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newbie question: how many variations of a protien exist

Frank O. Fackelmayer Frank at Fackelmayer.de
Tue May 28 06:21:50 EST 2002



Roger Anderson wrote:
> 
> How many variations of typical proteins exist for humans.
> 
> For example, hemoglobin, has 146 aminoacids (438 base paris needed to code).
> Now how many variants of this protein is allowed for it to still function,
> in humans for example. Ie how many aminoacids can be different, and the
> protein can be still called a hemoglobin protein.
> 

there is no clear answer to this question. It depends on many things.
For example, you can "change" an amino acid residue to a different one
without affecting the function at residue 100, but not at residue 101.
Whenever the change affects the structure of the protein in general, or
e.g. an active center of an enzyme, the affected protein will very often
(but not necessarily) become inactive. It is then a sematic question
whether the protein can still be called by its normal name (I would say so).
In addition, not all amino acid changes have comparable effects. It may
not harm a protein when you exchange Thr for Ser, or Ala for Leu,
because these amino acids are very similar in their chemical properties.
When you exchange Ser for Asp, this can mimic a phosphorylated Ser (and
thereby "switch on" an enzyme that is regulated by phosphorylation on
that particular residue). In contrast, when you exchange an amino acid
for one with different chemical properties (e.g. Phe for Cys), it may
(or may not, depending on the position of the change) ruin your protein completely.

It also depends on the importance of the protein. Some proteins are
luxury for the cells, many are essential. A good rule of thumb is that
the more conserved a protein is in evolution, the more important it is
and the lower the number of allowed "changes" will be. 
When you are interested in a particular protein, run database
comparisons with the same protein from other species. This will give you
hints as to where changes are tolerable.


> Other examples I am interested in is proteins needed for the brain. How many
> proteins and how large is a typical protein needed for a brain to function.

This question is also difficult to answer. Cell types differ in their
protein repertoire (that is what actually makes the difference!), and
tissues such as brain are composed of many different cell types. You may
find good estimates for the number of proteins in databases/websites
about proteomics.
There is also no typical size for a "brain protein"; all sizes from a
few amino acids (neuropeptides) to several hundred of kilodaltons will
be present.

> What type of variation is allowed for a typical protein of this function.

That would not be different from any other tissue, I suppose.


hope this helps,
Frank




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