Biochem - Ch 16 - Enzymes

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Biochem - Ch 16 - Enzymes
2011-10-30 13:52:18
Biochem Enzymes

Chapter 16 - Biochemistry - Enzymes
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  1. Biological catalysts are called ____________.
  2. What was enzymes?
    They are biological catalysts that accelerate the catalyzed reactions by lowering the reaction energy of the reaction.
  3. (T/F) Enzymes are very specific
    True. Some highly specific enzymes will differentiate between even closely related compounds. Moreso, enzymes can even distinguish between stereoisomers.
  4. (T/F) Enzymes also demonstrate catalytic specificity.
    True. Enzymes catalyzes the conversion of its substrate only along a particular reaction pathway.
  5. What are cofactors for enzymes?
    Enzymes may need cofactors for their activity.

    Cofactors can be metal ions or small organic molecules (coenzyme).
  6. What are coenzymes?
    A small organic molecule that is required for the catalytic activity of an enzyme. A coenzyme may be either a cosubstrate or a prosthetic group.
  7. What is a prosthetic group? What is a cosubstrate?
    A cofactor that is permanently (often covalently) associated with an enzyme.

    An example of a prosthetic group is a heme, which is tightly bound to cytochromes.

    A cosubstrate is a coenzyme that is temporarily bound to the enzyme.
  8. What is a cytochrome?
    A redox-active protein that carries electrons via a prosthetic Fe-containing heme group.
  9. What is an apoenzyme?
    An enzyme that is inactive due to the absence of a cofactor. It requires the coenzyme for its activity.

    A complete, catalytically active complex of apoenzyme with the coenzyme is called a holoenzyme.
  10. What is a holoenzyme?
    A complete, catalytically active complex of apoenzyme with the coenzyme is called a holoenzyme.

    Without the coenzyme, it would not be active (apoenzyme).
  11. What are the six general classes of enzymes?
    • Oxidoreductases - Catalyze oxidative-reductive reactions.
    • Transferases - Catalyze the reactions of a functional group transfer.
    • Hydrolases - Hydrolysis reactions.
    • Lyases - Catalyze the non-hydrolytic cleavage of chemical bonds or the addition of groups (often water molecules) to double bonds.
    • Isomerases - Catalyze the interconversion between isomers.
    • Ligases - Catalyze the joining of two large molecules.
  12. What are Oxidoreuctases?
    Enzymes that catalyze oxidative-reductive reactions. The reactions can be considered as the transfer of electrons of hydrogen atoms from the donor substrate to the acceptor.

    This includes dehydrogenases, which use coenzymes NAD+, NADP+, FAD, or FMN to catalyze dehydration reactions.

    Oxidases, which constitute another subclass of oxidoreductases, employ molecular oxygen as an electron acceptor.
  13. What are Transferases?
    Enzymes that catalyze the reactions of a functional group transfer.

    There are nine subclasses of transferases, which specify the type of the trasnferred group.

    The enzymes of the first subclass catalyze the transfer of one-carbon grounds, the third subclass is acyltransferases (transferring acyl groups), and so on.

    Other examples include phosphotransferases (kinases), aminotransferases (transferring amino groups) methyltransferases, and glycosyltransferases etc.
  14. What are Hydrolases?
    Enzymes involved in hydrolysis reactions. They are subdivided according to the type of the cleavable bond.

    Examples include esterases, which catalyze the hydrolysis of ester bonds, peptidases (peptide bonds), glycosylases, ATPases, and others.
  15. What are lyases?
    Enzymes that catalyze the non-hydrolytic cleavage of chemical bonds or the addition of groups (often water molecules) to double bounds.

    These enzymes can be subdivided into seven subclasses depending on the cleavable bond (C-C, C-O, C-N, C-S, C-halide, P-O, or others)

    Decarboxylase catalyzes the removal of a carboxyl group from its substrates

    Adenylate cyclase synthesizes 2',3'-cyclic AMP from ATP.
  16. What are Isomerases?
    Enzymes that catalyze the interconversion between isomers.

    • Subclasses include:
    • Racemization (racemases) and epimerization (epimerases); cis-trans isomerization; intramolecular oxidation-reduction, intermolecule transfer of a group (mutases); intramolecular lyases and others.
  17. What are ligases?
    Enzymes that catalyze the joining of two large molcules typically accompanied by the formation of a new chemical bond. This is typically done by hydrolysis of macroergic bonds in other molecules.

    DNA ligase is an example.
  18. What is the site that the enzyme interacts with the substrate?
    The active site. It is usually a cleft in the enzyme's structure, which contains amino acid residues responsible for the substrate specificity and binding of necessary ccofactors, as well as residues repsonsible for the catalysis.
  19. (T/F) To initate a reaction, an enzyme must come in contact with the substrate(s).
    True. To proovide specificity of substrate binding, the enzyme's active site must "fit" the substrate.

    Such complmentarity of the active site to the shape and functional group of the substrate does not allow other molecules, which differ even slightly, in structure from the substrate, to bind to the enzyme, and therefor undergo the enzyme-catalyzed reaction.
  20. What does lysozyme catalyze the hydrolysis of?
    • B(1-4) glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycans of cell walls.
    • It also catalyzes the hydrolysis of two N-acetylglucosamine moieties of chitin.