* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 7 7 The Behavior of Proteins: Enzymes Mechanisms and
Photosynthetic reaction centre wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Metabolic network modelling wikipedia , lookup
Adenosine triphosphate wikipedia , lookup
Mitogen-activated protein kinase wikipedia , lookup
Citric acid cycle wikipedia , lookup
Restriction enzyme wikipedia , lookup
Western blot wikipedia , lookup
Deoxyribozyme wikipedia , lookup
Multi-state modeling of biomolecules wikipedia , lookup
Biochemistry wikipedia , lookup
Metalloprotein wikipedia , lookup
Ultrasensitivity wikipedia , lookup
NADH:ubiquinone oxidoreductase (H+-translocating) wikipedia , lookup
Proteolysis wikipedia , lookup
Nicotinamide adenine dinucleotide wikipedia , lookup
Oxidative phosphorylation wikipedia , lookup
Biosynthesis wikipedia , lookup
Amino acid synthesis wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Catalytic triad wikipedia , lookup
Mary K. Campbell Shawn O. Farrell http://academic.cengage.com/chemistry/campbell Chapter 7 The Behavior of Proteins: Enzymes Mechanisms Enzymes, Mechanisms, and Control Paul D. Adams • University of Arkansas The catalytic activities of many enzymes are regulated by 1. Feedback inhibition (products – reactants) 2 Allosteric 2. All t i regulation l ti (allosteric ( ll t i iinhibitor hibit and d allosteric activator) 3. Covalent modification (phosphorylation) 4. Proteolysis (eg. Zymogen) 5. Coenzyme 1. Feedback Inhibition Formation of product inhibits its continued production Aspartate transcarbamoylase (ATCase) feedback inhibition product 2. Allosteric regulation • Allosteric: Greek allo + steric, other shape • Allosteric enzyme: enzyme: an oligomer whose biological activity is affected by other substances binding to it ese subs substances a ces change c a ge the ee enzyme’s y e s ac activity y by a altering e g • these the conformation(s) of its 4°structure • Allosteric effector: effector: a substance that modifies the behavior of an allosteric enzyme; may be an • allosteric inhibitor • allosteric activator Allosteric Enzymes • The key to allosteric behavior is the existence of multiple forms for the 4°structure of the enzyme y • allosteric effector (allosteric inhibitor or allosteric activator): ): a substance that modifies the 4° structure activator) of an allosteric enzyme • homo homotropic o ot ottropic op c effects: e ects tthe e subst substrate ate a also so se serves es as a positive (stimulatory) modulator, or activator; e.g., the binding g of aspartate p ((its substrate)) to ATCase. ((tends to be positive regulator) • hetero heterotropic tropic p effects: allosteric interactions that occur when different substances are bound to the protein; e.g., inhibition of ATCase by CTP and activation by ATP Allosteric Enzymes Allosteric enzymes do not obey Michaelis-Menten kinetics. • Two types of allosteric enzyme systems exist Note: for an allosteric enzyme, the substrate concentration at one-half Vmax is called the K0.5 ((與Km有所區隔) • K system: system y : an enzyme y for which an inhibitor or activators alters K0.5 • V system: system: an enzyme for which an inhibitor or activator alters Vmax but not K0.5 Substrate-activity y curves for representative p allosteric enzymes. (a) The sigmoid curve of a homotropic enzyme, in which the substrate also serves as a positive (stimulatory) modulator, or activator. i Note N the h resemblance bl to the h oxygen-saturation i curve off hemoglobin. K system positive modulator ( ll t i activator) (allosteric ti t ) negative modulator (allosteric inhibitor) The effects of a positive modulator (+) and a negative modulator (–) on an allosteric enzyme in which K0.5 0 is altered without a change in Vmax. The central curve shows the substrateactivity relationship without a modulator. V system Substrate-activity curves for representative allosteric enzymes. (c) A less common type of modulation, in which Vmax is altered and K0.5 is nearly constant. ATCase (Aspartate transcarbamoylase) • Rate of ATCase catalysis vs substrate concentration • Sigmoidal shape of curve describes allosteric behavior • CTP (allosteric inhibitor); ATP (allosteric activator) ATCase • Organization of ATCase • catalytic unit: 6 subunits organized into 2 trimers • regulatory unit: 6 subunits organized into 3 dimers Two Models Suggest Mechanisms for Cooperative Binding The 1st model: MWC model (concerted model) . The 2nd model, the sequential model . MWC model T form All subunits switch from TÆR in unison Sequential Model R form TÆR Transition only occur in subuints containing bond ligand The Concerted Model (MWC model) • Wyman, Monod, and Changeux - 1965 • The enzyme has two conformations • R (relaxed): the active form; binds substrate tightly • T (tight (ti ht or taut): t t) the th inactive i ti form; f binds bi d substrate b t