Proteins Explained: Definition and Classification with Key Examples

Proteins Explained: Definition and Classification with Key Examples

Define proteins, classify them, and give suitable examples.
Answer.

Proteins Definition

Proteins are polymers of amino acids.

They are the fundamental structural components of the body

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Classification Of Proteins

Proteins can be classified in four ways

• Classification based on shape and size
• Functional classification
• Classification based on chemical nature and solubility
• Nutritional classification

Classification based on Shape and Size

On the basis of shape and size, proteins are classified into 2 types – fibrous and globular

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Fibrous Proteins

When the axial ratio of length to width of a protein molecule is more than 10, it is called a fibrous protein eg, keratin and collagen

Globular Proteins

When the axial ratio of length to width of a protein molecule is less than 10, it is called a globular protein eg, haemoglobin and ribonuclease

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Functional Classification Of Proteins

Based on the functions they perform, proteins are classified as

• Structural proteins – they are involved in the formation of structures of the body e.g., keratin of hair and nail and collagen of bone
• Enzyme proteins – all enzymes are protein in nature, e.g., hexokinase, pepsin
• Transport proteins – proteins involved in the transport of substances ,e.g. –
  • Haemoglobin transports oxygen
  • Albumin transports bilirubin

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• Hormonal proteins- some of the hormones are protein in nature e.g. Insulin and growth hormone
• Contractile proteins – proteins which take part in muscle contraction. e.g. – Actin and myosin
• Storage proteins – proteins involved in storage of substances e.g. – Ferritin stores iron
• Genetic proteins – proteins involved in genetic function e.g. – Nucleoprotein
• Defence proteins – proteins involved in defence function e.g. – Immunoglobulins
• Receptor proteins – protein which act as receptors e.g. – Cytokine receptor, integrin
• Respiratory proteins – proteins involved in the function of respiration e.g. – Haemoglobin and cytochrome

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Classification Based On Chemical Nature And Solubility

According to this proteins are classified into 3 groups – simple, conjugated and derived

Simple Proteins

These are proteins which on complete hydrolysis yield only amino acids.

Example:

• Protamine – they are small molecules rich in arginine
• Histones – they are found in association with DNA
• Albumin – normal serum level is 3.5 to 5 gm %
• Globulin – normal serum level is 1.8 to 3.6 gm %
• Gliadin – it is rich in proline

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Nutritional Classification Of Proteins

From the nutritional point of view proteins are classified as

• Complete proteins
• Partially incomplete proteins
• Incomplete proteins

Complete Proteins

These proteins have all the essential amino acids in the required proportions by the human body to promote good growth e.g. egg albumin and milk casein

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Partially Incomplete Protein

These proteins are partially lacking one or more essential amino acids and hence can promote moderate growth e.g. – wheat and rice proteins (lack lysine and threonine)

Incomplete Proteins

These proteins completely lack one or more essential amino acids, hence do not promote growth at all e.g. – gelatin (lacks tryptophan), maize/corn (lacks tryptophan and lysine)