A protein is any of a large group of macromolecules that plays a central role in the structure and functioning of living cells. Together with nucleic acids, carbohydrates, and lipids, proteins form the biochemical basis of life on Earth.

 

Types of protein

Classified according to molecular structure, there are three main types of protein:

 

Fibrous protein: a fiber-like protein used for structural purposes in organisms. Examples include actin and collagen

Globular protein: a protein in which the polypeptide chains (see below) are folded together into a knot-like shape. Many examples are found among enzymes and hormones

Conjugated protein: essentially a globular protein that possesses non-living substances. E.g., hemoglobin, which contains heme.

Classified according to function, proteins fall into the following main groups:

Antibodies: proteins bind to specific foreign particles, such as viruses and bacteria, to help protect the body. Example: immunoglobulin

Enzymes: proteins that 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. Example: protease

Messengers: proteins, such as some types of hormones, that transmit signals to coordinate biological processes between different cells, tissues, and organs. that possesses non-living substances. Example: growth hormone

Structural component: proteins that provide structure and support for cells. On a larger scale, they also allow the body to move. Example: actin

Transport and storage: proteins that bind and carry atoms and small molecules within cells and throughout the body. Example: hemoglobin

 

Structure

Proteins are polymers made of hundreds or thousands of amino acid subunits linked by peptide bonds to give polypeptide chains. The specific amino acid sequence of a protein is referred to as its primary structure. Since, in principle, a protein can be formed from any long sequence of any or all of the 20 different amino acids found in terrestrial organisms, the potential diversity of proteins is enormous. For example, a protein containing 100 amino acids could have 20¹⁰⁰ (about 1 followed by 130 zeros) different sequences. This almost unlimited diversity is perhaps the most important property of proteins since it enables them to perform such a wide range of functions in living organisms.

 

The amino acid sequence of a protein encourages the formation of hydrogen bonds between nearby amino acids. This results in two distinct types of structure. In the first, hydrogen bonds are established between different parts of the same polypeptide chain which pulls the chain into a spiral shape, known as an alpha helix. In the second, hydrogen bonds form between two adjacent chains resulting in a pleated configuration called a beta sheet. These types of folding are referred to as the protein's secondary structure. A more complex pattern of folding organizes the protein into its final, unique, three-dimensional configuration, or tertiary structure. The details of this final configuration are determined by the chemical nature of the side groups of the amino acids making up the primary structure. Many proteins associate with other polypeptide chains in clusters. Each contributing chain is referred to as a subunit and the overall structure of the cluster, the quaternary structure.

 

Synthesis

The synthesis of proteins involves two main stages: transcription and translation. It takes place as a ribosomes moves along a messenger RNA mold and the correct transfer RNAs, loaded with their specific amino acids, move in to recognize the triplet sequence (Figure 1). The amino acids are then released from their RNA carriers and are linked together in their specific order. The proteins so formed may be structural, as in the case of the collagen found in our skin, or more actively functional, as with enzymes.

 

Other hormones are themselves among the chemical signals that trigger genes to dispatch particular messenger RNAs. Similarly, other chemical signals switch off genes and stop further protein production. The structural genes that carry the code information for specific proteins do not exist in isolation. Each is associated with controlling elements in a genetic unit. Countless genetic units linked together on the long DNA molecular strand. And the DNA itself is wrapped up in association with certain proteins.

 

Denaturation

Since many of the bonds holding a protein in its normal shape are weak, they are easily broken by changes in the protein's environment, including the pH, the temperature, or the concentration of ions in the surrounding solution. When this happens, the protein undergoes a process called denaturation; that is, it changes shape or may even unfold and, as a result, usually becomes biologically inactive. This has particularly serious consequences if the protein is an enzyme, since such substances regulate the metabolism of living cells.