Secrets of protiens

(h) Selenoproteins:
Selenium is a trace element and is found as a component of several pro­karyotic and eukaryotic enzymes that are involved in redox reactions. The selenium in these selenoproteins is incorporated as a unique amino acid, selenocysteine, during translation. A particularly important eukaryotic selenoenzyme is glutathione peroxidase. This enzyme is required during the oxidation of glutathione by hydrogen peroxide (H₂O₂) and organic hydroperoxides.

Protein molecules are large and have varying molecular weights. Their molecular weights cannot be determined by ordinary methods. The most important method for their determination is based upon the measurement of sedi­mentation rates in the ultracentrifuge. They can also be determined by osmotic pressure measurement, light scatter­ing, etc.

Molecular weights of some of the typical proteins from different sources are as follows: salmine (salmon sperm) – 5,600; cytochrome (heart) -156,000; lactalbumin (milk) -17,400; gliadin (wheat) – 27,400; pepsin (stomach) – 35,500; insulin (pancreas) – 40,900; haemoglobin (human) – 63,000; myogen (muscle) -150,000; thyroglobulin (thy­roid) – 628,000; myosin (muscle) -1,000,000; virus (tomato) – 7,600,000; mosaic virus (tobacco) – 60,000,000.
It has been shown by Svedberg that by a change in the protein concentration or in the pH or in the salt concentration of the medium, proteins in solution, may easily dissociated into smaller component units. If the original condition is resorted the fragments may re-associate and form the original molecule. This shows that proteins of higher molecular weights are built up by conglomeration of smaller units.
Virus Proteins:
These are nucleoproteins of special interest. Several varieties of virus proteins with very high molecular weights have been isolated in crystalline form, from the juice of plants suffering from virus diseases. The virus protein of tobacco mosaic disease has a molecular weight of about 60,000,000, an isoelectric point or pH 3.49 and contains about 5% nucleic acid. It is a highly interesting fact that even minute amount of pure recrystallised virus proteins induces the virus disease when injected into healthy plants.
One millionth of a milligram of tobacco mosaic protein invariably infects a healthy plant and from such infected plants large quantities of the same protein can be recovered. This astonishing property of the virus proteins, in directing the metabolism of the plant cells so that they synthesise more of the same protein, is unique in a chemical compound and is suggestive of the reproduction of a living organism rather than the elaboration of a non-living chemical mol­ecule. The evidence accumulated so far indicates that these giant molecules are purely of chemical nature although we have no absolute proof that they are dead.