1. Enzymes
Regulatory enzymes are usually the enzymes that are
the rate-limiting, or committed step, in a pathway,
meaning that after this step a particular reaction
pathway will go to completion
There are five primary forms of enzyme regulation:
substrate availability, allosteric, post-translational
modification, interaction with control proteins
2. Properties of Enzymes
In general, chemical reactions that release energy can
occur without input of energy
The oxidation of glucose releases energy, but the
reaction does not occur without an input of energy
Activation energy: the energy required to start such a
reaction
Enzymes lower the activation energy so reactions can
occur at mild temperatures in living cells
3. Enzymes
Provide a surface on which reactions take place
Active site: the area on the enzyme surface where the
enzyme forms a loose association with the substrate
Substrate: the substance on which the enzyme acts
Enzyme-substrate complex: formed when the
substrate molecule collides with the active site of its
enzyme
Enzymes generally have a high degree of specificity
Endoenzymes (intracellular)/exoenzymes (extracellular)
5. Enzyme Components
Biological catalysts
Specific for a chemical reaction; not used up in that
reaction
Apoenzyme: Protein
Cofactor: Nonprotein component
Coenzyme: Organic cofactor
Holoenzyme: Apoenzyme plus cofactor
7. Properties of Coenzymes and Cofactors
Many enzymes can catalyze a reaction only if substances
called coenzymes, or cofactors are present
Apoenzyme: protein portion of such enzymes
Holoenzyme: nonprotein coenzyme or cofactor that is
active when combined with apoenzyme
Coenzyme: nonprotein organic molecule bound to or
loosely associated with an enzyme
Cofactor: an inorganic ion (e.g. magnesium, zinc) that
often improve the fit of an enzyme with its substrate
12. Each substrate binds to an active site, producing
an enzyme-substrate complex. The enzyme helps
a chemical reaction occur, and one or more
products are formed
13. Enzyme Classification
Oxidoreductase: Oxidation-reduction reactions
Transferase: Transfer functional groups
Hydrolase: Hydrolysis
Lyase: Removal of atoms without hydrolysis
Isomerase: Rearrangement of atoms
Ligase: Joining of molecules, uses ATP
15. Temperature and pH
Enzymes are affected by heat and extremes of pH
Even small pH changes can alter the electrical charges
on various chemical groups in enzyme molecules,
thereby altering the enzyme’s ability to bind its
substrate and catalyze a reaction
Most enzymes have an optimum temperature, near
normal body temperature, and an optimum pH, near
neutral, at which they catalyze a reaction most rapidly
The rate at which an enzyme catalyzes a reaction
increases with temperature up to the optimum T
19. Enzyme Inhibition
Competitive inhibitor: A molecule similar in
structure to a substrate can bind to an enzyme’s active
site and compete with substrate
Noncompetitive inhibitors: attach to the enzyme at
an allosteric site, which is a site other than the active site
noncompetitive inhibitors: distort the tertiary
protein structure and alter the shape of the active site
Feedback inhibition: regulates the rate of many
metabolic pathways when an end product of a pathway
accumulates and binds to and inactivates the first
enzyme in the metabolic pathway
22. Allosteric regulation of enzyme
activity
Allosteric regulation = the activation or inhibition
of an enzyme’s activity due to binding of an
effectors molecule at a regulatory site that is
distinct from the active site of the enzyme
Allosteric regulators generally act by increasing or
decreasing the enzyme’s affinity for the
substrate
25. modification enzymes
Can either activate it or inhibit it by altering the
conformation of the enzyme or by serving as a
functional group in the active site