R groups interactions

In the unfolded state, the peptide chain and its R groups interact with solvent water, and any measurement of the free energy change upon folding must consider contributions to the enthalpy change (AH) and the entropy change (A.S) both for the polypeptide chain and for the solvent ... 

Tertiary structure is due to / V I side-chain (R-group) interactions, which include hydrophobic interactions, covalent disulfide bridges, electrostatic salt bridges, and hydrogen bonds. 

List the R group interactions that maintain protein conformation. 

The a-carbon of each amino acid is also bonded to a hydrogen atom and a side chain, or R group. In a protein, the R groups interact with one another through a variety of weak attractive forces. These interactions participate in folding the protein chain into a precise three-dimensional shape that determines its ultimate function. They also serve to maintain that three-dimensional conformation. 

The question, then, is why is the super hot enzyme so stable at very high temperatures, while its low temperature counterpart is not. The answer lay in the tertiary structure of the enzyme. Adams observed that the three-dimensional structure of the super hot enzyme is held together by many more R group interactions than are found in the low-temperature version. These R group interactions, along with other differences, keep the protein stable and functional even at temperatures above 100°C. 

The tertiary structure results from interactions between the R side chains of the amino acid residues. These R-group interactions are of four types ... 

Figure 19.10 R-group interactions leading to tertiary protein structure.

I LEARNING CHECK 19.3 What kind of R-group interaction might be expected if the following side chains were in close proximity ... 

Look for peptide bonds, hydrogen bonds in peptide, R group interactions, two or more subunits... 

Enzymes are most active at their optimum pH, the pH that maintains the proper tertiary structure of the protein (see Figure 16.16). If a pH value is above or below the optimum pH, the R group interactions are disrupted, which destroys the tertiary structure and the active site. As a result, the enzyme no longer binds substrate, and no catalysis occurs. Small changes in pH are reversible, which allows an enzyme to regain its structure and activity. However, large variations from optimum pH permanently destroy the structure of the enzyme. 

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