Flashcards · 7405 · A Level Topic 31

Amino acids, proteins and DNA (A Level) flashcards

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Zwitterion: An ion with a positive and a negative charge and no overall charge, H₃N⁺–CHR–COO⁻.
Form of an amino acid in acid and in alkaline solution: Acid: H₃N⁺–CHR–COOH (overall +1). Alkali: H₂N–CHR–COO⁻ (overall −1).
Primary, secondary and tertiary structure of a protein: what holds each?: Primary: the amino acid sequence, held by peptide links. Secondary: α-helix or β-pleated sheet, held by hydrogen bonds between peptide links. Tertiary: the 3D fold, held by hydrogen bonds and S–S bonds between side chains.
α-helix: A coil; hydrogen bonds between peptide links on different turns of the same chain.
Why does an enzyme act on only one enantiomer of a substrate?: The active site is stereospecific: its binding groups are in fixed 3D positions, and only one enantiomer can line up with all of them.
Base pairing in DNA and the number of hydrogen bonds: A pairs with T (2 hydrogen bonds); G pairs with C (3 hydrogen bonds).
How does cisplatin act as an anticancer drug?: Its Cl⁻ ligands are replaced as Pt bonds to nitrogen atoms on guanine in DNA. Binding two guanines distorts the DNA, so it cannot replicate and the cell cannot divide.
How are colourless amino acids located on a TLC plate?: Spray with ninhydrin (purple spots) or use UV light on a fluorescent plate.