Proteins Definition, Properties, Structure, Classification, Functions

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Definition of Proteins

Proteins are the largest macromolecules in biology, found throughout every cell. They are also the most adaptable organic molecule in living system and is found in a many various kinds that range in size from small polymers to huge peptides. Proteins are polymers composed of amino acids, which are connected by peptide bonds. Proteins, the protein building blocks are the naturally occurring twenty amino acids. Proteins are, therefore, the multimers made up of amino acids.

Properties of Proteins

Solubility in Water

The relation between proteins and water is a complex. Proteins’ secondary structures relies mostly on the interaction of peptide bonds and water via hydrogen bonds. Hydrogen bonds can also be formed between proteins (alpha or beta forms) as well as water. The static ball with a high protein content is more soluble than structure of helical. In the tertiary structure water is responsible for the direction of chains and radicals that are hydrophilic to the outside of the molecule. In contrast, the hydrophobic radicals and chains tend to react with one and each other within the molecules (hydrophobic impact).

Denaturation and Renaturation

Proteins are denatured through agents like heat and urea which trigger the disintegration of polypeptide chains but without the hydrolysis of peptide bond. Denaturing agents damage secondary and tertiary structure, without altering the main structure. When a protein denatured is returned to its original condition when the denaturing agent is removed, this process is known as”renaturation.


A few of the agents that denaturize include

  • Physical agents: Heat, radiation, pH
  • Chemical agents: Urea solution which forms new hydrogen bonds in the protein, organic solvents, detergents.


If proteins are destroyed by the heat, they develop into insoluble aggregates referred to as coagulum. The majority of proteins aren’t heat coagulable. Only some, such as albumins, globulins , and globulins are co-coagulable.

Isoelectric point

The point of isoelectricity (pI) can be defined as the pH where the quantity of positive charges is equal to the amount of negative charges and the total charge of an amino acid is at zero. When exposed to an electric field, proteins don’t move towards either the cathode or the anode. this property is utilized to separate proteins.

Molecular Weights of Proteins

The molecular weight average for an amino acid can be estimated as 110. The number of amino acids present in a protein multiplied by is the approximate molecular mass of the protein. Different proteins have distinct amino acid compositions and their molecular weights are different. Proteins’ molecular weights vary between 5000 and 109 Daltons.

Posttranslational modifications

It happens following the time that the protein is made by the ribosome. The glycosylation, phosphorylation, ADP methylation, ribosylation the hydroxylation process, as well as acetylation alter the charge and interplay between the amino acid residues. This alters the 3-dimensional structure and therefore, the purpose of the protein.

Chemical Properties of Proteins

  1. Biuret test: If 2 ml of test solutions is mixed with the same volume of 10 percent NaOH as well as one drop of 10 percent CuSO4 solution the formation of a violet color is a sign of linking peptides.
  2. Ninhydrin test: The test is performed when 1 ml Ninhydrin solution are added to 1 ml of protein solution and then heated to a temperature of about 165°C, the appearance of violet-colored color is a sign of the presence of a-amino acids.

Protein Structure

  • Its linear pattern of amino acids inside a polypeptide chain determines three-dimensional structure of a protein. the structure of the protein is what determines its purpose.
  • All proteins are composed of the elements carbon hydrogen oxygen, nitrogen and sulfur. Some of these could also contain phosphorus Iodine, and trace amounts of metals, such as ions zinc, copper, as well as manganese.
  • A protein could contain 20 kinds of amino acids. Each amino acid is composed of an ammonia group at one point with an acid-based group at the opposite as well as a distinct side chain.
  • The backbone is identical in all amino acids, whereas the side chain is different between one amino acid and the next.
Protein Structure
Protein Structure

Proteins’ structure can be classified in four different levels of structure:

1. Primary Structure

The basic structure of proteins is made up from the amino acid chain that runs along the chain of polypeptides. Amino acids are joined through bonds between peptides. Since there aren’t dissociable protons in peptide bond and the charges that are affixed to a polypeptide chain is caused by the amino group at the N-terminal and the carboxyl group at the C-terminal as well as the amino acid residues. The structure that is the primary one determines the other levels of structure of proteins.

