Biomolecules and Enzymes

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In beta pleated sheets, the polypeptide backbone lies in the plane of the sheet, with the peptide groups forming hydrogen bonds between adjacent strands. The R-groups (side chains) alternate above and below this plane, minimizing steric hindrance between adjacent side chains.

Cysteine contains a sulfhydryl (-SH) group in its side chain. Two cysteine residues can oxidize to form a disulfide bond (-S-S-), creating cystine. The sulfur atom in cysteine's side chain is essential for this bond formation.

Ampholytes are molecules that can act as both acids and bases. Amino acids contain both an amino group (basic, can accept protons) and a carboxyl group (acidic, can donate protons), allowing them to function as either depending on pH.

Hemoglobin is a tetramer consisting of four polypeptide chains (2 alpha and 2 beta in adult hemoglobin). Each chain has a molecular weight of approximately 17,000 daltons, making the total approximately 68,000 daltons.

Myoglobin is a single-chain globular protein containing 153 amino acids in humans. It consists of 8 alpha helices surrounding a heme group and stores oxygen in muscle tissue.

Selenocystine (or selenocysteine) is not one of the 20 standard amino acids. It's the 21st amino acid, incorporated co-translationally at UGA stop codons in a special mechanism. It contains selenium instead of sulfur and is called the '21st amino acid'.

Arginine is purely glucogenic - it can only be converted to glucose via gluconeogenic pathways. Tyrosine and tryptophan are both glucogenic and ketogenic (can form ketone bodies). Isoleucine is also both glucogenic and ketogenic.

An alpha-helix has 3.6 amino acid residues per turn with a pitch (vertical rise per turn) of 5.4 Å. For 40 residues: (40 / 3.6) × 5.4 = 60 Å. This is a standard calculation in protein secondary structure analysis.

Valine is a hydrophobic amino acid, making it likely to be buried in the interior of water-soluble globular proteins to avoid contact with water. Aspartate and Glutamate are negatively charged (hydrophilic) and prefer surface locations. Glycine is small and flexible but not specifically excluded from surfaces.

A ligand is any molecule or ion that binds reversibly to a protein. Co-factors and co-enzymes are non-protein chemical compounds required for protein activity (often tightly bound). A motif is a structural pattern within a protein.

Proline is a structural disruptor of protein secondary structure because its side chain forms a ring with the backbone nitrogen, making it an imino acid. This rigid ring prevents the rotation and phi angle needed for alpha-helices and beta-sheets, often creating kinks or terminating secondary structure elements.

The amino acid sequence (primary structure) determines how a protein folds into its unique three-dimensional structure, which in turn determines its specific function. Differences in amino acid sequences between enzymes result in different tertiary structures and different functional properties. Secondary structures alone don't account for functional specificity.

Nonpolar amino acids have hydrophobic side chains that do not carry any electrical charge. These side chains are typically composed of carbon and hydrogen atoms, making them uncharged and water-repellent. Aromatic side chains can be nonpolar, but 'uncharged' is the more fundamental and accurate categorization.

Fibroin is the main structural protein in silk, produced by silkworms and spiders. Its secondary structure is dominated by beta-pleated sheets arranged in an antiparallel fashion, which gives silk its strength and flexibility. Triple helices are found in collagen, double helices in DNA, and helix-turn-helix is a DNA-binding motif.