Proteins are the primary building material of organisms. Different proteins have different functions. If we think of the body as a building, we can accept most of the materials and components in the construction, such as iron, brick, cement, windows and elevators, as protein in the organism.
Proteins are mainly composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S). However, phosphorus (P), iron (Fe), copper (Cu) and zinc (Zn) can also be found in the structure of proteins.
The building blocks of proteins are amino acids. While there are proteins consisting of only amino acids, proteins contain different molecules such as carbohydrates and fats.
Amino acids
Although nearly 200 amino acids are detected in nature, generally, 20 of these amino acids are found in the structure of proteins.
Amino acids are molecules consisting of carbon chains that contain at least one carboxyl group (-COOH) and at least one amino group (-NH2) in their structure (Proline has imino group (-NH)). The number of carbons in the chain usually varies between 2 and 11, depending on the type of amino acid.
Glycine, the simplest amino acid, consists of a carboxyl group and amino group bonded to 1 carbon atom and contains two carbon atoms.
A peptide bond is formed between the carboxyl group of one amino acid and the amino group of the other amino acid. In this way, the structure formed due to the combination of two amino acids is called a dipeptide and the network formed by the connection of three amino acids with a peptide bond is called a tripeptide. Amino acids unite and become peptides by creating peptide bonds in this way.
Proteins are peptides usually made up of 100 or more amino acids. However, some of them contain less than 100 amino acids.
The number of amino acids, chain length, and order contained in a protein is encoded in the organism’s DNA, producing that protein. This is why proteins are unique to living organisms.
Structure of proteins
The structure in which amino acids combine to form a chain is the proteins’ “primary structure.” However, this structure does not remain in a straight chain; The amino acids in the chain interact with each other and as a result of this interaction, folds occur in the chain.
As a result of the exchange, hydrogen bonds are usually formed between oxygen and nitrogen. Generally, a folding takes place in 4 amino acids. As a result, the protein gains a helix structure; This structure of proteins is called “secondary structure.” Myosin, keratin and epidermis are secondary structures.
The hydrogen bonds that provide the formation of the secondary structure are weak bonds, but the construction of too many hydrogen bonds in the network makes the secondary structure stable.
However, if two sulfur-containing amino acids coincide in the folds of the chain, a disulfide bond (S-S) is also established between these two amino acids. The disulfide bond in the secondary structure makes the structure much stronger. In general, proteins show secondary structures in two different ways: β-sheet and α-helix.
The secondary structure formed takes on a more complex structure by showing hydrogen bonds, disulfide bonds, van der Waals bonds and hydrophobic interactions. This structure of proteins is called “tertiary structure.” Globular proteins generally show a tertiary structure.
“Quaternary structure” emerges when more than one polypeptide chain comes together with these interactions, Hemoglobin is a quaternary structure.
Naturally, each protein has different molecular weights depending on the number of amino acids. Insulin weighs 6,000 Da while urease weighs 480,000 Da.
Classification of proteins
Proteins can be classified according to different parameters. These classifications are as follows;
I) According to the configuration
II) According to the chemical composition
III) According to the function
I) According to the configuration
1. Fibrous; are rod-shaped proteins that are insoluble in water. Collagen, keratin, elastin and tropomyosin can be given as examples.
2. Globular; are water-soluble round-shaped proteins. Serum albumin, hemoglobin, enzymes, antibodies and hormones can be given as examples.
II) According to the chemical composition
1. Simple; are proteins that release only amino acids and their derivatives when hydrolyzed. Examples are albumins, globulins, glutelins, prolamins, protamines, histones, collagen, keratin, fibrin, myosin, elastin and epidermis.
2. Conjugated; are proteins that release substances other than amino acids and their derivatives when they undergo hydrolysis. Examples include phosphoproteins, lipoproteins, glycoproteins, nucleoproteins, chromoproteins and metalloproteins.
III) According to the function
1. Enzymes; amylase and lactase can be given as examples.
2. Structurals; Examples include glycoproteins and lipoproteins.
3. Contractiles; myosin, actin and tubulin can be given as examples.
4. Hormones; Examples include insulin and growth hormones.
5. Carriers; Examples include hemoglobin, transferrin and serum albumin.
6. Storages; Hardeine and gliadin can be given as examples.
7. Protectives; Examples include antibodies and immunoglobulins.
8. Toxics; snake venom and ricin are examples.
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