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Unformatted text preview: Biology 1A Dr. Doudna 1 Pimentel 02/05/10 8am Lecture #8: Enzyme regula3on • Reading: Chapter 8, pp. 156‐162 • Lecture outline: How enzymes are controlled – Enzyme cofactors – Enzyme inhibitors – Allosteric regulaFon Overview • Cofactors help enzymes to funcFon eﬃciently • Enzyme inhibitors block acFvity through diﬀerent mechanisms: • CompeFFve • Non‐compeFFve • UncompeFFve • Enzymes can be regulated by structural changes allosteric inhibition Enzymes enhance rates by very LARGE amounts Lock and Key model - explains specificity of enzymes - however if the enzyme fits the substrate too well, the substrate is too stable and will not proceed with the reaction Induced ﬁt model of enzyme catalysis hMp://en.wikipedia.org/wiki/File:Induced_ﬁt_diagram.svg Help with structural changes or help with chemistry of the reaction Cofactors • Cofactors are nonprotein enzyme helpers • Cofactors may be inorganic (such as a metal in ionic form) or organic • An organic cofactor is called a coenzyme • Coenzymes include vitamins A Divalent Metal ion Binds in the Active Site
2ʼ-OH U75 U75 M2+
2+ * *
5ʼ-O M G1 G1 Ke et al. (2004) Nature 429:201 Cofactors are not covalently bonded; they are free to move on and off the enzyme Cofactors • Tightly‐bound cofactors are called prosthe3c groups. • Loosely‐bound cofactors are called coenzymes. • An inacFve enzyme lacking the cofactor is called an apo‐enzyme; the complete enzyme with its cofactor is called a holoenzyme. Cofactors • Organic cofactors oXen share a common chemical feature, can you see it? All have phosphate groups and adenines; utilized over and over by many enzymes. Coenzyme A NAD+ FAD Cofactors • NADH is reversibly oxidized to NAD+ during electron transport in cells. • Many enzymes contain common structural features that bind NAD+/ NADH Rossmann fold in lactate dehydrogenase Regula3on of enzyme ac3vity helps control metabolism • Chemical chaos would result if a cell’s metabolic pathways were not Fghtly regulated • A cell does this by switching on or oﬀ the genes that encode speciﬁc enzymes or by regulaFng the acFvity of enzymes very common in metabolism Feedback Inhibi3on • In feedback inhibi3on, the end product of a metabolic pathway shuts down the pathway • Feedback inhibiFon prevents a cell from wasFng chemical resources by synthesizing more product than is needed Fig. 8-22 Active site available Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell Feedback inhibition Intermediate A
Active site of Enzyme 2 enzyme 1 no longer binds threonine; Intermediate B pathway is switched off. Enzyme 3 Isoleucine binds to Enzyme 1 and induces structure change Intermediate C Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine) Enzyme Inhibitors • Compe33ve inhibitors bind to the acFve site of an enzyme, compeFng with the substrate • Noncompe33ve inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the acFve site less eﬀecFve • Examples of inhibitors include toxins, poisons, pesFcides, and anFbioFcs Fig. 8-19 Substrate Active site Competitive inhibitor Enzyme Noncompetitive inhibitor (a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition Not an exact fit with the enzyme or can be a better fit than the enzyme alters the active site of enzyme to prevent substrate from binding Co(NH3)63+ is a competitive inhibitor of Mg2+ in the Hepatitis delta virus ribozyme Co(NH3)63+ HIV protease inhibitors block the viral enzyme Required to produce individual HIV proteins ritonavir hMp://en.wikipedia.org/wiki/Enzyme_inhibitor Allosteric RegulaFon of Enzymes • Allosteric regula3on may either inhibit or sFmulate an enzyme’s acFvity • Allosteric regulaFon occurs when a regulatory molecule binds to a protein at one site and aﬀects the protein’s funcFon at another site Hemoglobin is a classic example of an allosterically regulated protein Allosteric Ac3va3on and Inhibi3on • Most allosterically regulated enzymes are made from polypepFde subunits • Each enzyme has acFve and inacFve forms • The binding of an acFvator stabilizes the acFve form of the enzyme • The binding of an inhibitor stabilizes the inacFve form of the enzyme Fig. 8-20 Allosteric enyzme Active site with four subunits (one of four) Regulatory site (one of four) Activator Active form Stabilized active form Oscillation NonInhibitor functional Inactive form active site Stabilized inactive form (a) Allosteric activators and inhibitors Substrate Inactive form Stabilized active form (b) Cooperativity: another type of allosteric activation Fig. 8-20a Allosteric enzyme with four subunits Active site (one of four) Regulatory site (one of four) Activator Active form Stabilized active form Oscillation NonInhibitor functional Inactive form active site (a) Allosteric activators and inhibitors Stabilized inactive form The Hb tetramer includes two α and two β subunits Strong interacFons between the α1 and β1 (and α2 / β2) subunits hold them together even in the presence of urea. Hb exists in 2 major conforma3ons O2 will bind Hb in either state, but has much higher aﬃnity for Hb in the R state. T state is stabilized by a greater number of ion pairs at the subunit interfaces. When O2 binds, Hb conformaFon changes to the R state. • Coopera3vity is a form of allosteric regulaFon that can amplify enzyme acFvity • In cooperaFvity, binding by a substrate to one acFve site stabilizes favorable conformaFonal changes at all other subunits Fig. 8-20b Substrate Inactive form Stabilized active form (b) Cooperativity: another type of allosteric activation Iden3ﬁca3on of Allosteric Regulators • Allosteric regulators are aMracFve drug candidates for enzyme regulaFon • InhibiFon of proteolyFc enzymes called caspases may help management of inappropriate inﬂammatory responses Fig. 8-21a EXPERIMENT
Active site Substrate Caspase 1 SH Known active form SH Active form can bind substrate binding site Allosteric Known inactive form inhibitor SH Allosteric S–S Hypothesis: allosteric inhibitor locks enzyme in inactive form Fig. 8-21b RESULTS Caspase 1 Active form Inhibitor Allosterically Inactive form inhibited form You should now be able to: 1. DisFnguish between the following pairs of terms: catabolic and anabolic pathways; kineFc and potenFal energy; open and closed systems; exergonic and endergonic reacFons 2. In your own words, explain the second law of thermodynamics and explain why it is not violated by living organisms 3. Explain in general terms how cells obtain the energy to do cellular work 4. Explain how ATP performs cellular work 5. Explain why an investment of acFvaFon energy is necessary to iniFate a spontaneous reacFon 6. Describe the mechanisms by which enzymes lower acFvaFon energy 7. Describe how allosteric regulators may inhibit or sFmulate the acFvity of an enzyme 8. Explain how some enzyme inhibitors work ...
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