Lipids, Amino Acids, and Proteins: Structure, Function, and Properties

Lipids

Functions of Lipids

• Energy Storage: Lipids serve as the primary energy reserve in the body.
• Structural Components: Lipid bilayers form the plasma membrane and other cell organelles.
• Protection: Lipids cushion vital organs, like the kidneys, and protect body surfaces (e.g., waxes on hair and fruit).
• Biocatalysis: While not all lipids are biocatalysts themselves, some play a role in biocatalyst synthesis.
• Transport: Lipids are emulsified and transported from the intestine to either storage in adipose tissue or sites of utilization.

Amino Acids

Amino acids are small organic compounds characterized by a carboxyl group and an amino group. They are solid, crystalline, water-soluble, have high melting points, and exhibit optical activity. Primary amino acids, the building blocks of proteins, have an average molecular mass of 120 u and feature an amino group attached to the same carbon as the carboxyl group.

Chemical Behavior

In aqueous solutions, amino acids exhibit amphoteric behavior, acting as both acids (donating protons from the carboxyl group) and bases (accepting protons in the amino group). This results in a bipolar ionic form.

Proteins

A protein is a polypeptide chain containing more than fifty amino acids. Protein structure is organized into four levels:

• Primary Structure: The linear sequence of amino acids.
• Secondary Structure: The spatial arrangement of amino acids (e.g., alpha-helices and beta-sheets).
• Tertiary Structure: The overall three-dimensional folding of the polypeptide chain, stabilized by interactions between amino acid side chains (e.g., myoglobin).
• Quaternary Structure: The arrangement of multiple polypeptide chains within a protein complex (present only in some proteins).

Properties of Proteins

• Solubility: Protein solubility is influenced by the proportion of polar amino acid residues, particularly those with charges, which form hydrogen bonds with water.
• Denaturation: Denaturation is the loss of tertiary and quaternary structure, sometimes even secondary structure, due to disruption of stabilizing bonds. This can be caused by changes in pH, temperature fluctuations, or molecular agitation.
• Specificity: Proteins interacting with other molecules possess specific three-dimensional structures and strategically placed amino acids, enabling them to differentiate between similar molecules.
• Buffering Capacity: Due to their amino acid composition, proteins exhibit amphoteric behavior and can act as buffers, accepting or donating protons to maintain pH stability.