Protein Properties, Functions, and Classification
Properties of Proteins
Proteins’ properties depend mainly on the nature of their amino acids. These properties include:
Behavior and Chemical Solubility
Proteins, like amino acids, are amphoteric substances. This means they can buffer pH variations. Their solubility depends mainly on their conformation. Generally, fibrous proteins are insoluble, and globular proteins are water-soluble. However, due to their high molecular mass, they form colloidal dispersions. Nonpolar amino acids are located inside the molecule, while polar, acidic, and basic amino acids, capable of forming hydrogen bonds with water molecules, are located on the outer periphery.
Specificity
Proteins have characteristics specific to each species, and within a species, some vary from one individual to another. This property is responsible for rejection in transfusions and transplants, where the protein acts as a foreign molecule, and the receiving body reacts against it. Some proteins (homologous proteins) perform the same function in different species. The differences between these proteins are greater the further apart these species are on the phylogenetic scale. Therefore, comparative studies of these proteins can determine the evolutionary relationship between species.
Denaturation
Denaturation is the process by which a protein loses its characteristic configuration (native conformation) and, subsequently, its properties, ceasing its biological activity. Denaturation occurs when the protein is subjected to unfavorable environmental conditions (increased temperature, pH variations, presence of certain substances, etc.) that can break bonds maintaining the secondary, tertiary, and quaternary structures, but not the primary structure. Under certain conditions, denaturation may be reversible, and the protein can return to its native conformation. This occurs when the denaturing factors are not very intense and do not act for long.
Protein Functions
- Structural: Proteins, especially fibrous ones, are part of most structures, both globular and organic.
- At the cellular level: Glycoproteins (form cell membranes), histones (in chromosomes), tubulin, and actin (forming the cytoskeleton, cilia, and flagella).
- At the organic level: Collagen (in tendons, bones, cartilage, etc.), elastin (found in certain organisms), keratin (present in the epidermis, nails, hair).
- Reserve: Some proteins act as amino acid reserves that will be used as nutrients for embryonic development.
- Ovalbumin (found in eggs)
- Casein (found in milk)
- Homeostatic: They help maintain the constant conditions of the internal environment. They play a role in maintaining osmotic balance and, due to their amphoteric character, act as pH buffering systems.
- Transport: Several proteins are responsible for carrying specific molecules inside the body.
- Permeases and pumps (membrane proteins that transport molecules to and from the cell).
- Hemoglobin (carries oxygen in the blood).
- Lipoproteins (transport insoluble lipids).
- Defensive or Protective Function: A group of proteins defends and protects the body against foreign substances.
- Immunoglobulins (antibodies).
- Hormonal Function: Some proteins are hormones that regulate important physiological processes.
- Insulin (regulates blood glucose levels).
- Myofibrils are responsible for muscle cell contraction.
- Catalyst Function: Some proteins facilitate and accelerate chemical reactions that take place in the body by decreasing the energy required for these reactions to occur. These are enzymes.