First of all, I am not an expert in proteins or pharmacology, so sorry if it is not the answer you expected or my information is not useful to you.

I have done a small theoretical calculation using COSMO-RS (quantum chemical method) of the differences between methionine and selenomethionine

Unless one has to take into account that the volume of selenium is higher than the atomic volume of sulfur, I did not find any significant differences in the structure or polarity of both compounds. There is also no difference in the interaction (solubility) of these compounds with others such as water or octanol.

I add the structures of both compounds with the electronic charge density on the surface. (Blue zone = hydrogen bond donor or acidic region, red zone = hydrogen bond acceptor region or basic zone, green zone = non-polar region)

Thanks Daniel Moreno Fernández. This is exactly what I wanted to know. If there is no structural difference between the two then it wont affect the functions much. So, whenever we find selenium in a protein we can replace it with sulfur because of this reason. 

Cheers,

Aditya

One of the most characteristic effects of selenium exposure in fish is the appearance of developmental abnormalities. This occurs mainly due to oxidative stress but also, due to the replacement of sulphur by selenium in building protein blocks. I believe there will be effects due to sulphur replacement by selenium. Check the work done by Janz et al., Muscatello et al., Palace et al. There is lot of information on this out there.