2. Secondary Structure

The secondary structure is comprised of various kinds of local conformations in which the atoms of side chains do not play a role. Secondary structures are formed through the regularity of hydrogen bonding among backbone elements. Secondary structures include b-sheets, a-helices, and various other folding patterns that are because of a regular design of the hydrogen bond. A secondary structural feature of proteins may include :

  • Alpha-helix
  • Beta-helix

The a-helix can be described as a right-handed spiral string. The side-chain substituents for the amino acid groups within an a-helix extend out to the outside. Hydrogen bonds form between oxygen from the C=O in each strand of peptide bonds along with the hydrogen from the N-H group in the peptide bond that is four amino acids beneath it within the helix. Side-chain substituents of amino acids are positioned in the same place as those N-H group. The hydrogen bonding that occurs in an sheet conformation is between the strands (inter-strand) instead of within the strands (intra-strand). The sheet conformation is comprised of strands laid together. The carbonyl oxygens present in one strand are hydrogen-bonded to amino hydrogens on the strand next to it. The two strands could be parallel or anti-parallel, based on whether the strand’s directions (N-terminus from C-terminus to N-terminus) are the opposite or the same. The anti-parallel ss-sheet is stronger due to the more aligned hydrogen bonds.

3. Tertiary Structure

The Tertiary structure of proteins refers to its general three-dimensional shape. The different kinds that interact between amino acid residues which create the three-dimensional form of a protein are electrostatic interactions, hydrophobic interactions and hydrogen bonds, each of which is non-covalent. Disulfide bonds that are covalent also exist.

It is created through the interactions of amino acid residues which could be found far from each other within the main structure of the polypeptide chain. Acid residues that have hydrophobic properties are more likely to accumulate in the inner part of globular proteins. There, they are able to exclude water, while hydrophilic residues typically reside on the surface of the protein, in which they are in contact with water.

4. Quaternary Structure

Quaternary structures refer to the interaction between two or more subunits in order to create functional proteins, employing the same forces that stabilize the Tertiary structure. It’s the arrangement of subunits within the protein, which is composed of multiple polypeptide chain.

Classification of Proteins

Based on their chemical structure, nature shape, and solubility proteins are classified as:

  1. Simple proteins: they consist of amino acid residues. When hydrolyzed the proteins produce only amino acids as their constituents. The protein is further classified into:
    1. Fibrous protein: Keratin, Elastin, Collagen
    2. Globular protein: Albumin, Globulin, Glutelin, Histones
  2. Conjugated proteins: They’re mixed with non-protein moiety. Eg. Nucleoprotein, Phosphoprotein, Lipoprotein, Metalloprotein, etc.
  3. Derived proteins: Derived proteins are the degraded and derivatives of conjugated and simple proteins. They can include :
  • Primary derived protein: Proteans, Metaproteins, Coagulated proteins
  • Secondary derived proteins: Proteosesn or albunoses, peptones, peptides.

Functions of Proteins

Proteins are essential for repair and growth and their roles are endless. They also play a huge variety of biological functions. They are among the most crucial final product of the information pathways.

  • Proteins, comprised of amino acid, are used in a variety of roles within your body (e.g. as structural components, enzymes hormones, as well as antibodies).
  • They function as structural elements like hair keratin and nail collagen, bone collagen and bone, etc.
  • Proteins are the molecules that allow genetic information to be expressed.
  • They carry out their tasks through the transport of carbon dioxide via hemoglobin as well as specific enzymes found in red cells.
  • They are involved in the homeostatic regulation on the amount of blood circulation and the interstitial fluids via the plasma proteins.
  • They play a role in the process of blood clotting via fibrinogen, thrombin and other protein-related factors.
  • They protect against infections through proteins.
  • They transmit hereditary traits through nucleoproteins in the cell’s nucleus.
  • Ovalbumin, glutelin, etc. are storage proteins.
  • Myosin, Actin are an important contractile protein that is essential to muscle contraction.
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