Proteins Structure Functions and Biological Roles

Protein is a very complicated molecule that is found in all living things. Proteins have a high nutritional value and play a direct role in the chemical reactions that keep life going. Proteins were identified as important biomolecules by chemists in the early nineteenth century. A Swedish chemist Jöns Jacob Berzelius, invented the term ‘Protein’ in 1838, a word derived from the Greek proteios, which means "the first position". One species' proteins differ from those of another. They are also organ-specific; for example, muscle proteins differ from brain and liver proteins within the same organism.

Classification of Protein 

Classification of protein is done based on the solubility and biological functions of protein which is described below in brief. 

Classification By Solubility

The biological function of proteins had not yet been established. An attempt was made to classify proteins based on their chemical and physical properties after two German chemists, Emil Fischer and Franz Hofmeister, independently stated in 1902 that proteins are essentially polypeptides consisting of many amino acids. (It wasn't until the 1920s that the protein character of enzymes was established.) Proteins were generally categorized based on their solubility in a variety of solvents. However, this classification is no longer valid because proteins with very distinct structure and function can have comparable solubilities, while proteins with the same function and structure can have very distinct solubilities. However, the words connected with the previous classification are still extensively used. They're listed below.

Albumins are proteins that are soluble in water and ammonium sulfate-saturated water. Globulins, on the other hand, are salted out (i.e. precipitated) when ammonium sulphate is half-saturated. Pseuglobulins are globulins that are soluble in salt-free water while euglobulins are globulins that are insoluble in salt-free water. Plant proteins prolamins and glutelins are both water-insoluble; the prolamins dissolve in 50 to 80 percent ethanol, whereas the glutelins dissolve in acidified or alkaline solution. The term protamine refers to a group of proteins found in fish sperm that contain about 80% arginine and are hence very alkaline. The insoluble proteins of animal organs are referred to as scleroproteins. Keratin, an insoluble protein found in epithelial tissues including skin and hair, and collagen, a connective tissue protein, are two among them. Because conjugated proteins are complex protein molecules with both protein and nonprotein components, they are referred to as conjugated proteins. The prosthetic group is the nonprotein component. Mucoproteins, which contain carbohydrates as well as protein; lipoproteins, which contain lipids; phosphoproteins, which are high in phosphate; chromoproteins, which contain pigments such as iron-porphyrins, carotenoids, bile pigments, and melanin; and finally, nucleoproteins, which contain nucleic acid.

The preceding classification has a flaw in that many, if not all, globulins contain minor amounts of carbohydrate, making the distinction between globulins and mucoproteins blurry. Furthermore, there is no prosthetic group that can be identified in phosphoproteins; they are simply proteins in which part of the hydroxyl groups of serine are phosphorylated (i.e., contain phosphate). Finally, globulins contain proteins that serve a variety of functions, including enzymes, antibodies, fibrous proteins, and contractile proteins.

Classification by Biological Functions

The old categorization is in such poor shape, it is preferable to classify proteins according to their biological function. However, because a protein might have multiple functions, this classification is far from optimal. Myosin, for example, is a contractile protein that also functions as an ATPase (adenosine triphosphatase), an enzyme that hydrolyzes adenosine triphosphate (removes a phosphate group from ATP by introducing a water molecule). Another issue with functional classification is that a protein's exact function is frequently unknown. As long as the substrate (the exact substance on which it works) is unknown, a protein cannot be considered an enzyme.

The Role of the Active Site 

The assumption that the compound on which an enzyme operates (substrate) must mix with it in some way before catalysis can take place is an ancient one that is now backed up by a lot of experimental evidence. Collisions between substrate molecules and enzymes occur when they are combined. Enzymes are big molecules with molecular weights ranging from several thousand to several million (based on the weight of a hydrogen atom as one). Enzymes normally act on substrates with molecular weights of several hundred. Because of the size disparity, only a small portion of the enzyme comes into touch with the substrate; this area is referred to as the active site. Each subunit of an enzyme usually has one active site that can bind the substrate.

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Enzyme Active Site 

The shape of an enzyme (i.e., the structure of the active site) and thus the specificity of the enzyme is determined by the amino acid sequence. The physical or chemical forces that attract the substrate to an enzyme's surface can be physical or chemical. Electrostatic bonds can form between oppositely charged groups—the circles on the enzyme that have plus and minus signs are attracted to their polar opposites in the substrate molecule. Electrostatic bonds can form between totally positively or negatively charged groups (ionic groups), as well as partially charged groups (i.e., dipoles). Hydrophobic bonds may also be involved in the attraction forces between substrate and enzyme, in which the oily, or hydrocarbon, sections of the enzyme (shown by H-labeled circles) and the substrate are pulled together in the same way that oil droplets coalesce in water.

Did You Know?

Protein is an essential component for human growth and survival. Proteins are the most commonly found molecule in the body, apart from water. Protein is found in all cells of the body and is the most important structural component of all cells, particularly muscle cells. Body organs, hair, and skin are also included. Glycoproteins, for example, are proteins that are used in membranes. Amino acids are employed as precursors of nucleic acid, coenzymes, hormones, immunological response, cellular repair, and other life-sustaining compounds when broken down into amino acids. Protein is also required for the formation of blood cells.