It may finally be recalled that conditions during heating, such as pH
and ionic composition, greatly influence denaturation rate. In some liquids,
the pH changes significantly during prolonged heating. Another variable
may be the presence of a suitable ligand, especially a substrate for an
enzyme. Many enzymes are far more heat stable in the presence of substrate.
Question
The low-temperature inactivation of a proteolytic enzyme presented in Figure 7.11c
was observed to proceed faster when the enzyme preparation had been further
purified. What would be the explanation?
Answer
As mentioned, the inactivation was due to autodigestion. Molecules still in the
native, i.e., enzymatically active, conformation can split unfolded ones. We thus have
N
?
/U
NþU/?NU?NþU 1 þU 2where U 1 and U 2 are the enzymatically inactive fission products. However, besides
NþU/?NU, the competing association
NþP/?NPwill occur, where P means any other protein that can be attacked by the enzyme. This
would slow down the splitting of U, the more so for a larger [P]. Since purification of
an enzyme preparation essentially means removal of other proteins, it will diminish
competition and hence increase splitting of U.
7.3 SOLUBILITY
The solubility of a component is defined as its concentration in a saturated
solutioncsat, i.e., in a solution in contact and in equilibrium with crystals or
a fluid phase of the component. Such a situation can often not be realized
for a protein, because crystals cannot be obtained, and determination of the
solubility then is somewhat questionable. Nevertheless, fairly consistent
results can generally be obtained.