Standards for categorization of Proteins:
The major criteria utilized in categorization of proteins comprise:
ii) structural complexity
Categorization on Basis of Function:
Catalytic proteins are class of proteins which accelerate rate of biological reactions. This class is by far largest, as virtually all steps in metabolism are catalyzed by enzymes which are generally proteinuous by nature.
Transport proteins carry particular substances from one place to another. Hemoglobin, for instance, transports oxygen from lungs to various parts of body and carbon dioxide back to lungs for exhalation. Serum albumin, another transport protein, carries fatty acids from adipose tissues to different organs. Transmembrane proteins such as Na+/K+ ATPase, glucose and amino acid transporters, are proteins found in cellular membranes and they assist to transport ions and metabolites across such membranes.
These proteins assist in formation and maintenance of biological structures. They give power and protection to cell and tissues. Examples of structural proteins comprise α-keratin found in hair, horns and fingernails and collagen located in bone, tendons, connective tissues, cartilage and hide. Others are elastin (in ligaments) and fibroin (in spider's web).
Proteins which assist to control skill of other proteins to perform their physiological functions are known as regulatory proteins. Insulin is the good example. It assists to control glucose metabolism in animals. Other hormones which are regulatory proteins comprise somatotropin and thyrotropin.
Storage proteins give reservoir of necessary nutrient. As nitrogen is limiting nutrient for development, organisms have exploited proteins as the source of nitrogen in their time of need. For instance, ovalbumin, protein of egg white, gives developing embryo with the source of nitrogen in its development within egg. Casein of milk serves as major nitrogen source for mammalian infants. Again, few seeds of higher plants have proteins which serve as good sources of nitrogen in germination.
Scaffold proteins give anchorage for other proteins. They help binding of such proteins with other structures in cell.
Protective and Defensive Proteins:
Immunoglobulins defend body by recognizing, binding and neutralizing foreign molecules. Thrombin and fibrinogen are blood clothing proteins whose significance is in prevention of continuous blood loss. Other proteins such as phospholipase found in snake venom play the rather protective role.
Classification on the Basis of Structural Complexity:
Simple proteins have only amino acids. Albumins of egg, protamines, globulins, lectins are all simple proteins.
Conjugated proteins have lastingly related chemical components, additionally to amino acids. Few subclasses of this group of proteins comprise: glycoproteins, lipoproteins, and metalloproteins etc.
Classification on the Basis of Shape:
Globular Proteins: In terms of shape, globular proteins are spherical or oval. They are simply soluble. E.g: albumins and globulins.
Fibrous Proteins: Fibrous proteins are lengthened or needle shaped. These proteins are mainly insoluble and are extremely resistant to digestion. E.g., Elastin, collagen and keratin.
Classification on the Basis of Nutritional Importance:
Complete Proteins: They have all the necessary amino acids in proportion they are needed. Good example is casein found in milk. This class of protein is nutritionally rich and is also known as first class protein.
Incomplete Proteins: Incomplete proteins lack at least one necessary amino acid and as such, can't promote development in growing individuals. For instance protein from cereals lacks lysine.
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