serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may contain thousands of different proteins, each with a unique function. Their structures, like their functions, vary greatly. They are all, however, polymers of amino acids, arranged in a linear sequence and connected together by covalent bonds.
Amino acids are the monomers that make up proteins (Figure 1). Each amino acid has the same fundamental structure, which consists of a central carbon atom, also known as the alpha (α) carbon, bonded to an amino group (NH2), a carboxyl group (COOH), and to a hydrogen atom. Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group.
| Function | Examples | Description |
| Defense | Immunoglobulins | Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body. |
| Enzyme | Digestive enzymes such as amylase, lipase, pepsin, trypsin | Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA. |
| Messenger | Insulin, thyroxine | Messenger proteins, such as some types of hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs. |
| Structural component | Actin, tubulin, keratin | These proteins provide structure and support for cells. On a larger scale, they also allow the body to move. |
| Transport/ storage | Hemoglobin, albumin, Legume storage proteins, egg white (albumin) | These proteins bind and carry atoms and small molecules within cells and throughout the body. Some provide nourishment in early development of the embryo and the seedling |
| Contractile | Actin, myosin | Affect muscle contraction. |
You may have noticed that “source of energy” was not listed among the function of proteins. This is because proteins in our diet are typically broken back down into individual amino acids that our cells then assemble into our own proteins. Humans are actually unable to build some amino acids inside our own cells – we require them in our diet (these are the so-called “essential” amino acids). Our cells can digest proteins to release energy, but will usually only do so when carbohydrates or lipids are not available.
The functions of proteins can be very diverse because they are made up of are 20 different chemically distinct amino acids that form long chains, and the amino acids can be in any order. The function of the protein is dependent on the protein’s shape. The shape of a protein is determined by the order of the amino acids. Proteins are often hundreds of amino acids long and they can have very compl