Although all atoms of a protein molecule in water are always in constant rapid r

Question

Although all atoms of a protein molecule in water are always in constant rapid random motion (translation, rotation and to some degree vibration) at T = 310 K (37 C), most natural proteins have well-defined 3D structures in water at 37 C. If a protein molecule had only kinetic or thermal energy, it would not, in theory, be able to form a well-defined 3D structure since the various parts of the molecules very likely would move apart from one another at any given moment. Therefore, there must be some other energy terms or molecular forces that hold the various parts of the molecules together. All atoms are composed of a nucleus with positive charge and an outer electron shell with negative charge. The covalent bonds are formed by the sharing of an electron pair between two atoms. Based on what we have learned about internal energy (U) and physical forces, postulate what kind of energy (e.g. gravitational or electrostatic potential energy) might be involved and the nature of the physical force or forces (e.g. gravitational force?) involved in the formation and maintenance of the 3D structure of a protein molecule in water at 37 C. Please also briefly describe your reasoning.

Explanation / Answer

Protein structures are primarily governed by hydrophobic interactions. The protein molecule in water have its hydrophobe groups out of contact with water, i.e., in contact with each other. In this way a force of association is provided which is not so much that of attraction between hydrophobe groups, which is always weak, but that of repulsion of the groups out of the water medium.

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