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Proteins

Proteins are important molecules in cells. Proteins are the major component of the dry weight of cells. The name protein is derived from a Greek word Proteios which means pre-eminent or first. This name was fist suggested in 1838 by a Swedish chemist Berzelius. He suggested it to a Dutch chemist Mulder and he referred it to the complex organic substances found in the cells of living beings. 

Proteins are the most abundant intracellular macro-molecules. Proteins are connected intimately with all chemical and physical activity, which constitutes the life of the cell. Proteins are present in and vital to all living cells. They provide structure, protection to the body of multicellular organism in the form of skin, hair, callus, cartilage, ligaments, muscles, tendons. Proteins regulate and catalyze the body chemistry in the form of hormones, enzymes, immunoglobulins etc. 

 

What are Proteins?

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Proteins are known as building blocks of life. Proteins are biomolecules, usually large in size, that consists of one or more chains of amino acids. Proteins perform variety of  functions like catalyzing metabolic reactions, replication of DNA, response to stimuli, and transporting molecules. Proteins differ from each other mainly in the sequences of amino acids.

Characteristics of Proteins

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General Characteristics of Proteins are as follows:
  • Proteins are organic substances, they are made up of nitrogen and also, oxygen, carbon an d hydrogen. 
  • Proteins are the most important biomolecules, they are the fundamental constituent of the cytoplasm of the cell. 
  • Proteins are the structural elements of body tissues. 
  • Proteins are made up of amino acids. 
  • Proteins gives heat and energy to the body and also aid in building and repair. 
  • Only small amounts of proteins are stored in the body as they can be used up quickly on demand. 
  • Proteins are considered as the bricks, they make up bones, muscles, hair and other parts of the body. 
  • Proteins like enzymes are functional elements that take part in metabolic reactions. 
  • Antibodies, blood haemoglobin are also made  of proteins. 
  • Proteins have a molecular weight of 5 to 300 kilo-daltons. 

Properties of Proteins

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The general properties of proteins are similar to those of the amino acids:

Physical Properties of Proteins
  • Proteins are colorless and tasteless.
  • They are homogeneous and crystalline. 
  • Proteins vary in shape, they may be simple crystalloid structure to long fibrilar structures. 
  • Protein structures are of two distinct patterns - Globular proteins and fibrilar proteins. 
  • Globular proteins are spherical in shape and occur in plants. Fibrilar proteins are thread-like, they occur generally in animals. 
  • In general proteins have large molecular weights ranging between 5 X 103 and 1 X 106
  • Due to the huge size, proteins exhibit many colloidal properties. 
  • The diffusion rates of proteins is extremely slow. 
  • Proteins exhibit Tyndall effect. 
  • Proteins tend to change their properties like denaturation. Many a times the process of denaturation is followed by coagulation. 
  • Denaturation may be a result of either physical or chemical agents. The physical agents include, shaking, freezing, heating etc. Chemical agents are like X-rays, radioactive and ultrasonic radiations. 
  • Proteins like the amino acids exhibit amphoteric property i.e., they can act as acids and alkalies.
  • As the proteins are amphoteric in nature, they can form salts with both cations and anions based on the net charge. 
  • The solubility of proteins depends upon the pH. Lowest solubility is seen at isoelectric point, the solubility increases with increase in acidity or alkalinity. 
  • All the proteins show the plane of polarized light to the left, i.e., laevorotatory. 
Chemical Properties of Proteins
  • Proteins when hydrolyzed by acidic agents, like conc.HCl yield amino acids in the form of their hydrochlorides. 
  • Proteins when are hydrolyzed with alkaline agents leads to hydrolysis of certain amino acids like arginie, cysteine, serine, etc., also the optical activity of the amino acids is lost. 
  • Proteins with reaction with alcohols gives its corresponding esters. This process is known as esterification. 
  • Amino acids reacts with amines to form amides. 
  • When free amino acids or proteins are said to react with mineral acids like HCl, the acid salts are formed. 
  • When amino acids in alkaline medium reacts with many acid chlorides, acylation reaction takes place. 
  • Sanger's reaction - Proteins react with FDNB reagent to produce yellow colored derivative, DNB amino acid. 
  • Xanthoproteic test - On boiling proteins with conc. HNO3, yellow color develops due to presence of benzene ring.
  • Folin's test - This is a specific test for tyrosine amino acid, where blue color develops with phosphomolybdotungstic acid in alkaline solution due to presence of phenol group. 

