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Proteins

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Proteins

Postby rikajaj » Tue Sep 25, 2007 1:57 am

Proteins often are composed of multiple polypeptide chains. For example, ADH (alcohol dehydrogenase) is a protein composed of two of the same polypeptide chains, a homodimer. Consider the case of an individual that is heterozygous for the ADH gene. The two alleles differ by only one nucleotide, and only one amino acid is different between the two polypeptides they encode. The polypeptide specified by each allele can form a functional dimer with itself, or with the polypeptide specified by the other allele. How many different proteins will be produced in the heterozygote? How much of each form will exist? Why?
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Postby blcr11 » Tue Sep 25, 2007 2:58 pm

Unless I'm missing something, this is a simple matter of counting. The heterozygote makes two distinguishable chains, call them alpha and beta. If all types of dimers are equally likely, there are three types of dimers possible: pure alpha, pure beta and alpha-beta heterodimers. If each are equally likely, then 33.3% of the mixture should be represented by each type of dimer. I guess you can argue there are five types of proteins, the two monomers and the three dimers, but there is no way to know how many monomers are present without knowing more information (like turnover rates and association constants, etc).
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Postby kotoreru » Tue Sep 25, 2007 4:10 pm

Sounds as though you just did somebody's homework for them there blcr11...
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Postby MrMistery » Wed Sep 26, 2007 5:26 pm

congrats blcr11 :)
And be careful, ADH is the accepted abreviation of antidiurhetic hormone, which is only one polipeptide chain with 9 aminoacids.
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Postby blcr11 » Tue Oct 02, 2007 10:16 am

Well, if they copied my answer verbatim without thinking it through for themselves, they probably got it wrong, because I think I did it wrong. I was acting as if the chains leapt into existence as pre-made dimers, which is, obviously, not the case. They are made as individual molecules that later associate to form dimers. If you look at it as a pseudo-Punnet square, then 25% of the dimers are of each homodimer and 50% are heterodimers (alpha associates with beta or beta associates with alpha). There are four possible associations, two of them are the same heterodimer, so there are three distinguishable proteins, ignoring residual monomers. Sorry to mislead anyone, but this is a caveat emptor board.
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