Biochem Exam 3

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Lecture 13
Allosteric Enzymes
* DO NOT show Michaelis-Menten kinetics
* Show a sigmoidal curve instead of a hyperbolic curve * allosteric derived from the Greek – meaning other shape * characterized by sigmoidal kinetics (as opposed to Michaelis-Menten kinetics) * show cooperativity

* quaternary structure
* Note: hemoglobin shows allosteric behavior although it is not an enzyme * conformational change usually involves binding small effector molecules that are not substrates – these bind to an allosteric site on the enzyme that result in rapid changes in enzyme activity Allosteric behavior: an important characteristic of regulatory enzymes * respond to environmental signals – adaptive response

* small changes in allosteric modifiers can result in large changes in activity * this type of regulation may explain why enzymes are much larger than would be expected based on catalysis alone * catalyze the committed (rate-limiting step) in biochemical pathways Phosphofructokinase-1 (PFK-1), a glycolytic enzyme, shows allosteric behavior * Fructose-2,6-biphosphate as an allosteric modifier of PFK-1 Aspartate Transcarbamoylase: coupling of inhibition by pyrimidine nucleotide (CTP) with stimulation by purine (ATP) nucleotide balances the 2 nucleotide pools

Models to explain allosteric behavior: Concerted vs. sequential * concerted
* Principles
* all subunits exist as either T or R
* substrate binds more readily to R
* multiple active sites on different polypeptide chains * binding of S disrupts T, R equilibrium in favor of R, Cooperatively * accounts for sharp increase in Vo in plot of V vs. [S] * regulatory molecules alter the T, R equilibrium

* positive effectors stabilize R state: negative effectors stabilize T state * Allosteric enzymes become more or less sensitive to substrate concentration depending on the activator or inhibitor, respectively * Sequential

* Subunits of allosteric enzyme undergo sequential changes in conformation on binding substrate. Binding of substrate to one subunit favors conformational change in adjacent subunit

Lecture 14
Enzyme Regulation Critical for adaption

Mechanisms of enzyme regulation
* catalytic activity of an enzyme is regulated so that the amount of a product is sufficient to meet the needs of the cell * enzymes can exhibit a range of activity dependent on the physiological state of the cell * example of adaption

* Mechanisms
* Substrate concentration – has limitations
* Amount of enzyme
* Increase de novo synthesis; gene activation can be slow process * Protein degradation
* Allosteric control – rapid process
* Activators/inhibitors; feedback inhibition
* Reversible covalent modification: phosphorylation and other types * Zymogen activation – generally irreversible
Phosphorylation: an important covalent modification that modulated protein activity * Kinases: add phosphate to serine, threonine, tyrosine hydroxyl groups * Phosphatases: remove the phosphate(s) added by the kinases * both can be very specific for the proteins that they modify

Phosphorylation/dephosphorylation rapidly alters the activity of an enzyme * changes in phopsorylation/dephosphorylation are initiated by some external signal (for example, a hormone or neurotransmitter) * addition of a phosphate to a protein

* introduces negative charges
* disrupt or form electrostatic interactions
* can form H bonds
* addition of a phosphate to a protein is a covalent bond * reversible through the action of phosphatases
* activity may be increased or decreased – effect is specific to the enzyme ** enzymes can be regulated by other covalent modifications on AAs * zymogen activation: irreversible conversion of an inactive precursor to active...
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