Biomolecules Unveiled: Lipids, Steroids, and Protein Structures

Steroids: Structure and Biological Roles

Steroids are lipids derived from a complex chemical nucleus called sterane. This group of steroids includes cholesterol. This compound is a vital part of cell membranes, providing fluidity. It also serves as a precursor for many large molecules, such as sex hormones (e.g., testosterone) and adrenocortical hormones (e.g., aldosterone, cortisol), bile acids, and vitamin D.

Saponifiable Lipids: Esters and Their Hydrolysis

Saponifiable lipids are esters that, upon hydrolysis, yield fatty acids and an alcohol (like glycerol). When heated with an alkali, they form soaps. These lipids can be simple (e.g., glycerides and cerides) or complex (e.g., phosphoglycerides and sphingolipids).

Unsaponifiable Lipids: Terpenes and Steroids

Unsaponifiable lipids do not yield fatty acids upon hydrolysis and therefore cannot form soaps. This group includes terpenes and steroids.

Triacylglycerols: Energy Storage Lipids

Triacylglycerols (also known as triglycerides) are simple saponifiable lipids composed solely of carbon, hydrogen, and oxygen atoms. They are formed by the esterification of three fatty acid molecules with one glycerol molecule. These molecules are insoluble, have low density, and are nonpolar. In the body, they primarily function as a reserve of food energy.

Proteins: Essential Biomolecules

Proteins are biomolecules primarily composed of carbon, hydrogen, oxygen, and nitrogen, and sometimes sulfur. They constitute about 70% of the dry matter in cells, making them the most abundant biomolecules.

The Peptide Bond: Linking Amino Acids

The peptide bond is a covalent link established between the carboxyl group of one amino acid and the amino group of another, with the release of a water molecule. This is a condensation process. In turn, this bond can be hydrolyzed, separating the amino acids. A peptide bond has some double bond character, making it flat and rigid, with four atoms involved in the bond lying in the same plane.

Primary Protein Structure: Amino Acid Sequence

The primary structure corresponds to the sequence of amino acids (the polypeptide sequence). It defines both the specific amino acids that make up the protein and their precise order. This structure's rigidity is maintained by the peptide bonds.

Secondary Protein Structure: Alpha Helix and Beta Sheet

The secondary structure describes the stable spatial arrangement adopted by the amino acid chain. There are two main types: the alpha helix and the beta-pleated sheet. Both can coexist within a single protein, though one may predominate. Certain combinations of these two structures are remarkably stable and are found in many proteins with diverse functions.

Alpha Helix Structure

The alpha helix, named after its prevalence in keratin (a protein abundant in epidermal cells), consists of a spiral folding of the polypeptide chain. This folding is stabilized by hydrogen bonds formed between the NH group (part of a peptide bond) of an amino acid and the CO group (part of another peptide bond) of the fourth amino acid that follows in the linear chain.

Beta-Pleated Sheet Structure

In the beta-pleated sheet, the polypeptide chain does not form a helix but rather a folded, accordion-like or zigzag structure. This arrangement is caused by hydrogen bonds between segments of the same chain or between different polypeptide chains, forming cross-bridges. The amino acid side chains (R groups) are arranged alternately above and below this structure.