Introduction of Proteins
We know that Proteins of all the macromolecules found in cell biology, the proteins are chemically and physically more diverse. It is a very important constituent of all cell parts forming more than 50 % of the cell's dry weight. The term protein was coined by Dutch chemist G.J.Mulder (1802-180) and is derived from the Greek word proteios, which means "of the first rank".
Proteins serve the structural material of protoplasm and play essential roles in the living system's organisms. It forms enzymes of globular proteins with specialized to serve as a catalyst in virtually all types of biological activities of the cells. Other proteins are antibodies, transport proteins, storage proteins, contraction proteins, and some hormones. Chemically, proteins are polymers, and their others name of amino acids.
I. Amino acids. Nobel Laureate Email Fischer (1902) discovered that all proteins consist of chains (linear sequence) of smaller units that he named amino acids. There are about 20 different amino acids these are given below:
According to the 20 - naturally occurring amino acids
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| Structural Formula of Amino Acids (Proteins) |
A. Aliphatic amino acids
1. The Monoamino monocarboxylic amino acids
2. The Monoamino dicarboxylic
3. The Diamino-monocarboxylic
4. The Hydro containing amino acids
5. The Sulphur containing amino acids
6. The Aromatic amino acids
C. Secondary amino acids
D. Amino acid amides
Which occur and their constituents of naturally occurring proteins in all types of cellular organisms. An organic compound of cell organisms proteins there presents one or more amino groups, carboxyl groups that are called amino acids, and that occur freely in the cytoplasmic matrix of proteins and constitute it so we called amino acid pool.
II. Formation of proteins. Because a molecule of the amino acid contains both basic or amino and acidic or carboxylic groups, it can behave as an acid and base at a time. The molecules of such organic compounds that contain both acidic and basic properties are known as amphoteric molecules.
Types of proteins
Many different methods and process that have been used to classify of all types of proteins and no method of their classification being entirely satisfactory:
A. Classification based on biological functions. According to their biological functions, proteins are of two main types:
1. Structural proteins which include keratin, the major proteins component of hair, wool fur, nail, beak, feathers, hooves, and a cornified layer of skin: and collagen, abundant in skin, bone, tendon, cartilage, and other connective tissues.
2. Functional proteins that include all types of cell enzymes that serve as a catalyst in protein metabolism, hormonal proteins of a cell, and respiratory pigments of the cell, etc.
B. Classification based on the shape of proteins. According to the shape of proteins, it has two major types of proteins and their recognized:
1. Fibrous proteins. It is water-insoluble and their thread-like proteins having greater length than diameter. Fibrous proteins contain secondary protein structure and occur in those cellular.
2. Globular proteins. Its proteins are water-soluble as well as its shape like that roughly spheroidal and ovoidal shape. They readily go into colloidal suspension. It has a tertiary protein structure and is usually functional proteins of globular proteins. For example - cell enzymes, cell hormones, and immunoglobulins. The action of macro- filaments, and tubulins of microtubules are also globular proteins.
C. Classification based on solubility characteristics. According to this criterion proteins can be classified into two main types:
1. Simple proteins. These proteins contain only amino acids in their molecules and they are of the following types:
(i) Albumins (v) Scleroproteins
(ii) Globulins (vi)Histones
(iii) Glutelines (vii) Protamines
(iv) Prolamines.
2. Conjugated proteins. These proteins consist of simple proteins in combination with some non- protein components, called prosthetic groups. Its prosthetic groups of conjugated proteins are permanently associated with the molecules of conjugated proteins that are covalent and non- covalent linkages with the side chains of certain amino acids. Conjugated proteins are of the following types that are:
1. Chromoprotein 2. Glycoproteins
3. Lipoproteins 4. Nucleoproteins
5. Metalloproteins 6. Phosphoproteins
Structural levels of proteins
The proteins as-synthesized and there linear sequence of amino acids in all types of cells, polymerized of proteins by the elimination of water between successive products of amino acids to the form of the peptide bond, and here existing as a randomly coiled chain without specific shape and possessing no biological activity. Proteins within seconds of synthesis are completed the proteins fold into a specific part in the three-dimensional form, which is the same for all molecules of proteins of the same type of proteins in all cells and which now is capable of doing catalysis. According to structural levels of proteins and their mode of folding that are four levels of protein organization has been recognized are:
1. Primary protein structure. The primary protein structure is defined as the particular sequence of amino acids found in the proteins. It is determined by the covalent peptide bindings between amino acids. The primary structure also includes other covalent linkages in proteins, for example, the linkage that may exist between sulfur atoms of cysteine amino acids located in the chain of the protein insulin. The first protein to have its primary structure determined was insulin, the pancreatic hormones that regulate glucose metabolism in mammals. Insulin has a molecular weight of 5,800 daltons and contains 51 amino acids.
2. Secondary protein structure. The secondary structure of the protein is any regular repeating organization of the polypeptide chain. These are 3 types of structures are:
(i) Helical structure
(ii) Pleated sheet structure
(iii) Extended configuration
Most fibrous proteins have a secondary structure. In globular protein, too, it is not uncommon for half of all the residues of each polypeptide to be organized into one or more specific secondary structures.
3. Tertiary protein structure. Tertiary protein structure refers to a compact structure in which the helical and non- helical regions of a polypeptide chain are folded back on themselves. Its structure is typical of the globular structure of the protein and it is the non-helical region and that permits the folding. Their folding of a polypeptide chain structure and it is not random but occurs in a specific fashion type structure. thereby imparting certain steric properties to the proteins. For example, enzyme folding brings other active amino acids, which are otherwise scattered along the chain, and may form a distinctive cavity or cleft in which the substance is bound.
4. Quaternary protein structure. In proteins that are composed of two or more polypeptide chains, the quaternary structure refers to the specific orientation of these chains with respect to one another and the nature of the interactions that stabilize this orientation. The individual polypeptide chains of the proteins are called subunits and the active proteins itself is called multimer.