An antibody molecule has a molecular weight of 150 kD when analyzed by SDS polyacrylamide-gel electrophoresis (SDS-PAGE). Treatment with mercaptoethanol, a reducing agent that breaks disulfide linkages, yields polypeptides with molecular weights of 50 kD and 25 kD, and both of these bands are similar in intensity and thickness. Based on this gel pattern as described, what can we conclude about the molecular composition of the antibody molecule

The antibody has a molecular weight of 150 kD and consists of four subunits: two subunits of 50 kD and two subunits of 25 kD, which are bonded by covalent bonds.

Explanation:

Sodium dodecyl sulfate (SDS) is a detergent used to alter and denature proteins before gel electrophoresis. SDS can break hydrophobic interactions and hydrogen (H) bonds, but this detergent does not break down disulfide bonds, which is a type of covalent bond between two sulfur atoms. Moreover, mercaptoethanol is a reducing agent used to break disulfide bonds. In this case, it is expected that mercaptoethanol can break disulfide bonds between the four subunits of the mature antibody, thereby yielding two bands corresponding to the 50 kD and 25 kD bands (two subunits of 50 kD + two subunits of 25 kD = 150 kD).

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