Proteins, Functions of proteins, Structure of Proteins, Protein formation, Amino acids, Types of amino acids, Structure of amino acids, Classification of amino acids, Properties of amino acids


Proteins are the most abundant organic molecules of the living system are commonly known as the building blocks of the body.

All proteins are polymers of amino acids which are linked together by the peptide bond which is formed between the amino group of one amino acid and the carboxyl group of another amino acid.

Although about 700 amino acids occur in nature but only 20 of them are abundant in the human body.


⇒ Proteins help in growth and repair of body tissues.

⇒ Many proteins act as biocatalyst,

⇒ There are various hormones which are made up of proteins, for example, Insulin.

⇒ Proteins also provide energy (1 gram of protein gives 3.9 kilocalories).

⇒ Proteins are required for various body processes like nutrients transport and muscle contraction.

⇒ They keep skin hair and nails healthy.

⇒ The help in protecting our body from antigen by forming antibodies.


An amino acid contains two functional groups that are the amino group and the carboxyl group. The amino group is basic while the carboxyl group is acidic in nature.

All amino acids except Glycine exist in two optically active forms that are those having amino group to the right are as D- forms and those having amino group to the left are as L- forms.


All amino acids found in the proteins are of L- series, many of the L- amino acids can rotate the plane polarized light towards the right (Dextro-rotatory) and many others can rotate the light towards left (Laevo-Rotatory).


A.) Classification on the basis of side chain that is -R- group

I.) Aliphatic Amino Acids

  • Glycine (Gly, G)
  • Alanine (Ala, A)
  • Valine (Val, V)
  • Leucine (Leu, L)
  • Isoleucine (Ile, I)

II.) Hydroxyl Group-Containing Amino Acids     

  • Serine (Ser, S)
  • Threonine (Thr, T)

Tyrosine also contains hydroxyl group

III.) Sulphur Containing Amino Acids

  • Cysteine (Cys, C)
  • Methionine (Met, M)

IV.) Acidic Amino Acids And Their Amide

  • Aspartic Acid (Asp, D)
  • Asparagine (Asn, N)
  • Glutamic Acid (Glu, E)
  • Glutamine (Gln, Q)

V.) Basic Amino Acids

  • Lysine (Lys, K)
  • Arginine (Arg, R)
  • Histidine (His, H)

VI.) Aromatic Amino Acids 

  • Phenylalanine (Phe, F)
  • Tyrosine (Tyr, Y)
  • Tryptophan (Trp, W)

VII.) Imino Acids

  • Proline (Pro, P)

B.) Classification based on the Nutritional requirement:


The amino acids which are not synthesized by the body and therefore need to be supplied through diet are known as essential amino acids, for example, Leucine, Valine, Isoleucine, Threonine, Tryptophan,  Histidine, Arginine, Lysine, Methionine, Phenylalanine.


The body can synthesize about 10 amino acids to meet biological needs hence they need not be consumed in diet these are the non-essential amino acids.

Arginine and Histidine can be synthesized by adults but not by growing children considered as semi-essential amino acids.


The structure of protein can be divided into four levels of organization –  

  • 1° (Primary) Structure
  • 2° (Secondary) Structure
  • 3° (Tertiary) Structure
  • 4° (Quaternary) Structure

⇒ Primary Structure of Proteins

It describes the linear sequence of amino acids which are held together by peptide bond

⇒ Secondary Structure of Proteins

  • It refers to the spatial arrangement of polypeptide chain
  • Two types of secondary structure are mostly identified that is alpha helix and beta sheet-like structure.
    • The alpha-helix structure is the simplest arrangement of the polypeptide chain. In this, the chain is well arranged in a helical manner which is stabilized by extensive hydrogen bonding. Each turn of alpha-helix contains around 3.6 amino acids. The right-handed helix is more stable than the left-handed helix.
    • The beta sheet-like structure –  In the beta configuration, the polypeptide chain is extended in a zigzag manner rather than helical structure. The adjacent polypeptide chain in the beta sheet can be either parallel or anti-parallel. Beta sheets are stabilized by interchain hydrogen bonding.

⇒ Tertiary Structure of Proteins

It refers to the three-dimensional arrangement of polypeptide chains. It is a compact structure with hydrophobic side chain held interior while hydrophilic groups are on the surface of protein molecules. Beside hydrogen bonding, disulfide Bond, Ionic bond and hydrophobic interactions contribute to the stability of the tertiary structure.

