Proteins are polymers of amino acids and can be classified based on their structure, shape and solubility. They have four levels of organization - primary, secondary, tertiary and quaternary structure. The primary structure is the linear sequence of amino acids joined by peptide bonds. Secondary structure involves folding into alpha helices or beta sheets held by hydrogen bonds. Tertiary structure is the compact 3D structure formed by interactions between secondary structures. Quaternary structure refers to proteins with more than one polypeptide chain subunit.

1. PROTEINS
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2. CONTENTS
Definition
Classification of proteins
Chemistry of proteins
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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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16. Reference
• Textbook of biochemistry – D M VASUDEVAN
• Fundamentals of biochemistry – Dr. U. Sathyanarayana
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