Proteins may be classified as simple proteins and conjugated proteins.

1. Simple protein

Simple protein consists of only amino acids or their derivatives. When hydrolysed by acids, alkalies or enzymes, simple proteins yield only amino acids or their derivatives. They include the following groups.

(a) Albumins:

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These are water soluble-proteins found in all body cells and also in the blood stream. Examples are lacto albumin found in milk and serum albumin found in blood.

(b) Globulins:

These are insoluble in water but are soluble in dilute salt solutions of strong acids and bases. Examples of globulins are lactoglobulin found in milk and ologlobulin.

(c) Glutelins:

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These are soluble in dilute acids and alkalies. The protein glutenin from wheat is an example. They occur only in plant material.

(d) Prolamines:

These are soluble in 70-80% alcohol. They include fliadin from wheat and zein from corn. They are found only in plant material.

(e) Albumunoids:

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Albuminoids or seleroproteins are insoluble in all neutral solvents and in dilute alkalies and acids. They are found in connective tissues and in hair and nails. Examples are keratin, found in the cornified layers of the skin, cortex of hair and nails, and collagen which is found in the white fibres of areolar tissue.

(f) Histones:

These are water soluble proteins in which basis amino acids predominate. They are rich in arginine or lysine. In eukaryotes the DNA of the chromosomes is associated with histones to form nucleoproteins.

(g) Protamines:

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These are water soluble basic polypeptides with a low molecular weight (about 4,000 daltons). They are very rich in the amino acid arginine. The polypeptide chain consists of 28 amino acid residues, which include 19 arginines and 8-9 non basic amino acids. Protamines are found bound to DNA in spermatozoa of some fishes. Examples of protamines are salmine (in salmon) and sturine (in sturgeons)

(2) Conjugated proteins:

These consist of simple proteins in combination with some non-protein component. The non-protein groups are called prosthetic groups. Conjugated protein includes the following group.

(a) Nucleoproteins:

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(Protein + nucleic acid). Nucleoproteins are proteins in combination with nucleic acids. In trout spermatozoa nucleoproteins constitute 90% of the solid material, and in certain erythrocyte nuclei almost 100% nucleoproteins are combinations of nucleic acids with the basic simple protein protamine. Nuclehistones are combinations of nucleic acids with the simple basic protein histone. In addition there are several acidic proteins, the non histone proteins.

(b) Glycoproteins:

(Protein+Carbohydrate): Glycoproteins are proteins link to carbohydrate. In most glycoproteins the linkage is between asparagines (Ans) and N-acetyl-D-glocosamine (GICNAC). Salivary glands and mucous glands of the digestive tract, however, secrete mucoproreins in which the linkage is between N-acetylglycosamine and serinel threonine of the protein. Glcoproteins are of two main catagories, intracellular and secretory. Intracellular glycoproteins are present in cell membranes and have an important role in membrane interaction and recognition. Examples of secretory glycoproteins are: Plasma glycoproteins, secreted by the liver, thyroglobulin, secreted by the thyroid gland, immunoglobins, secreted by plasma cells, ovoalbumins, secreted by the oviduct in the hen, ribonuclease, the enzyme which breaks down RNA, and deoxyribonuclease, the enzyme which breaks down DNA.

(c) Phosphoproteins (Protein+phosphate):

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Phosphoproteins are proteins in combination with a phosphate-containing radical other than a nucleic acid or a phospholipid. Examples of phosphoproteins are casein of milk and ovovitelline in eggs.

(d) Chromoproteins:

These are proteins in combination with a proshetic group that is a pigmen. Examples are the respiratory pigments haemoglobin and haemocyanin, visual purple or rhodopsin found in the rods of the eye, flavoproteins and cytochromes.

(e) Lipoproteins:

These are proteins conjugated with lipids. There are four types of lipoproteins, high density lipoproteins (HDL) or a-lipoproteins, low density lipoproteins (VLDL) or pre-β lipoproteins and chylomicrons.

(f) Metalloproteins:

These are proteins conjugated to metal ion (s) which are not part of the prosthetic group. They include caeruloplasmin, an enzyme with oxidase activity that may transport copper in plasma, and siderophilin that is found to iron.