⇒ Quaternary Structure of Proteins

There are various proteins which contain two or more than two polypeptide chains the arrangement of these chains in three dimensions Complex constitutes the quaternary structure


I.) On The Basis Of Functions

a.) Catalytic Proteins – Catalytic proteins are the proteins that act as a Biocatalyst and catalyzed various biochemical reactions, for example, Amylase and Polymerase etc.

b.) Regulatory Proteins – Regulatory proteins are the proteins which are required for the regulation of various body processes, for example, Insulin, Growth Hormone etc.

c.) Structural Proteins – Structural proteins are those proteins which help in maintaining the structure of cells and tissues, for example, Collagen, Elastin, and Keratin etc.

d.) Protective Proteins – Protective proteins are those which help in the immune response of the organism by neutralizing foreign molecules or antigens, for example, Antibodies, Interferon etc.

e.) Transport Proteins – Transport proteins help in the transport of various molecules and ions, for example, Myoglobin, Hemoglobin etc.

f.) Genetic Proteins – Genetic proteins these are nuclear proteins which are associated with DNA, for example, Histone proteins etc.

g.) Storage Proteins – Storage proteins help in the storage of other molecules, for example, Ferritin etc.

h.) Motor proteins – Motor proteins are also called as contractile proteins they have the ability to convert chemical energy into mechanical energy, For example, Actin and Myosin are responsible for the contraction and relaxation of muscles.

II.) On The Basis Of Shape

a.) Fibrous Proteins – Fibrous proteins have the rod-like structure and resemble fibers in shape, for example, Elastin, Keratin, and Collagen etc.

b.) Globular Proteins – Globular proteins are spherical in nature and are more Complex than fibrous proteins, for example, Enzymes, Hormones, Antibodies, Hemoglobin, and Myoglobin etc.

III.) On The Basis Of Composition

a.) Simple Proteins – Simple protein is composed of only amino acids, for example, Albumin, Globulin, Collagen, Histone proteins etc.

b.) Derived Proteins – Derived proteins are the denatured products of simple and compound proteins resulting from the action of heat or chemical agents, for example,  Coagulated Egg white, Albumose from Albumin etc.

c.) Compound Proteins – Besides the amino acids, proteins contains a non-protein group known as the prosthetic group for example Metalloproteins, Phosphoproteins, Lipoproteins, and Glycoproteins etc.

IV.) On The Basis Of Nutritional Qualities

a.) Complete Proteins – Complete proteins have all the 10 essential amino acids in the required proportions to promote good growth, for example, Egg Albumin and Milk Casein etc.

b.) Partially Incomplete Proteins – These proteins are partially lacking one or more essential amino acids hence they promote moderate growth, for example, Wheat and Rice Proteins etc.

c.) Incomplete proteins – These proteins completely lack one or more essential amino acids hence they do not promote growth at all, for example, Zein Protein etc.


Physical Properties of Proteins

a.) Colours And Taste – Proteins are colorless and usually tasteless

b.) Shape – Proteins May be globular and fibrous in shape

c.) Molecular Weight – Proteins generally have the large molecular weight which depends on the number of amino acids residues each amino acid on an average contributes to a molecular weight of about 110 – 120, for example, Insulin – the molecular weight is 5733 and the number of the amino acids is 51.

d.) Solubility Proteins form colloidal solution instead of the true solution in water which is due to the huge size of protein molecules.

e.) Amphoteric Nature – Like amino acids proteins are amphoteric that is they act as both acids and alkalis.

f.) Precipitation of Proteins – This is the process of downstream processing of proteins. Protein can be separated out at the isoelectric point or by salting out.

  • Precipitation At Isoelectric Point – The proteins are least soluble at isoelectric pH. thus they will get easily precipitated when the pH is adjusted to isoelectric point.
  • Precipitation by salting out – By the addition of the salt such as  Ammonium Sulphate dehydration of proteins molecules occurs. This causes increase protein to protein interaction results in the aggregation and precipitation. 

Chemical Properties Of Proteins

a.) Biuret Reaction

The biuret test detects the presence of protein in the body fluids. Compounds containing peptide bond forms purple color, when treated with alkaline 0.2% copper sulfate solution. This reaction is termed as biuret reaction.

b.) Ninhydrin Reaction

Ninhydrin is a powerful oxidizing agent and causes oxidative decarboxylation of Alpha-amino acid producing CO2, NH3, and Aldehyde. Ninhydrin, when reacts with amino acids, forms blue colored Complex that is the Ruheman’s purple. Proline gives yellow color with Ninhydrin.

c.) Xanthoproteic Test

This test is specific for aromatic amino acid, yellow color developed on boiling the proteins with concentrated nitric acid due to the presence of benzene ring.

d.) Nitroprusside Test

This test is specific for cysteine. Red color develops with Sodium nitroprusside in dilute Ammonium hydroxide solution.

e.) Sakaguchi Test

This test is specific for Arginine. Red color develops with alpha-naphthol and NaClO3

f.) Millon’s Test

This test is specific for tyrosine. Red color develops when proteins are heated with Hg(NO)3.

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