|MadSci Network: Biochemistry|
Answer: In general the solubility of proteins in [pure] water is limited, in order to improve their solubility, the preferred solution contains a [buffer] salt to have [a stable pH] an ionic strength of about 0.1-03. molar. In doing so, the ionic charges of any protein is compensated and solubility problems do not occur. In the case of pepsin it may even be more beneficial as at the pH values where this enzyme is functional [pH range 2-4] many protein substrates are poorly soluble [e.g. caseins], therefore this salting in effect - as it is called - is useful. Besides that, due to the specificity of pepsin, which cleaves peptides bonds next to hydrophobic amino acid residues like phenylalanine, tyrosine and others, the peptides formed have usually a rather hydrophobic character. To avoid their precipitation in a pure aqueous solution, salt is essential to keep these degradation products in solution too. So [buffer] salts are used to maintain the optimal environment for the enzyme to function and to keep the enzyme, the substrate and the degradation products in solution.
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