Protein_Science_-_August_1995_-_Spassov_-_The_optimization_of_protein_solvent_interactions__Thermostability_and_the_role_of

This document discusses the optimization of protein-solvent interactions, particularly focusing on thermostability and the roles of hydrophobic and electrostatic interactions in proteins from thermophilic organisms.

Authors: Velin Z. Spassov, Andrej D. Karshikoff, Rudolf Ladenstein
Publication: Protein Science (1995), Volume 4, Pages 1516-1527
Affiliations:
- Centre for Structural Biochemistry, Karolinska Institutet, Stockholm, Sweden
- Institute of Biophysics, Bulgarian Academy of Sciences, Sofia, Bulgaria

Analyzed protein-solvent interactions using a parameter based on solvent-accessible area in native vs. unfolded structures.
Investigated 183 nonhomologous proteins with known structures.
Found no difference in solvent-accessible area between monomeric and oligomeric proteins.
Indicated that proteins above 28 kDa likely form domain structures or oligomers rather than enlarged single domains.

Recent advancements in understanding protein thermostability correlate with the increase in sequence data and 3D structures.
Identified that properties responsible for higher denaturation temperatures in thermostable proteins remain to be fully elucidated.
Key Example: Thermostable ferredoxin from Clostridium thermosaccharolyticum differs from its mesophilic counterpart at two amino acid positions.
Other examples show that small sequence changes can significantly impact thermostability.

Primary Interactions: Charged and hydrophobic interactions influence overall thermal stability.
Theoretical Basis: The unique native structure corresponds to the minimum free energy, determined by the balance of interactions:
- Van der Waals forces
- Hydrogen bonds
- Charge-charge interactions
- Hydrophobic effects
Stress on electrostatic interactions and contributions to stability through hydrogen bonds and hydrophobic aggregates.

Established that solvent-accessible areas are linearly tied to protein molecular mass, with differing slopes for monomeric (a = 45.7, x = 0.732) and oligomeric proteins (a = 36.9, x = 0.773).
Notable linear dependence in solvent-accessible areas recognized as a fundamental characteristic, confirming previous studies.

The optimization of protein-solvent interactions is critical; specific analysis methods developed to quantify hydrophobic interactions.
Proteins from thermophilic organisms exhibit greater optimization in hydrophobicity, with high efficiency in minimizing hydration of apolar groups.
Optimization Characteristics: Involves hydrophobic parameter ( t_h ) and hydrophilic parameter ( t_p ), both show compensatory behavior concerning protein size.

Two critical masses identified:
- ~1,000 atoms critical for forming a sufficient hydrophobic core.
- ~2,000 atoms represent the upper limit for single-domain proteins—additional mass favors domain formations or oligomerization.

Point mutations exhibit minimal impact on optimization parameters unless mass replacements occur.
Hydrophilic interactions primarily influenced by hydrogen bonding with reduced solvent accessibility.
Analysis discusses membrane-bound proteins showing distinctive optimization patterns according to structural environments.

Thermophilic proteins active at high temperatures demonstrate specific adaptations in structures affecting thermal stability.
Optimization parameters reflected not only environmental adaptations but also inherent protein qualities relating to evolutionary pressures.

The study outlines principles underlying the stabilizing factors of thermostable proteins in terms of molecular interactions and structural composition.
Supports the theory that understanding hydrophobic and charge interactions can further elucidate the abilities of enzymes and proteins from extremophilic organisms.