Protein Properties, Denaturation and Biological Importance
Protein Properties
1. Solubility
Proteins are soluble or insoluble depending on their conformation. When a protein adopts a globular conformation, the radicals (R groups) of the amino acids are oriented outward. The ionized R groups establish weak bonds (hydrogen bridges) with the water molecules. When a protein solubilizes, a shell of water molecules surrounds it, and this first layer of water molecules can prevent other proteins from approaching, which leads to its insolubilization. This property is the basis of the possible hydration of proteins in the buffer systems of living organisms.
2. Amphoteric Capacity
Proteins are amphoteric: they have a complex ionization and can neutralize the variations in pH. They can behave as an acid or as a base, both with a gain or loss of protons where they are found.
3. Denaturation
A protein is denatured when it suffers a fracture of its bonds. The tertiary and secondary structures are lost while the primary structure is preserved. In these cases the protein becomes a filament, unfolds into linear chains, and these chains become entangled, forming insoluble fibrous structures with water. The agents that denature a protein include excessive heat, high salinity, molecular agitation, and others. The visible effect of this phenomenon is that proteins become soluble or insoluble and lose most of their biological activity.
Proteins denature when heated above 50 and 60 °C, and others denature below 10 to 15 °C. Denaturation can be reversible in some cases and irreversible in many others.
4. Specificity
Each species of living being can make its own proteins, and within the same species there are differences between individuals. Glucides and lipids are not specific, since they are common to all living proteins. Specificity explains some biological phenomena, such as the compatibility or incompatibility of transplants between bodies and biological grafts.
Differences Between Glucides and Lipids
Lipids are energetic compounds, not only because they have a very important structural component: they are the major component of cell membranes. They are formed by carbon, hydrogen and oxygen, and differ from glucides in proportion. A major property of lipids is their low polarity: they are poorly soluble in water and soluble in organic solvents such as alcohol, benzene, ether, and others.
Saturated and Unsaturated Fatty Acids
Saturated fatty acids, such as butyric acid (4 carbons, CH3-(CH2)2-COOH), palmitic acid (16 carbons) and stearic acid (18 carbons), have only single bonds and no free double bonds. Their melting point is high and they are solid at room temperature.
Unsaturated fatty acids, such as oleic acid (18 carbons, one double bond) and linoleic acid (18 carbons, two double bonds), and linolenic acid (omega-3), contain double bonds. Because of these double bonds, they are liquid at room temperature. The first peculiarity to know is that unsaturated fatty acids are rigid.
Importance of Tertiary Structure
The tertiary structure determines the function of the protein. When a protein attains its tertiary structure, it has its greatest biological activity.