Lehninger Chapter 5: Protein Function PDF

10/2/2023 5 Protein Function 1 1 Proteins Function by Interacting Dynamically with Other Molecules • two types of interactions: – protein acting as a reaction catalyst, or enzyme, alters the chemical configuration or composition of a bound molecule – neither the chemical configuration nor the co...

10/2/2023 5 Protein Function 1 1 Proteins Function by Interacting Dynamically with Other Molecules • two types of interactions: – protein acting as a reaction catalyst, or enzyme, alters the chemical configuration or composition of a bound molecule – neither the chemical configuration nor the composition of the bound molecule is changed 2 2 Principle 1 (1 of 4) The functions of many proteins involve the reversible binding of other molecules. A molecule bound reversibly by a protein is called a ligand. A ligand may be any kind of molecule, including another protein. The transient nature of protein-ligand interactions is critical to life, allowing an organism to respond rapidly and reversibly to changing environmental and metabolic circumstances. 3 3 1 10/2/2023 Principle 2 (1 of 4) A ligand binds a protein at a binding site that is complementary to the ligand in size, shape, charge, and hydrophobic or hydrophilic character. The interaction is specific: the protein can discriminate among the thousands of different molecules in its environment and selectively bind only one or a few types. A given protein may have separate binding sites for several different ligands. These specific molecular interactions are crucial in maintaining the high degree of order in a living system. 4 4 Principle 3 (1 of 2) Proteins are flexible. Changes in conformation may be subtle, reflecting molecular vibrations and small movements of amino acid residues throughout the protein. Changes in conformation may also be more dramatic, with major segments of the protein structure moving as much as several nanometers. Specific conformational changes are frequently essential to a protein’s function. 5 5 Principle 4 (1 of 2) The binding of a protein and a ligand is often coupled to a conformational change in the protein that makes the binding site more complementary to the ligand, permitting tighter binding. The structural adaptation that occurs between protein and ligand is called induced fit. 6 6 2 10/2/2023 Principle 5 (1 of 4) In a multisubunit protein, a conformational change in one subunit often affects the conformation of other subunits. 7 7 Principle 6 (1 of 3) Interactions between ligands and proteins may be regulated. 8 8 5.1 Reversible Binding of a Protein to a Ligand: OxygenBinding Proteins 9 9 3 10/2/2023 Principle 1 (2 of 4) The functions of many proteins involve the reversible binding of other molecules. A molecule bound reversibly by a protein is called a ligand. A ligand may be any kind of molecule, including another protein. The transient nature of protein-ligand interactions is critical to life, allowing an organism to respond rapidly and reversibly to changing environmental and metabolic circumstances. 10 10 Heme Prosthetic Group • heme = proteinbound prosthetic group – present in myoglobin and hemoglobin – consists of a complex organic ring structure, protoporphyrin, with a bound Fe2+ atom 11 11 Oxygen Can Bind to a Heme Prosthetic Group • oxygen: – poorly soluble in aqueous solutions – diffusion through tissues is ineffective over large distances – transition metals have strong tendency to bind (iron, copper) 12 12 4 10/2/2023 Coordination Bonds of Iron • six coordination bonds: – four to nitrogen atoms in the flat porphyrin ring – two perpendicular to the porphyrin 13 13 Principle 2 (2 of 4) A ligand binds a protein at a binding site that is complementary to the ligand in size, shape, charge, and hydrophobic or hydrophilic character. The interaction is specific: the protein can discriminate among the thousands of different molecules in its environment and selectively bind only one or a few types. A given protein may have separate binding sites for several different ligands. These specific molecular interactions are crucial in maintaining the high degree of order in a living system. 14 14 Perpendicular Coordination Bonds • two perpendicular coordination bonds: – one is occupied by a sidechain nitrogen of a highly conserved proximal His residue – one is the binding site for molecular oxygen (O2) • Fe2+ binds O2 reversibly • Fe3+ does not bind O2 15 15 5 10/2/2023 Globins Are a Family of OxygenBinding Proteins • globins = widespread protein family – highly conserved tertiary structure: eight α-helical segments connected by bends (globin fold) – most function in O2 transport or storage 16 16 Types of Globins • four types in humans and other mammals: – myoglobin = monomeric, facilitates O2 diffusion in muscle tissue – hemoglobin = tetrameric, responsible for O2 transport in the bloodstream – neuroglobin = monomeric, expressed largely in neurons to protect the brain from low O2 or restricted blood supply – cytoglobin = monomeric, regulates levels of nitric oxide, a localized signal for muscle relaxation 17 17 Myoglobin Has a Single Binding Site for Oxygen • myoglobin: – 153 residues + one molecule of heme – bends named after the α-helical segments they connect • His93 = ninety-third residue from the amino terminal end • His F8 = eighth residue in α helix F 18 18 6 10/2/2023 Protein-Ligand Interactions Can Be Described Quantitatively • a simple equilibrium expression describes the reversible binding of a protein (P) to a ligand (L): (5-1) 19 19 when given Association Constant • association constant (Ka) = provides a measure of rd - Ra= ta the affinity of the ligand L for