Disulfide

... complex made up of six protein subunits: a single copy of the A subunit (part A), and five copies of the B subunit (part B), connected by a disulfide bond. The five B subunits form a five-membered ring that binds to GM1 gangliosides on the surface of the intestinal epithelium cells. The ...

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MALDI-TOF mass spec comes to mind to check if the disulfide is broken. I suppose it depends what instruments are at hand.

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That depends on particular enzyme. Some may dimerize after e.g. phosphorylation, glycosylation or other modification, some may dimerize due to disulfide bridge, some may have even other mechanism.

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As canalon said, the program calculates MW of only one subunit, but obviously, they are connected by disulfide bonds to form tetramer.

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... The computer will just the size of the sequence you fed it and calculate a size based on that. It does not magically analyse the protein to infer disulfide bonds and such.

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