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Proteins Introduction

Biomolecules are mostly organic compounds, e.g. nucleotides, nucleic acids, amino acids, proteins, lipids, carbohydrates, vitamins, bonnones, etc.; a few are inorganic, e.g. H20, salts, mineral ions, etc. Some are small molecules (micromolecules), e.g. minerals, water, amino acids, sugars (mono and disaccharides), lipids, nucleotides, whereas, others are larger in size (macromolecules), e.g., proteins, nucleic acids, polysaccharides.

 

 

INTRODUCTION

Protein word is derived from the Greek word, which means “of primary importance” or “of the first rank”. These are linear polymers of amino acids and are macromolecules, which consist of mainly C, H, 0, N and in some cases S that is present in the amino acids cysteine and

methionine. Their molecular weight ranges from 12,000 daltons to several millions daltons.  Molecular weight of proteins is determined by ultracentrifugation method. About l ,000 -2,000 types of proteins are found in bacteria and 1,00,000 types in human cell. Most of the proteins have less than 2000 amino acids. The proteins can be degraded by Sanger’s reagent (2,4-dinitrofluoro benzene). They are denatured by heat, UV, alcohol, acetone, acid, detergents, disinfectants, soap, etc, in which the 3-D structure of protein is lost and it becomes nonfunctional and, thus, gets precipitated.

 

Small proteins are soluble in water and are not coagulated by heat, e.g. histones; but as the size of globular proteins increases, solubility in water decreases and heat coagulability increases, e.g. egg albumin, serum globulins, etc. Fibrous proteins are insoluble in water. Glutelins are found in wheat and rice seeds.

 

Only plants and some microbes can synthesize proteins from inorganic nitrogenous materials. All proteins are levorotatory due to a-amino acids. Acid-base properties of proteins are due to terminal -NH2, —COOH and R-groups.

 

Protein Tests: Some of the important methods to test the proteins are:

 

(i) Biuret Test: It is the test of peptide bonds:

Protein+ 10% of NaOH

Biuret

Heat l

1 drop CuS04

        Reddish-violet colour

 

(ii) Millon’s Test :                    Protein+ Million’s reagent

  [HgN03 + Hg(N03)2 + HN03 + HN02] l

Red precipitate

 

(iii) Ninhydrin Reaction :

a-Amino acid +Ninhydrin

 Blue colour

 

(iv) Xanthoproteic Reaction :         Protein + cone. HN03

 

(like skin)

 Yellow colour