3. PROTEINS
DEFINITION
• Protein is derived from a Greek word PROTOS which
means ‘the first or the supreme’.
• Proteins are extremely complicated and nitrogenous
molecule made up of variable number of amino acid residue
joined to each other by a specific covalent bond called a
peptide bond.
• Proteins contain Carbon, Hydrogen, Oxygen and Nitrogen as
major components while Sulphur and Phosphorus are minor
components.
• Nitrogen is characteristic of proteins, on an average the
nitrogen content of ordinary proteins is 16% by weight.
• The term protein is generally used a polypeptide containing
more than 50 amino acids.
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4. CLASSIFICATION
• The classification of protein is based on the amino acid composition, structure, shape and solubility
properties.
PROTEINS
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5. 1.FIBROUS PROTEIN
• These proteins are lipid soluble
• They are elongated polypeptide chain generate filamentous sheet like structure
• E.g. : Elastin, Keratin
2.GLOBULAR PROTEIN
• These proteins are water soluble
• They are spherical polypeptide chain
• E.g. : Hemoglobin, Insulin
3.INTERMEDIATE PROTEIN:
• This type of protein have cytoskeleton structure
• They provide support structure to the cells
• E.g. : Desmin, Keratin
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6. 4.SIMPLE PROTEIN:
• They have only amino acids, no other non protein components.
• Eg: Albumin, Globulins
5.CONJUGATED PROTEIN:
• They have both amino acids and non protein components.
• Eg: Phosphoprotein, Glycoprotein
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7. CHEMISTRY OF PROTEIN
• Proteins are polymers of L – α – amino acids.
• The structure of proteins is rather complex which can be divided into 4 levels of organizations such
as
1.Primary structure
2.Secondary structure
3.Tertiary structure
4.Quarternary structure
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8. PRIMARY STRUCUTRE
• Primary structure is linear, ordered and 1 dimensional.
• Covalently linked by peptide bonds
• Starts from amino terminal (n) end
• Ends in carboxy terminal (c) end
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9. SECONDARY STRUCTURE
• The conformation of polypeptide chain by twisting or
folding is referred to as secondary structure.
• The folding of the polypeptide chain into specific
coiled structure held together by Hydrogen bonds is
called secondary structure of protein.
• Secondary structure results in mainly two forms
1. Alpha – helical structure
2. Beta – sheet structure
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10. 1.Alpha helical structure
• Α helix is the most common spiral structure of
protein.
• It has a rigid arrangement of polypeptide chain.
• It is stabilized by extensive hydrogen bonding.
• It is formed between H atom attached to peptide N,
and O atom attached to peptide C.
• These type of protein have great strength and
elasticity.
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11. 2. Beta – sheet structure
• Beta – sheet structures are composed of
two or more segments of fully extended
peptide chains.
• In β sheets, the hydrogen bonds are
formed between the neighbouring
segments of polypeptide chains.
• This form is fully expended and can’t be
further stretched and they are inelastic.
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12. TERTIARY STRUCTURE
• The three – dimensional arrangement of protein
structure.
• It is a compact structure with hydrophobic side chains
held interior while the hydrophilic groups are on the
surface of the protein molecule.
• It is formed when alpha helices and beta sheets are held
together by week interactions.
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13. QUATERNARY STRUCTURE
• Formed by those protein having more than one
peptide chain subunit.
• Each peptide have its own primary, secondary,
tertiary structure.
• Depending on the number of monomers, the
protein may be termed as dimer, tetramer etc,
• Each polypeptide chain is termed as subunit or
monomer.
• Eg: Hemoglobin,
Creatine kinase (CK) – dimer,
Lactate dehydrogenase (LDH) – tetramer.
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