Tyrosine hydrogen bonds make a large contribution to protein stability

Area of Science:

• Protein biochemistry
• Structural biology
• Biophysics

Background:

• Protein stability is crucial for biological function.
• Tyrosine residues, through their hydroxyl (-OH) groups, can participate in hydrogen bonding, potentially influencing protein stability.
• Understanding the specific contributions of individual amino acid residues is key to deciphering protein folding and stability.

Purpose of the Study:

• To investigate the role of hydrogen bonds formed by tyrosine hydroxyl groups in protein stability.
• To quantify the impact of specific tyrosine mutations on the stability of Ribonuclease Sa (RNase Sa) and RNase Sa3.

Main Methods:

• Site-directed mutagenesis was used to create 16 tyrosine-to-phenylalanine mutants in RNase Sa and RNase Sa3.
• Protein stability was measured for each mutant.
• Crystal structures of two selected mutants were determined.

Main Results:

• Mutating tyrosine to phenylalanine resulted in decreased stability for most mutants, with an average decrease of 2.0 kcal/mol for residues involved in hydrogen bonds and 0.4 kcal/mol for those not.
• A few mutations led to increased stability, suggesting complex contributions.
• Structural analysis confirmed that mutated tyrosine's hydrogen bonding partners formed new interactions with water, indicating favorable internal interactions in the folded state.

Conclusions:

• Tyrosine residues, particularly their -OH groups, significantly contribute to protein stability through hydrogen bonding and favorable interactions within the protein core.
• The burial of polar groups, like tyrosine -OH, in the hydrophobic interior of folded proteins is energetically favorable compared to interactions with water.
• These findings underscore the importance of specific amino acid interactions in maintaining protein structure and function.