Phospholipids, Steroids, and Amino Acids: Building Blocks of Life
Phospholipids
Phospholipids, also known as phosphoglycerides, are the major lipid components of biological membranes. The simplest phospholipid is phosphatidic acid, composed of glycerol, two fatty acids, and phosphoric acid. Other phospholipids have a polar compound containing at least one amino or alcohol group linked to phosphatidic acid. Phospholipids are amphipathic, meaning they have both polar and nonpolar regions. The polar region includes the phosphoric acid and alcohol, while the nonpolar region contains the fatty acid chains. This amphipathic nature is crucial for their biological function.
All cell membranes consist of a phospholipid bilayer. The nonpolar tails of both layers face each other, while the polar heads face the aqueous external and internal environments. Different types of phospholipids are classified based on the aminoalcohol present: cephalin (phosphatidylethanolamine) if the aminoalcohol is ethanolamine, lecithin (phosphatidylcholine) if it contains choline, and phosphatidylserine if it contains serine.
Unsaponifiable Lipids and Steroids
Unsaponifiable lipids do not contain fatty acids and do not undergo saponification. Although less abundant than saponifiable lipids, they include biologically active compounds. Steroids are derivatives of a saturated tetracyclic hydrocarbon called sterane or cyclopentanoperhydrophenanthrene. Steroids differ in the type and location of functional groups and the presence of double bonds in the rings.
Sterols are an important group of steroids characterized by an alcohol group (-OH) at carbon 3 and a hydrocarbon chain at carbon 17. Cholesterol, found in animal cell plasma membranes and blood plasma, is a key sterol. In other eukaryotes, similar sterols exist. Cholesterol is the precursor for other steroids, including:
- Bile acids: emulsify fats in the small intestine, aiding lipase action and absorption.
- Sex hormones: testosterone (male), estradiol and progesterone (female).
- Adrenal cortex hormones: cortisol and aldosterone.
- Vitamin D: regulates calcium and phosphate metabolism in vertebrates.
Amino Acids
Amino acids consist of a central carbon atom (alpha-carbon) bonded to a carboxyl group (-COOH), an amino group (-NH2), and an R group (side chain) that varies for each amino acid. The alpha-carbon is asymmetric, resulting in two optical isomers (enantiomers), classified as D and L based on glyceraldehyde as a reference. Only L-isomers are found in proteins.
In aqueous solutions, amino acids ionize into dipolar ions, acting as both acids and bases (amphoteric).
Peptides and Peptide Bond Formation
Peptides and proteins are macromolecules formed by linking amino acids through peptide bonds. A peptide bond results from the loss of water between the carboxyl group of one amino acid and the amino group of another. Two linked amino acids form a dipeptide, three form a tripeptide, and so on. Oligopeptides have a moderate number of amino acids, while polypeptides have many.
Protein Structure
Primary Structure
The primary structure is the sequence of amino acids in the polypeptide chain. The number, type, and order of amino acids are unique to each protein. The chain has a free amino terminus and a free carboxyl terminus. By convention, amino acids are numbered starting from the amino terminus. The primary structure is a chain of articulated planes because peptide bonds cannot rotate, and the carbon, nitrogen, and oxygen atoms involved are coplanar.