Organic Chemistry

Module 6. Proteins

Lesson 17
DEFINITION AND CLASSIFICATION OF PROTEINS

17.1 Introduction

• The terms ‘protein’ -coined by Dutch physiological chemist G. J. Mulder in 1838.
• The term - derived from Greek word “ π protas” - meaning- the first or the top most position or the eminent of the significant
• Protein- a common constituent of all biological materials , without which life is not possible - an essential constituent of all living cells.
• Average 2/3rd of total dry of the cell - composed of protein
• A complex nitrogenous organic compound – a polymer of amino acids - therefore defined as

“Protein may be defined as high molecular weight polymers of low molecular weight monomers known as amino acids, which are linked to gather by peptide bonds”

17.2 Elementary Composition

• Important elements – C,H, O and N
• Sometimes - S, P and I

Table 17.1 Elementary composition

• Some time Fe, Zn, Cu, Mn, Mg, etc found present

• Not directly attached with amino acids of proteins
• Attached to non-protein substances to form complex which inturn attach to amino acid

17.3 Classification

• Classified on the basis of composition, shape of molecules and solubility

17.3.1 On the basis of composition

Three groups- simple, conjugated and derived proteins
1. Simple proteins

• Consist of only amino acids- do not contain other class of compounds

2. Conjugated proteins

• Consist of amino acids as well as other class of compounds
• Further classified into six subgroups

Table 17.2

• Obtained from simple or conjugated protein as derivative - generally hydrolysis products - proteose, peptone, peptide and ultimately α-amino acids.
  

The sequence is characterized by decreasing order of molecular weight with increasing order of solubility in water

17.3.2 On the basis of shape of molecules

Two main groups : fibrous proteins and globular proteins
1. Fibrous proteins

• Long and thread or ribbon like, lie side by side to form fibers
• Generally insoluble in water - because of strong intermolecular attraction
• Serve as chief structural material of animal tissues
• e.g. keratin, myosin, collegen etc

2. Globular proteins

• Spherical in shape
• Generally soluble in water of aqueous solution of acid, base or salt
• Involve in physiological processes of animal body
• e.g. enzymes, some hormones, haemoglobin, etc.

17.3.3 On the basis of solubility

• Albumin – soluble in distilled water, dilute salt , acid and base solutions
• Globulin- insoluble in distilled water, but soluble in dilute salt, acid and base solutions
• Protamine and Histones- highly soluble in distilled water - because small molecules, stable to heat i.e. not coagulated . Protamine soluble in NH₄OH, whereas Histones insoluble NH₄OH.
• Glutelins- insoluble in distilled water and alcohol but soluble in dilute acid and base solution.
• Prolamine- insoluble in distilled water, but soluble in dilute acid, dilute base and 70-80% alcohol
• Scleroproteins- insoluble in most of the solvents- water, dilute acid, dilute base, dilute salt etc. They are fibrous proteins

17.4 Functions of Proteins

• Nutritional and physiological

17.4.1 Nutritional functions

• Primary and important function - role as a source of essential amino acids - for synthesis of new cells and maintenance of wear and tear
• Supply energy - 4.4 kcal/g

17.4.2 Physiological functions

1. Structural - fibrous protein- keratin (hair), myosin (muscles), etc
2. Catalyst- enzymes – in digestion, absorption etc
3. Transport- haemoglobin (O₂) , Albumin (fatty acids, amino acids)
4. Regulation - hormones- insulin- digestion
5. Buffering- albumin maintain pH and osmotic pressure
6. Protection - immunoglobulins (antibodies)