Solubility properties of proteins

Proteins consist of polypeptide chains, each with a specific sequence of amino acid side chains. The amino acid side chains (or residues if we include the peptide portion) can be placed in different classes according to their structure. The simplest classification on that basis is into polar and nonpolar classes. We will be dealing with solubility in aqueous solution, so the polar residues will tend to be soluble and nonpolar residues will tend to be insoluble. 

Proteins have a balance between polar and nonpolar residues and this balance will largely determine the solubility. If all of the residues were nonpolar (e.g., polyleucine), we would expect the protein to be insoluble. Within the two main classes of residues, there are degrees of polarity (hydrophilicity) and nonpolarity (hydrophobicity). The most polar residues are the ionized ones, such as glutamic acid at high pH and lysine at low pH. Attempts have been made to use amino acid composition of proteins to predict solubility. As a result, several parameters have been introduced and we will briefly examine fhese. 

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A possible transfer of mannose from retinyl mannosyl phosphate to polypeptide was first indicated by the studies of Maestri and De Luca (1973), though these did not prove that retinyl mannosyl phosphate was the mannosyl donor. In a more detailed analysis Rosso etaL (1977) used a radioactive label largely free of dolichyl mannosyl phosphate in a cell-free system and obtained a clear-cut incorporation of mannose from retinyl mannosyl phosphate into a substance with the solubility properties of protein and which was labile to proteolysis. Some free mannose, its 1-phosphate and retinyl mannosyl phosphate were also present at the end of the incubation, but no large mannolipid was detected. Dolichy CJmannosyl phosphate did not give any transfer of [ C] mannose to protein and further support for a distinctive retinol pathway came from the effects of EDTA. This totally failed to block transfer from retinyl [ mannosyl phosphate, under conditions where block... 

The preceding theory provides a fundamental foundation for understanding the solubility properties of proteins. However, to gain a greater understanding, we need to invoke molecular theory. Let us look at two simple examples where we can combine thermodynamic principles with molecular properties. 

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