Protein Classification

Protein Classification BI 171 [Table]

Different Classification Schemes of Proteins Based on Function, Shape, and Physicochemical Properties
Kinds of Proteins Description and/or Examples
    1. Enzymes (catalytic proteins) Lactase, ribonuclease, pyruvic dehydrogenase, fumarase, proteinase
    2. Structural proteins Collagen, elastin, keratin
    3. Regulatory or hormonal proteins Insulin, adrenaline
    4. Transport proteins Hemoglobin, myoglobin
    5. Genetic proteins Nucleoproteins, histones
    6. Immune Proteins Gamma Globulin, IG's, (Ab's)
    7. Contractile Proteins Actin, myosin
    8. Storage Proteins Zein, ovalbumin, casein

    1. Fibrous Proteins - Long thread-like molecules whose helical strands often form fibers or sheets; often insoluble in water. E.g. Collagen, elastin, keratin
    2. Globular Proteins - Generally soluble in aqueous media; spheroid or ovoid shape; further classified on the basis of solubility - water soluble, heat coaguble, include proteins in blood serum, egg white, milk, and in certain plants (pea, wheat);soluble in dilute salt solutions include proteins in blood serum, egg white, and in plants such as peanuts.

    1. Simple Proteins - Yield only amino acids on hydrolysis
      1. Albumins, Globulins, Glutelins Soluble in dilute acids and alkalies
      2. Scleroproteins Nonsoluble proteins such as Collagen
      3. Prolamines Soluble in alcohol, insoluble in water; found in seeds of plants
      4. Histones Soluble in water, dilute acids, and alkalies; contain a large portion of basic amino acids such as globin of hemoglobin
      5. Protamines Basic proteins which are essentially large polypeptides
    2. Conjugated Proteins Yield amino acids and nonprotein products upon hydrolysis
      1. Glyco- or mucoproteins Proteins plus carbohydrates; e.g. mucin of saliva
      2. Lipoproteins Proteins plus a lipid; e.g. liporutellin of egg yolk
      3. Chromoproteins Proteins plus a pigmented prosthetic group e.g. hemoglobin, myoglobin
      4. Metalloproteins Proteins plus a metal element such as iron, magnesium, copper, or zinc; e.g. ferritin (Fe), tyrosine oxidase (Cu), alcohol dehydrogenase (Zn).
      5. Nucleoproteins Proteins plus nucleic acid; e.g. nucleohistone

Like fats, amino acids are formed within living cells using sugars as starting materials. But while fats are made up only of carbon, hydrogen, and oxygen atoms, all available in the sugar and water of the cell, amino acids also contain nitrogen. Most of the earth's supply of nitrogen exists in the form of gas in the atmosphere. Only a few microorganisms are able to incorporate nitrogen from the air into compounds - ammonia, nitrites, and nitrates - that can be used by living systems. Hence, the proportion of the earth's nitrogen supply available to the living world is very small. Plants incorporate the nitrogen in ammonia, nitrites, and nitrates into carbon-hydrogen compounds to form amino acids. Animals are able to synthesize some of their amino acids, using ammonia as a nitrogen source. The amino acids they cannot synthesize, the so-called ESSENTIAL AMINO ACIDS, must be obtained either directly or indirectly from plants. For adult human beings, the essential amino acids are: LYSINE, TRYPTOPHAN, THREONINE, METHIONINE, PHENYLALANINE, LEUCINE, VALINE, and ISOLEUCINE. People who eat meat usually get enough protein and the correct balance of amino acids. People who are vegetarians, whether for philosophical, esthetic, or economic reasons, have to be careful that they get enough protein and, in particular, the essential amino acids. Until recently, agricultural scientists concerned with the world's hungry people concentrated on developing plants with a high caloric yield. Increasing recognition of the role of plants as a major source of amino acids for human populations has led to emphasis on the development of high-protein strains of food plants and of plants with essential amino acids, such as "high-lysine" corn. Another approach to the right balance of amino acids is to combine certain foods. Beans, for instance, are likely to be deficient in tryptophan and in the sulfur containing amino acids, but they are a good to excellent source of isoleucine and lysine. Rice is deficient in isoleucine and lysine but provides an adequate amount of the other essential amino acids. Thus, rice and beans in combination make just about as perfect a protein menu as eggs or steak, as some nonscientists seem to have known for quite a long time.