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3.4.Classification
Unit 3 - Proteins and Amino acids
3.4.Classification
Proteins may be classified into three main groups according to their shape, solubility and chemical composition:
a. Fibrous proteins: insoluble animal proteins which are generally very resistant to digestive enzyme breakdown. Fibrous proteins exist as elongated filamentous chains. Examples of fibrous proteins include the collagens (main protein of connective tissue), elastin (present in elastic tissues such as arteries and tendons), and keratin (present in hair, nails, wool and hooves of mammals).
b. Globular proteins: include all enzymes, antigens and hormone proteins. Globular proteins can be further subdivided into albumins (water soluble, heat-coagulable proteins which occur in eggs, milk, blood and many plants); globulins (insoluble or sparingly soluble in water, and present in eggs, milk, and blood, and serve as the main protein reserve in plant seeds); and histones (basic proteins of low molecular weight, water soluble, occur in the cell nucleus associated with deoxyribonucleic acid - DNA).
c. Conjugated proteins: these are proteins which yield non-protein groups as well as amino acids on hydrolysis. Examples include the phosphoproteins (casein of milk, phosvitin of egg yolk), glycoproteins (mucous secretions), lipoproteins (cell membranes), chromoproteins (haemoglobin, haemocyanin, cytochrome, flavoproteins), and nucleoproteins (combination of proteins with nucleic acids present in the cell nucleus).
d. Derived proteins: These are proteins derived from simple or conjugated proteins by physical or chemical means. Examples are: denatured proteins and peptides.
a. Fibrous proteins: insoluble animal proteins which are generally very resistant to digestive enzyme breakdown. Fibrous proteins exist as elongated filamentous chains. Examples of fibrous proteins include the collagens (main protein of connective tissue), elastin (present in elastic tissues such as arteries and tendons), and keratin (present in hair, nails, wool and hooves of mammals).
b. Globular proteins: include all enzymes, antigens and hormone proteins. Globular proteins can be further subdivided into albumins (water soluble, heat-coagulable proteins which occur in eggs, milk, blood and many plants); globulins (insoluble or sparingly soluble in water, and present in eggs, milk, and blood, and serve as the main protein reserve in plant seeds); and histones (basic proteins of low molecular weight, water soluble, occur in the cell nucleus associated with deoxyribonucleic acid - DNA).
c. Conjugated proteins: these are proteins which yield non-protein groups as well as amino acids on hydrolysis. Examples include the phosphoproteins (casein of milk, phosvitin of egg yolk), glycoproteins (mucous secretions), lipoproteins (cell membranes), chromoproteins (haemoglobin, haemocyanin, cytochrome, flavoproteins), and nucleoproteins (combination of proteins with nucleic acids present in the cell nucleus).
d. Derived proteins: These are proteins derived from simple or conjugated proteins by physical or chemical means. Examples are: denatured proteins and peptides.
Last modified: Tuesday, 30 August 2011, 8:58 AM