Protein Classification

Different Classification Schemes of Proteins
	Based on Function, Shape, and Physicochemical Properties

Kinds of Proteins                         Description and/or Examples            

CLASSIFICATION BASED UPON FUNCTION

Enzymes (catalytic proteins)        Lactase, ribonuclease, pyruvic
                                          dehydrogenase, fumarase, proteinase 
Structural proteins                 Collagen, elastin, keratin
Regulatory or hormonal proteins     Insulin, adrenaline
Transport proteins                  Hemoglobin, myoglobin
Genetic proteins                    Nucleoproteins, histones
Immune Proteins                     Gamma Globulin, IG's, (Ab's)
Contractile Proteins                Actin, myosin
Storage Proteins                    Zein, ovalbumin, casein


CLASSIFICATION BASED UPON SHAPE


Fibrous Proteins -                   Long thread-like molecules whose helical
                                          strands often form fibers or sheets; often
                                          insoluble in water. E.g. Collagen,
                                          elastin, keratin   
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.


CLASSIFICATION BASED UPON PHYSICOCHEMICAL PROPERTIES


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