Classifications of Proteins

  1. Simple Proteins
    1. Albumins: blood (serumbumin); milk (lactalbumin); egg white (ovolbumin); lentils (legumelin); kidney beans (phaseolin); wheat (leucosin). Globular protein; soluble in water and dilute salt solution; precipitated by saturation with ammonium sulfate solution; coagulated by heat; found in plant and animal tissues.
    2. Globulins: blood (serum globulins); muscle (myosin); potato (tuberin); Brazil nuts (excelsin); hemp (edestin); lentils (legumin). Globular protein; sparingly soluble in water; soluble in neutral solutions; precipitated by dilute ammonium sulfate and coagulated by heat; distributed in both plant and animal tissues.
    3. Glutelins: wheat (glutenin); rice (oryzenin). Insoluble in water and dilute salt solutions; soluble in dilute acids; found in grains and cereals.
    4. Prolamines: wheat and rye (gliadin); corn (zein); rye (secaline); barley (hordein). Insoluble in water and absolute alcohol; soluble in 70% alcohol; high in amide nitrogen and proline; occurs in grain seeds.
    5. Protamines: sturgeon (sturine); mackerel (scombrine); salmon (salmine); herring (clapeine). Soluble in water; not coagulated by heat; strongly basic; high in arginine; associate with DNA; occurs in sperm cells.
    6. Histones: Thymus gland; pancreas; nucleoproteins (nucleohistone). Soluble in water, salt solutions, and dilute acids; insoluble in ammonium hydroxide; yields large amounts of lysine and arginine; combined with nucleic acids within cells.
    7. Scleroproteins: Connective tissues and hard tissues. Fibrous protein; insoluble in all solvents and resistant to digestion.
      1. Collagen: connective tissues, bones, cartilage, and gelatin. Resistant to digestive enzymes but altered to digest gelatin by boiling water, acid, or alkali; high in hydroxylrpline.
      2. Elastin: Ligaments, tendons, and arteries. Similar to collagen but cannot be converted to gelatin.
      3. Keratin: Hair, nails, hooves, horns, and feathers. Partially resistant to digestive enzymes; contains large amounts of sulfur, as cystine.

  2. Conjugated Proteins
    1. Nucleoproteins: cytoplasm of cells (ribonucleoprotein); nucleas of chromosomes (deoxyribonucleoprotein) viruses, and bacteriophages. Contains nucleic acids, nitrogen, and phosphorus. Present in chromosomes and in all living forms as a combination of protein with either RNA or DNA.
    2. Mucoprotein: saliva (mucin); egg white (ovomucoid). Proteins combined with amino sugars, sugar acids, and sulfates.
    3. Glycoprotein: bone (osseomucoid); tendons (tendomucoid); carilage (chondromucoid). Containing more than 4% hexosamine, mucoproteins; if less than 4%, then glycoproteins.
    4. Phosphoproteins: milk (casein); egg yolk (ovovitellin). Phosphoric acid joined in ester linkage to protein.
    5. Chromoproteins: hemoglobin; myoglobin; flavoproteins; respiratory pigments; cytochromes. Protein compounds with such nonprotein pigments as heme; colored proteins.
    6. Lipoproteins: serum lipoprotein; brain, nerve tissues, milk, and eggs. Water-soluble protein conjugated with lipids; found dispersed widely in all cells and all living forms.
    7. Metallo proteins: ferritin; carbonic anhydrase; ceruloplasmin. Proteins combined with metallic atoms that are not parts of a nonprotein prosthetic group.

  3. Derived Proteins
    1. Proteans: edestan (from elastin) and myosan (from myosin). Results from short action of acids or enzymes; insolvent in water.
    2. Proteases: intermediate products of protein digestion. Soluble in water; uncoagulated by heat; and precipitated by saturated ammonium sulfate; result from partial digestion of protein by pepsin or trypsin.
    3. Peptones: intermediate products of protein digestion. Same properties as proteases except that they cannot be salted out; of smaller molecular weight that proteases.
    4. Peptides: intermediate products of protein digestion. Two or more amino acids joined by a peptide linkage; hydrolyzed to individual amino acids.