Elementary plant Biochemistry and Biotechnology (2+1)

A. Classification based on solubility and composition

Classification based on solubility and composition

 
• According to this classification, proteins are divided into three main groups as simple, conjugated and derived proteins

Simple proteins

• Simple proteins yield on hydrolysis, only amino acids.These proteins are further classified based on their solubility in different solvents as well as their heat coagulability

Albumins

• Albumins are readily soluble in water, dilute acids and alkalies. coagulated by heat.Seed proteins contain albumin in lesser quantities. Albumins may be precipitated out from solution using high salt concentration, a process called 'salting out'. They are deficient in glycine.Serum albumin and ovalbumin (egg white) are examples.

Globulins

• Globulins are insoluble or sparingly soluble in water, but their solubility is greatly increased by the addition of neutral salts such as sodium chloride. These proteins are coagulated by heat. They are deficient in methionine. Serum globulin, fibrinogen, myosin of muscle and globulins of pulses are examples.

Prolamins

• Prolamins are insoluble in water but soluble in 70-80% aqueous alcohol. Upon hydrolysis they yield much proline and amide nitrogen, hence the name prolamin.They are deficient in lysine. Gliadin of wheat and zein of corn are examples of prolamins.

Glutelins

• Glutelins are insoluble in water and absolute alcohol but soluble in dilute alkalies and acids. They are plant proteins e.g., glutenin of wheat.

Histones

• Histones are small and stable basic proteins. They contain fairly large amounts of basic amino acid, histidine.They are soluble in water, but insoluble in ammonium hydroxide. They are not readily coagulated by heat. They occur in globin of hemoglobin and nucleoproteins.

Protamines

• Protamines are the simplest of the proteins. They are soluble in water and are not coagulated by heat. They are basic in nature due to the presence of large quantities of arginine. Protamines are found in association with nucleic acid in the sperm cells of certain fish.Tyrosine and tryptophan are usually absent in protamines.

Albuminoids

• These are characterized by great stability and insolubility in water and salt solutions. These are called albuminoids because they are essentially similar to albumin and globulins.They are highly resistant to proteolytic enzymes. They are fibrous in nature and form most of the supporting structures of animals. They occur as chief constituent of exoskeleton structure such as hair, horn and nails.