Protein Classification

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Classification of Proteins Based on Shape

They are grouped under two categories globular and fibrous.
Globular or Corpuscular Proteins 
Globular proteins have axial ratio less than 10. They are compactly folded and coiled. and possess a relatively spherical or ovoid shape. They are usually soluble in water and in aqueous media. Example Insulin, plasma albumin, globulin enzymes. 

Fibrous or Fibrillar Proteins
These proteins have axial ratio more than 10, hence, they resemble long ribbons or fibres in shape. They are mostly found in animals, and are not soluble in water or in solution of dilute acids. Fibrous proteins aid in protection and structural support. 
Example: Collagen, Keratin, Elastins, Fibroin.


Classification of Proteins Based on Composition and Solubility

Simple Proteins or Holoproteins 
These proteins are made of only one type of amino acid, as structural component, on decomposition with acids, they liberate constituent amino acids. They are mostly globular type of proteins except for scleroproteins, which are fibrous in nature. 

Simple proteins are further classified based on their solubility.

Protamines and histones - These proteins occur only in animals and are basic proteins. The possess simple structure and low molecular, are water soluble and are not coagulated by heat. The are strongly basic in character due to the high content of lysine, arginine. Example: Protamines - salmine, clupine, cyprinine; Histones - nucleoshistones, globin.

Albumins - They are widely distributed in nature, mostly seen in seeds. They are soluble in water and dilute solutions of acids, bases and salts. Example: Leucosine, legumeline, serum albumin. 

Globulins - They are of two types, pseudoglobulins which are soluble in water, other is euglobulins which are insoluble in water. They are coagulated by heat. Example: Pseudoglobulin, serum globulin, glycinine. etc.
 
Scleroproteins or Albuminoids - These occur mostly in animals and are commonly known as animal skeleton proteins, they are insoluble in water, and in dilute solution of acids, based and salts.

Conjugated or Complex Proteins or Heteroproteins
These are proteins that are made of amino acids and other organic compounds. The non-amino acid group is termed as prosthetic group. 

Complex proteins are further classified based on the type of prosthetic group present.

Metalloproteins - These are proteins linked with various metals. Example: casein, collagen, ceruloplasmin, etc.  

Chromoproteins - These are proteins that are coupled with a colored pigment. Example: Myoglubin, hemocyanin, cytochromes, flavoproteins, etc. 

Glycoproteins and Mucoproteins - These proteins contain carbohydrates as the prosthetic group. Example: Glycoproteins - egg albumin, serum globulins, serum albumins; Mucoproteins - Ovomucoid, mucin etc.

Phosphoproteins - These proteins are linked with phosphoric acid. Example: casein. 

Lipoproteins - Proteins forming complexes with lipids are lipoproteins. Example: lipovitellin, lipoproteins of blood. 

Nucleoproteins - These are compounds containing nucleic acids and proteins. Example: Nucleoproteins, nucleohistones, nuclein

Derived Proteins 
These are proteins that are derived from the action of heat, enzyme or chemical reagents.
 
Derived proteins are of two types, primarily derived proteins and secondary derived proteins. Primary derived proteins are derivatives of proteins, in which the size of the protein molecule is not altered materially, while in secondary derived proteins, hydrolysis occurs, as a result the molecules are smaller than the original proteins. 

Primary derived proteins are classified into three types - Proteans, Infraproteins and Coagulated proteins. Example: edestan, coagulated eggwhite.
Secondary derived proteins are further classified into 3 types - Proteoses, Peptones and Polypeptides.

Classification of Proteins on Biological Function

Proteins depending upon their physical and chemical structure and location inside the cell, they perform various functions. Proteins are grouped as follows, based on their metabolic function they perform.

Enzymic Proteins 
They are the most varied and highly specialized proteins with catalytic activity. Enzymes catalyze a variety of reactions. Example: Urease, catalase, cytochrome C, etc.

Structural Proteins 
Theses proteins aid in strengthening or protecting biological structures. Example: Collagen, elastin, keratin, etc.

Transport or Carrier Proteins 
These proteins help in transport of ions or molecules in the body. Example: Myoglobin, hemoglobin, etc.

Nutrient and Storage Proteins
These proteins provide nutrition to growing embryos and store ions. 

Contractile or Motile Proteins 
These proteins function in the contractile system. Example: Actin, myosin, tubulin, etc.

Defense Proteins 
These proteins defend against other organisms. Example: Antibodies, Fibrinogen, thrombin.

Regulatory Proteins 
They regulate cellular or metabolic activities. Example: Insulin, G proteins, etc.

