Lipids, Amino Acids and Proteins: Structure and Function

Lipids: Structure and Function

Lipids are organic compounds composed of carbon, hydrogen and oxygen. They are insoluble in water and soluble in apolar organic solvents such as ether. They are structural elements, mainly in membranes, and include triglycerides, which are energy-rich, serve as energy reserves, and as vitamins and hormonal substances. Vitamins and hormones derived from lipids are also important.

Lipids can be saponifiable or insaponifiable. Saponifiable lipids contain fatty acids in their molecules and form soaps through saponification with a strong base, and they are classified as follows:

• Fats and oils: saturated fats are usually solid at room temperature, while unsaturated fats are usually liquid. Waxes are esters of a long-chain fatty acid and a monoalcohol, and they have protective functions.
• Phospholipids: derived from glycerol esterified with phosphate at carbon 3. They are amphipathic molecules, with a hydrophilic polar region formed by the phosphate group and other substituents, and a hydrophobic region formed by two fatty acids.
• Sphingolipids: similar to phospholipids, they form part of nervous tissue. They are composed of one fatty acid and one polar aminoalcohol.

Insaponifiable lipids do not contain fatty acids and cannot form soaps. They are very varied and are derived from the polymerization of isoprene units. Examples are steroids, which include cholesterol, certain hormones and vitamin D, as well as fatty acid derivatives such as prostaglandins, which act as vasodilators and intervene in inflammation and blood coagulation.

Amino Acids and Peptide Bonds

Amino acids are organic molecules of low molecular weight that, when linked together, form proteins. They are composed of carbon, hydrogen, oxygen and nitrogen, and are characterized by a carboxyl group and an amino group bonded to an alpha carbon atom, along with a variable R group. There are 20 standard amino acids, which are the building blocks of proteins, such as valine and lysine.

Amino acids are joined by peptide bonds, covalent bonds formed between the carboxyl group of one amino acid and the amino group of another. Some amino acids have properties that allow the formation of double bonds, which gives the peptide chain rigidity and enables turns.

Protein Properties and Functions

Key Properties

• Proteins may be globular (soluble) or fibrous (insoluble).
• Denaturation: changes in temperature or pH lead to loss of secondary, tertiary and quaternary structures, and proteins lose their biological activity and specificity.
• Buffer capacity: proteins help resist changes in pH.

Biological Functions

• Transport: for example, hemoglobin transports oxygen.
• Contractile: actin and myosin enable muscle contraction.
• Protective: for example, thrombin participates in blood clotting.
• Hormonal: for example, insulin regulates blood glucose.
• Structural: for example, tubulin forms microtubules.
• Enzymatic: enzymes are biochemical catalysts that speed up chemical reactions in living cells.
• Homeostatic: proteins help maintain the balance of the internal environment.