Page 83 - Color Atlas of Biochemistry

P. 83

74 Biomolecules

Protein folding When the urea and thiol are removed by
dialysis (see p. 78), secondary and tertiary
Information about the biologically active (na- structures develop again spontaneously. The
tive) conformation of proteins is already en- cysteine residues thus return to a suf ciently
coded in their amino acid sequences. The na- close spatial vicinity that disulfide bonds can
tive forms of many proteins arise spontane- once again form under the oxidative effect of
ously in the test tube and within a few mi- atmospheric oxygen. The active center also
nutes. Nevertheless, there are special reestablishes itself. In comparison with the
auxiliary proteins (chaperonines) that sup- denatured protein, the native form is aston-
port the folding of other proteins in the con- ishingly compact, at 4.5 2.5 nm. In this state,
ditions present within the cell (see p. 232). An the apolar side chains (yellow) predominate
important goal of biochemistry is to under- in the interior of the protein, while the polar
stand the laws governing protein folding.This residues aremainlyfound on thesurface.This
wouldmakeit possibleto predict theconfor- distribution is due to the “hydrophobic effect”
mation of a protein from the easily accessible (see p. 28), and it makes a vital contribution to
DNAsequence(seep. 260). the stability of the native conformation (B).

A. Folding and denaturation of B. Energetics of protein folding
ribonuclease A
The energetics of protein folding are not at
The folding of proteins to the native form is present satisfactorily understood. Only a sim-
favored under physiological conditions. The plified model is discussed here. The confor-
native conformation is lost, as the result of mation of a molecule is stable in any given
denaturation,atextreme pH values,at high conditions if the change in its free enthalpy
temperatures, and in the presence of organic during folding (∆G fold )is negative(seep.16).
solvents, detergents, and other denaturing Themagnitude of thefolding enthalpy is af-
substances, such as urea. fected by several factors. The main factor
Thefactthata denaturedprotein can spon- working against folding is the strong increase
taneously return to its native conformation in the ordering of the molecule involved. As
was demonstrated for the first time with ri- discussed on p. 20, this leads to a negative
bonuclease, a digestive enzyme (see p. 266) change in entropy of ∆S conf and therefore to
consisting of 124 amino acids. In the native a strongly positive entropy term –T ∆S(violet
form (top right), there are extensive pleated arrow). By contrast, the covalent and nonco-
sheet structures and three α helices. The eight valent bonds in the interior of the protein
cysteine residues of the protein are forming have a stabilizing influence. For this reason,
four disulfide bonds. Residues His-12, Lys-41 the change in folding enthalpy ∆H fold is neg-
and His-119 (pink) are particularly important ative (red arrow). A third factor is the change
for catalysis. Together with additional amino in the system’s entropy due to the hydropho-
acids, they form the enzyme’s active center. bic effect. During folding, the degree of order
The disulfide bonds can be reductively in the surrounding water decreases—i. e.,
cleaved by thiols (e. g., mercaptoethanol, ∆S water is positive and therefore –T ∆Sis
HO-CH 2 -CH 2 -SH). If urea at a high concentra- negative (blue arrow). When the sum of these
tion is also added, the protein unfolds com- effects is negative (green arrow), the protein
pletely. In this form (left), it is up to 35 nm folds spontaneously into its native conforma-
long. Polar (green) and apolar (yellow) side tion.
chains are distributed randomly. The dena-
tured enzyme is completely inactive, because
the catalytically important amino acids (pink)
are too far away from each other to be able to
interact witheachother and withthe sub-
strate.

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