6 - Enzymes I: Introduction to enzymes Flashcards
Louis Pasteur
fermentation requires living cells (1850)
Live yeast + sugar = alcohol
Cell-free yeast extract + sugar = no alcohol
Wilhelm Kühne
proposed the term “enzyme” (1878)
Buchner brothers
fermentation in cell-free extracts from yeast (1897)
agreed cells contained a biological catalyst
Biological catalysts; functions
• Regulate the flow of molecules through all metabolic
pathways
• Build structures of cells (and break them down again)
• Break down invading pathogens and parasites - E.g. lysozyme – tears, egg white etc
Biological catalysts; uses
Lipases, proteases; stains - Cellulase; ‘bobbles’ - Low temp, less water
Fondant centres – invertase breaks down sucrose in the fondant centre and prevents it from crystallising
Rennet – calf
stomach; now GM microorganisms, Mixture – mainly Chymosin – curdles casein in milk
Cotton and cellulosic textile manufacture – stonewashed denim (cellulase), de-starching - amylase
Biological catalysts
- Enzymes increase reaction rates without enzyme – thermodynamically unfavourable e.g. prolyl isomerase – cis to trans (x 1 million!)
- Enzymes remain unchanged after the reaction repeat again & again; a few may be ‘modified’ (later)
- Chemicals acted upon = substrates, end point = products
- Most - but not all - enzymes are proteins: ‘Ribozymes’ (ribonucleic acid enzymes) – ‘self-splicing’ RNA (removes introns
Substrate-specificity
- Low specificity: Subtilisin - a bacterial protease that cleaves peptide bonds indiscriminately
- Moderate specificity: Trypsin - a digestive enzyme that cleaves peptide bonds on the carboxyl sides of Lys or Arg
- High specificity: Thrombin - a blood clotting enzyme that cleaves between Arg and Gly within specific amino acid sequence motifs
Enzyme classification
- Most enzyme names end in “-ase”: ATPase (breaks down) ATPsynthase (synthesises)
- Exceptions are proteolytic enzymes that end with “-in” e.g. trypsin, pepsin
Enzyme Commission (EC) numbers
International Union of Biochemistry and Molecular Biology :Enzyme Commission
EC numbers take the form : 1.2.3.4
• First digit from 6 major classes (bulk descriptor of what the enzyme does)
• Second and third digits give further details for the type of reaction (bonds, groups)
• Fourth digit gives substrate (general descriptor, e.g. amino acid, nucleic acid, lipid, etc)
Measurements of enzyme activity
Activity of an enzyme indicated by rate of reaction: amount of product produced per unit time
• SI Unit - katal (kat) : The amount of an enzyme that converts 1 mol of substrate per second under standard* assay conditions – BIG numbers!
• International activity unit (U) : An amount of enzyme that will catalyse the transformation of 1 micromole of the substrate per minute under standard* assay conditions.
Thermodynamics – Free energy
• Breaking bonds requires energy, forming bonds liberates energy However, even most exergonic reactions need an initial ‘spark’ of energy to kick start them (collision and bond destabilisation e.g. initial bonds in glucose and ATP opposite need to be broken)
Exergonic reaction
Releases energy to surroundings
• Will occur spontaneously if there is enough ‘Activation energy’
Endogonic reaction
reactions that require overall input of energy
Thermodynamics – Activation energy
Initial input or ‘spark’ of energy = Gibbs free energy of activation (DG‡)
Thermodynamics – Activation energy; energy profile diagram
• The difference between *free energy of the products and the free energy of the reactants is the Delta G (total reaction change in free energy)
• At the summit the molecules are at an unstable point, the transition state
• The difference between the free energy of the reactants and the transition state is the Delta G‡ (change in Gibbs free energy of activation)
Delta G‡ is limiting