1. "How mother nature fine tunes protein stability?"

Most proteins are globular proteins in which the chain of amino acids folds
to give a roughly spherical shape. Consequently, as globular proteins become
larger, they bury a larger fraction of their side chains and peptide groups.
Based on what we have learned about the stabilizing forces, proteins should
become more stable as the size increases. This is not observed: small and
large proteins have about the same stability. This suggests that evolution
has strategies to keep large proteins from becoming too stable. In an
attempt to learn what this strategy is, we compare a small globular protein
with just 36 amino acids to a large globular protein with 341 amino acids.
Our goal is to gain a better understanding of the means used to keep large
proteins from becoming too stable. Our analysis suggests that burying
charged groups in the interior of globular proteins is the primary strategy
used to fine tune protein stability. Preliminary results from experimental
studies designed to test this idea will be discussed.

2. " Factors determining the pK values of the ionizable groups in proteins"

The content and pK values of the ionizable groups of proteins determine the
net charge on the protein and make contributions to such important
properties as solubility, stability and activity. We have just completed an
experimental study of the most important factors that determine the pK
values of the ionizable groups of proteins. We will discuss these studies.
The pKs depend on the local environment. Groups on the surface of the
protein, exposed to solvent, typically have pKs that are close to the
intrinsic values observed in model peptides with small perturbations due
mainly to electrostatic interactions with other charged groups on the
protein. In contrast, the pKs of buried ionizable groups may be
substantially perturbed, upward or downward, from model compound values. In
addition to electrostatic interactions with other charged groups, these
buried groups may be perturbed by the unfavorable Born self energy incurred
when a charged group is removed from water and buried in the protein
interior in an environment with a lower dielectric constant and by hydrogen
bonding. We will show that the pK of a buried carboxyl group can be raised
as high as 9 by the Born self energy or lowered to 0.5 by hydrogen bonding.