  The rigidity and flexibility of homologous psychrophilic(P), mesophilic(M)
and thermophilic(T) proteins have been investigated at the global and local
levels in terms of packing factor and atomic fluctuations obtained from
B-factors. For comparison of atomic fluctuations, correction of errors by
considering errors in B-factors from all sources in a consolidated manner and
conversion of the fluctuations to the same temperature have been suggested and
validated. Results indicate no differences in the global values like average
packing factor among the three classes of protein homologs but at local levels
there are differences. Comparison of homologous proteins triplets show that the
average atomic fluctuations at a given temperature obey the order P>M>T.
Packing factors and the atomic fluctuations are anti-correlated suggesting that
altering the rigidity of the active site might be a potential strategy to make
tailor made psychrophilic or thermophilic proteins from their mesophilic
homologs.