Toxic Proteins 
These proteins hydrolyze or degrade enzymes. Example: snake venom, ricin. 

Structure of Proteins

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There are four structural levels of organization to describe the complex macromolecule, protein based on the degree of complexity of of the molecule. They are Primary Structure, Secondary structure, Tertiary structure and Quaternary structure. 

Primary Structure of Protein 
  • Primary structure of protein is the linear sequence of amino acids that make up the polypeptide chain. 
  • his sequence is given by the sequence of nucleotide bases of the DNA in the genetic code. 
  • The amino acid sequence determines the positioning of the different R groups relative to each other. 
  • The positioning determines the way the protein folds and the final structure of the molecule. 
Primary Structure Protein

Secondary Structure of Protein 
  • The linear, unfolded structure of polypeptide chain assumes helical shape to produce the secondary structure. 
  • The secondary structure refers to the regular folding pattern of twists and kinks of the polypeptide chain. 
  • The regular pattern is due to the hydrogen bond formation between atoms of the amino acid backbone of the polypeptide chain. 
  • The most common types of the secondary structure are the alpha helix and the ÃŸ pleated sheet. 
Secondary Structure Of Protein

Tertiary Structure of Protein 
  • Tertiary structure of proteins is the three dimensional structure formed by the bending and twisting of the polypeptide chain.
  • The linear sequence of polypeptide chain is folded into compact globular structure.
  • The folding of the polypeptide chain is stabilized by weak, noncovalent interactions. 
  • These interactions are hydrogen bonds and electrostatic interactions.
  •  Hydrogen bonds are formed when hydrogen atom is shared with two other atoms.
  • Electrostatic interactions between charged amino acid chains. 
  • Electrostatic interactions are between positive and negative ions of the macromolecules.
  • Hydrophobic interactions, disulphide linkages and covalent bonds also contribute to tertiary structure. 
Tertiary Structure Protein

Quaternary Structure of Protein 
  • Some proteins contain more than one polypeptide chains, this association of polypeptide chains refers to the quaternary structure.
  • Each polypeptide chain is called a subunit. 
  • The subunits can be same or different ones. 
  • Example: Haemoglobin the oxygen carrying component of blood is made up of two polypeptide chains, one with 141 amino acids and the other is a different type of 146 amino acids. 
Quaternary Structure Protein

Function of Proteins 

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Below is the list of proteins functions.
  • Proteins are seen in muscles, hair, skin and other tissues, they constitute the bulk of body's non-skeletal structure. Example: The protein keratin is present in nails and hair.
  • Some proteins are hormones and regulate many body functions. Example: Insulin hormone is a protein and it regulated the blood sugar level. 
  • Some proteins act enzymes, they catalyze or help in biochemical reactions. Example: Pepsin and Tripsin.
  • Some proteins act as antibodies, they protect the body from the effect of invading species or substances. 
  • Proteins transport different substances in blood of different tissues. Example: Haemoglobin is a oxygen transport protein. 
  • Contractile proteins help in contraction of muscle and cells of our body. Example: Myosin is contractile protein. 
  • Fibrinogen a glycoprotein helps in healing of wounds. It prevents blood loss and inhibits passage of germs. 

Types of Proteins

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The types of proteins are as follows:
  • Hormones are the proteins based chemicals that are secreted by the endocrine glands. Hormones are chemical messengers, that transmit signals from one cell to the other. 
  • Enzymatic proteins accelerate the metabolic activity in the cells. 
  • Structural proteins are necessary components of the body. Structural proteins like collagen forms connective framework in body tissues, and keratin is main component of hair, skin and nails. 
  • Defensive proteins like antibodies and immunoglobulins are the core part of the body's immune system. 
  • Storage proteins store mainly mineral ions in the body, like potassium, iron etc.
  • Transport proteins carry vital materials to the cells. 
  • Receptor proteins are located on the outer part of the cells, they control the substances that leave and enter the cell. 
  • Contractile proteins control the strength and speed of heart and muscle contractions. 

Examples of Proteins

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Examples of fibrous proteins:

Actin, Collagen, Elastin, Fibronectin, Keratin, Myosin, Tropomyosin, Tubulin, etc

Examples of globular proteins:

Albumins, Alpha globulin, Beta globulin, Cadherin, Fibrin, Gamma globulin, Haemoglobin, Immunoglobins, Myoglobin, Selectin, Serum albumin, Thrombin etc. 

Examples of membrane proteins:

Estrogen receptor, Glucose transporter, Histones, Hydrolases, Oxidoreductases, P53, Rhodopsin, etc.
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