BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 5. STRUCTURE, PROPERTIES, AND FUNCTIONS OF PROTEINS

5.7. Physicochemical Properties of Proteins and Methods for Their Isolation

5.7.1. Physicochemical Properties of Proteins

Individual Proteins differ in their physicochemical properties: molecular shape, net molecular charge, The ratio of polar and non-polar groups On the surface of the native protein molecule, and the degree of resistance to Denaturing Agents.

As previously noted, based on molecular shape, proteins are classified into globular and fibrous. Globular proteins have a more compact Structure, with most of their hydrophobic residues hidden within the Hydrophobic core, and they are significantly more soluble in Body Fluids than Fibrous proteins (with Membrane Proteins being an exception).

Proteins are macromolecular compounds, yet they can vary greatly in molecular weight, ranging from 6,000 to 1,000,000 Da and higher. The Molecular Weight of a protein depends on the number of amino acid residues in the polypeptide chain, and for Oligomeric Proteins, it also depends on the number of protomers (or subunits) that form it.

Proteins contain residues of Lysine, Arginine, Histidine, glutamic acid, and aspartic acid, which possess functional groups capable of ionization (ionogenic groups). In addition, the N- and C-termini of The polypeptide chains contain $\alpha$-amino and $\alpha$-carboxyl groups that are also capable of ionization. The net charge of a protein molecule depends on the ratio of ionized anionic residues of Glu and Asp to cationic residues of Lys, Arg, and His.

The degree of ionization of the functional groups in these residues depends on the pH of the medium. At a solution pH of approximately 7, all ionogenic groups of the protein are in an ionized state. In an acidic environment, an increase in proton (H+) concentration leads to the suppression of carboxyl group dissociation and a decrease in the negative charge of proteins:

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In an alkaline environment, the binding of excess OH^- with protons formed during the dissociation of NH3 to produce Water leads to a decrease in the positive charge of proteins:

The pH value at which a protein acquires a net zero charge is called the isoelectric point and is designated as pI. At this point, the number of positively and negatively charged groups on the protein is equal, meaning the protein is in an isoelectric state.

In most cellular proteins, anionogenic groups (-СОО^-) predominate; the isoelectric point of these proteins lies at weakly acidic pH values, whereas the isoelectric point of proteins dominated by cationogenic groups lies at alkaline pH values. The most striking example of such intracellular proteins, rich in arginine and lysine, are Histones, which are components of Chromatin.

Proteins with a net positive or negative charge are more soluble than those in the isoelectric state. The net charge increases the number of water dipoles capable of binding to the protein molecule and prevents contact between like-charged molecules, thereby increasing Protein solubility. Charged proteins can move in an electric field: negatively charged anionic proteins will move toward the positively charged anode (+), while cationic proteins will move toward the negatively charged cathode (-). Proteins in the isoelectric state do not move in an electric field.

Polar residues predominate on The surface of most intracellular proteins, but the ratio of polar to non-polar groups varies among different individual proteins. For instance, the protomers of oligomeric proteins often contain hydrophobic residues in their contact regions with one another. The surfaces of proteins that function within or attach to membranes are also enriched in hydrophobic residues. Such proteins are more soluble in Lipids than in water.

The Solubility of proteins in water depends on all of their aforementioned properties: shape, molecular weight, magnitude of charge, and the ratio of polar and non-polar functional groups on the protein surface. Furthermore, it is determined by the COMPOSITION OF THE solvent, i.e., the presence of other solutes in the solution.

For example, some proteins dissolve better in a weak salt solution than in distilled water. At the same time, an increase in the concentration of neutral salts can promote the precipitation of certain proteins. Denaturing agents present in the solution also reduce protein solubility.