A-Level Chemistry AQA Notes
3.3.13 Amino acids, proteins, and DNA (A-Level)
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Proteins
- Proteins are sequences of amino acids joined by peptide links
- Peptide linkages are formed from condensation reactions
- Hydrolysis of the peptide link produces the constituent amino acids
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- The secondary and tertiary structures are a consequence of various types of intermolecular forces
- Hydrogen bonds – these stabilise both the secondary and tertiary structures
- London and dipole-dipole forces – these stabilise the tertiary structure
- Disulphide bonds – these are only important when the amino acid cysteine is part of the protein in the tertiary structure
Enzymes
- An enzyme is a protein-based biological catalyst.
- Every enzyme has an active site, which is a part of the three-dimensional tertiary protein structure of an enzyme that is responsible for its catalytic activity
- The lock and key theory of enzyme activity states that the substrate must fit exactly in the active site, with the correct orientation, in order for a reaction to happen. The wrong stereoisomer is not catalysed (stereospecific).
- Enzyme inhibitors have similar shapes to the substrate and prevent the substrate from binding the enzyme
- Computers can be used to design drugs which can fit into the active site
Deoxyribonucleic Acid (DNA)
- DNA is made up of nucleotide monomers: a phosphate group, 2-deoxyribose pentose sugar and a base. The base can be adenine, cytosine, guanine or thymine.
- A strand of DNA is a polymer of nucleotides linked by phosphodiester bonds between the phosphate group of one nucleotide and the 2-deoxyribose of another nucleotide. Resulting in a sugar-phosphate backbone.
- The DNA double helix is composed of two complementary DNA strands held together by hydrogen bonds between complementary base pairs
- Adenine and thymine form two hydrogen bonds
- Cytosine and guanine form three hydrogen bonds
Anticancer Drugs
- The Pt (II) complex cisplatin is used as an anticancer drug. It prevents DNA replication in cancer cells by a ligand replacement reaction with DNA in which a bond is formed between platinum and a nitrogen atom on adjacent guanines
- The DNA strands are then kinked, so they cannot unravel properly to replicate
- Cisplatin can have adverse side effects because it also binds to healthy cells. However, it is still used as a treatment because the advantages outweigh the disadvantages
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