Sites of Hydration in Proteins

Hydration of amino add side-chain and polypeptide main-chain atoms. In Thble 23.4, the hydration of the 15 well-refined proteins given in Thble 19.1 is analyzed in terms of functional groups in main-chains and in side-chains. About 44% of the water molecules are bound to side-chain atoms, 42% to main-chain C=0 and only 14% to main-chain N-H. 

Protein ACT CPA CYTC DHFR ECR INS LYS MB PUP PCY RNASE TLN TRYP Total 

Number of water molecules in each category Protein ligands lO 91 65 12 74 31 80 31 111 22 13 20 45 31 626 

The hydration of amino acid side-chains is more difficult to evaluate because they display higher thermal motion relative to main-chain atoms. Therefore the thermal motion of the associated water molecules is also higher and they are frequently not observed in the electron density maps. For this reason, the apparent hydration of main-chain atoms is more extensive compared with that of side-chain atoms although, on average, the polar side-chains that cover the periphery of the 

Side-chain Number of side-chains in sample Number of water molecules hydro-gen-bonded Number of water molecules  

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