Classification Of Proteins- Conjugated Proteins

Conjugated Proteins consist of simple proteins in combination with some non-protein component. the non-protein groups are called prosthetic groups. Conjugated proteins include the following groups:-

  • Nucleoproteins (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%. Nucleoprotamines are combinations of nucleic acids with the basic simple protein protamine. Nucleohistones are combinations of nucleic acids with the simple basic protein histone. In addition there are several acidic proteins, the non-histone proteins.
  • Glycoproteins ( Protein + carbohydrate) These are proteins linked to carbohydrate. In most glycoproteins the linkage is between asparagine (Asn) and N-acetyl-D-glycosamine (GlcNAc). Salivary glands and mucous glands of the digestive tract, however secrete mucoproteins in which the linkage is between N-acetylglycosamine and serine/threonine of the protein. Glycoproteins are of two main categories, intrcellular 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.
  • Phosphoproteins.(protein + phosphate). Phosphoproteins are proteins in combination with  phosphate-containing radical other than a nucleic acid or a phospholipid. Examples of phosphoproteins are casein of milk and ovavitellin in eggs.
  • Chromoproteins. These are proteins in combination with a prosthetic group that is pigment. Examples are the respiratory pigments haemoglobin and haemocyanin, visual purple or rhodopsin found in the rods of the eye, flavoproteins and cytochromes.
  • Lipoproteins. These are proteins conjugated with lipids. There are four types of lipoproteins, high density lipoproteins (HDL) α-lipoproteins, low density lipoproteins (LDL) or β-lipoproteins very low density lipoproteins (VLDL) or pre β lipoproteins and chylomicrons.
  • 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 bound to iron.