t less tightly • in i th the absence b off substrate, b t t mostt enzyme molecules l l are in the T (inactive) form • the h presence off substrate b shifts hif the h equilibrium ilib i ffrom the T (inactive) form to the R (active) form • in changing from T to R and vice versa, all subunits change conformation simultaneously; all changes are concerted t d (一致) ( 致) Sequential Model (Cont’d) • Main Feature of Model: • the binding of substrate induces a conformational change from the T form to the R form • the change in conformation is induced by the fit of the substrate to the enzyme, enzyme as per the induced-fit induced fit model of substrate binding • sequential model represents cooperativity 3. Reversible Covalent modification— Phosphorylation • The side chain -OH groups of Ser, Thr, and Tyr can form phosphate p p esters • Phosphorylation by ATP can convert an inactive precursor into an active enzyme protein kinases protein phosphatases Example: Membrane Transport Source of PO4 is ATP • When ATP is hydrolyzed, energy released that allows other energetically unfavorable reactions to take place • PO4 is donated to residue in protein by protein kinases 4. Proteolysis-- Zymogen • For some enzymes, an inactive precursor called a zymogen is cleaved to form the active enzyme. • Chymotrypsinogen (zymogen, inactive enzyme) • synthesized and stored in the pancreas • a single polypeptide chain of 245 amino acid residues cross linked by five disulfide (-S-S-) ( S S ) bonds • when secreted into the small intestine, the digestive y trypsin yp cleaves a 15 unit p polypeptide yp p from the Nenzyme terminal end to give π-chymotrypsin Activation of chymotrypsin • Activation of chymotrypsinogen by proteolysis Chymotrypsin -- a serine protease -- Because Ser-195 and His-57 are required for activity, th mustt b they be close l tto each h other th iin th the active ti site it -- Catalytic triad: His 57, Asp 102, Ser195 -- catalyzes the carboxyl side of aromatic side chains (Tyr, Phe, Trp) Chymotrypsin (Cont’d) • The active site of chymotrypsin shows proximity of 2 reactive a.a. Active Sites and Transition States • Enzyme catalysis • an enzyme provides an alternative pathway with a lower activation i i energy • the transition state often has a different shape than either the substrate(s) or the product(s) • “True nature” of transition state is a chemical species that is intermediate in structure between the substrate and the product. • • T Transition iti state t t analog: l a substance b t whose h shape h mimics i i th thatt off a transition state In 1969 Jenks proposed that • an immunogen would elicit an antibody with catalytic activity if the immunogen mimicked the transition state of the reaction • the first catalytic antibody or abzyme was created in 1986 by Lerner and Schultz *(Biochemical Connections, p. 196) Induced fit model Enzyme Specificity • Absolute specificity specificity: catalyzes the reaction of one unique substrate to a particular product • Relative R l i specificity: specificity ifi i : catalyzes l the h reaction i off structurally related substrates to give structurally related l d products d • Stereospecificity: catalyzes a reaction in which one stereoisomer is reacted or formed in p preference to all others that might be reacted or formed Asymmetric binding • Enzymes can be stereospecific p ((Specificity p y where optical activity may pay a role) • Binding sites on enzymes must be asymmetric 5. Coenzymes • Coenzyme: a nonprotein substance that takes part in an enzymatic y reaction and is regenerated g for further reaction • metal ions- can behave as coordination compounds. (Zn2+, Fe2+) • organic compounds, many of which are vitamins or are metabolically related to vitamins (Table 7.1). NAD+/NADH • Nicotinamide adenine dinucleotide ((NAD+) is used in many redox reactions in biology. • Contains: 1) nicotinamide ring 2) Adenine ring 3) 2 sugar-phosphate groups NAD+/NADH (Cont’d) • NAD+ is a two-electron oxidizing agent, and is reduced to NADH • Ni Nicotinamide i id ring i iis where h reduction-oxidation d i id i occurs • The B6 vitamins are coenzymes involved in amino group transfer from one molecule to another. • Important in amino acid biosynthesis Some important nouns § Proenzyme (Zymogen ) : the inactive or nearly inactive precursor of an enzyme, converted into an active ti enzyme b by proteolysis. t l i § Holoenzyme: An active, complex enzyme consiting of an apoenzyme and a coenzyme Holoenzyme = apoenzyme +coenzyme (or cofactor) § Coenzyme: low molecular weight organic molecules (NAD+, FAD+…) § Cofactor: metal ions, ions is required for holoenzyme’s holoenzyme s activity (metalloenzyme) § Prosthetic g group: p the nonprotein p component p of a conjugated protein, as the heme group in hemoglobin §A ti ti factor: §Activation f t only l affect ff t the th rate t off reaction ti