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PP5: Regulatory Mechanisms (Allosteric Interactions; enzyme activity…
PP5: Regulatory Mechanisms
Allosteric Interactions; enzyme activity regulated by binding of effector molecules (which can activate or inhibit enzymatic activity)
Homotypic Allostery (2nd substrate molecule)
Example: O2 binding in cooperative kinetic interactions to hemoglobin; binding of O2 to one subunit of Hb changes the conformation and increases the other subunits' affinity for O2
Heterotypic Allostery (different molecule)
Aspartate Transcarbamoylase: ATP is a positive effector; increases rate of rxn because the product is needed for DNA replication, so the cell will have a high energy state prior to this
Aspartate Transcarbamoylase: CTP is a negative effector; reduces rate because it is a product of the pathway (feedback inhibition)
CTP and ATP compete for the same binding site on the enzyme
sigmoidal shape of unaffected rxn indicates cooperative kinetic interactions with the substrate, aspartate
Protein Modifications
Covalent addition of chemical groups to proteins to control their activity levels
Example: Phosphorylation Cascade
Signal amplification--one molecule of epinephrine can initiate significant breakdown of glycogen for energy
Cascade sequence allows for greater regulation and control
glycogen phosphorylase breaks down glycogen stores when the organism needs glucose for usable energy
Proteolytic Activation
Example: Chymotrypsin is produced as chymotrypsinogen, inactive form; is activated when it gets to the stomach by trypsin who cleaves it--first cut enables autocleavage
Irreversible cleaving of a protein to activate it; can only be deactivated by degadation
Isozymes: related forms of an enzyme that differ functionally
Example: fetal hemoglobin vs adult hemoglobin; fetal Hb has 2 alpha subunits and 2 gamma subunits; the gamma subunits have a higher affinity for oxygen than the beta subunits in adult Hb