the protein – higher Ka = higher affinity – equivalent to the ratio of the rates of the forward (association) and the reverse (dissociation) reactions that form the PL complex (5-2) 20 20 [L] Remains Constant (5-2) (5-3) • when [L] >>> [ligandbinding sites], the binding of the ligand by the protein does not appreciably change [L] 21 21 7 10/2/2023 Binding Equilibrium (5-4) (5-5) x = y/(y + z) describes a hyperbola 22 22 Graphical Representations of Ligand Binding (5-5) • [L] at which ½ of the available ligand-binding sites are occupied (Y = 0.5) corresponds to 1/Ka 23 23 Dissociation Constant • dissociation constant (Kd) = reciprocal of Ka – equilibrium constant for the release of ligand – lower Kd = higher affinity (5-6) (5-7) • when [L] = Kd, ½ of the ligandbinding sites are occupied (5-8) 24 24 8 · d = the dissociation (mstant represents the ligand concentration at which half of the avaliable receptor binding SitS are occupied It iS a measure of affinity between the ligand and the recepter ... . higher - weak binding , I affinity 10/2/2023 Representative Kd Values 25 25 Binding of O2 to Myoglobin (5-9) • Kd equals the [O2] at which ½ of the available ligandbinding sites are occupied, or [O2]0.5: (5-10) 26 26 Partial Pressure of O2 (5-11) 27 27 9 10/2/2023 Principle 3 (2 of 2) Proteins are flexible. Changes in conformation may be subtle, reflecting molecular vibrations and small movements of amino acid residues throughout the protein. Changes in conformation may also be more dramatic, with major segments of the protein structure moving as much as several nanometers. Specific conformational changes are frequently essential to a protein’s function. 28 28 Protein Structure Affects How Ligands Bind • carbon monoxide (CO) binds free heme more than 20,000 times better than does O2 – differences in the orbital structures affect binding geometries 29 29 Myoglobin’s Distal His Increases Heme’s Affinity for O2 • hydrogen bond between the imidazole side chain of His E7 and bound O2 electrostatically stabilizes the Fe-O2 polar complex • 20,000-fold stronger binding affinity of free heme for CO compared with O2 declines to ~40-fold Rotates to open and close the pocket 30 30 10 10/2/2023 Hemoglobin Transports Oxygen in Blood • erythrocytes (red blood cells) transport O2 – formed from hemocytoblasts (precursor stem cells) – main function is to carry hemoglobin • arterial blood = ~96% saturated with O2 • peripheral blood = ~64% saturated with O2 31 31 Principle 5 (2 of 4) In a multisubunit protein, a conformational change in one subunit often affects the conformation of other subunits. 32 32 all missed In person R Hemoglobin Subunits Are Structurally Similar to Myoglobin "Hgb" ~ • hemoglobin: - 4 submits – tetrameric protein with 4 heme groups – adult hemoglobin has two globin types: two α chains (141 residues each) and two β chains · (146 residues each)alpolicies monomer ↳ tetramer 33 33 11 10/2/2023 Structural Conservation of Globins -> hum & Mys of put entip eachother Low sequence similarity 31 structure myoglobin High structural similarity the in us humo . figure shows the evaitich infivence on the 34 globin Fold 34 The Quaternary Structure of Hemoglobin • strong interactions between unlike subunits – hydrophobic effect – hydrogen bonds the submits – ion pairs (salt bridges) interact ↳ charged aminoacias • α1β1 (and α2β2) interface involves >30 residues Show • α1β2 (and α2β1) interface involves 19 residues 35 35 Principle 4 (2 of 2) The binding of a protein and a ligand is often coupled to a conformational change in the protein that makes the binding site more complementary to the ligand, permitting tighter binding. The structural adaptation that occurs between protein and ligand is called induced fit. 36 36 12 - 10/2/2023 My0- de storage Hemo- pickup * It can & drop off bind On in either state Hemoglobin Undergoes a Structural Change on Binding Oxygen - Greater number of ion pairs hemoglobin: – R state = O2 has a " higher affinity for hemoglobin – T state = more stable when O2 is absent, predominant conformation of deoxyhemoglobin V= caries Why by and • two conformations of · ↓R hemoglobin drop has to bind 02 off ↳T (iniuys) · T state = higher on pairs 37 37 Ion Pairs Stabilize the T State • T state is stabilized by a greater number of ion pairs, -> when O2 is binded to any 4 of the monomers many of which lie at the α1β2 (and α2β1) interface salt Bridge 38 38 Conformational Change in Hemoglobin • O2 binding to hemoglobin in the T state triggers a conformational change to the R state - > so that all 4 subunits can now – αβ subunit pairs slide past each other and rotate – the pocket between the β subunits narrow – some ion pairs that stabilize the T state break and ↳ to allow the some new ones form bind 02 submits to slick 39 39 13 10/2/2023 The T → R Transition ↳ becomes move Anotice gets the pocket smallet planar 40 40 Changes in Conformation Near Heme "pucker" "Planar" - stabolizes the Onbinding 41 41 Principle 5 (3 of 4) In a multisubunit protein, a conformational change in one subunit often affects the conformation of other subunits. 42 42 14 10/2/2023 Hemoglobin Binds Oxygen Cooperatively • Hemoglobin must bind oxygen efficiently in the lungs (pO2 = 13.3 kPa) and release efficiently in tissues (pO2 = 4 kPa) ○ Myoglobin would be unsuited; would not release in tissues due to high affinity • Hemoglobin solves problem by undergoing transition from low-affinity state (T) to high-affinity state (R) Hemo - works be · bi transitich from + > R ... need to monomeric globins stive their raes which relates to their , SAMCAWT 43 · 43 Hemoglobin Binds Oxygen Cooperatively • hemoglobin ↓Pickitup has a hybrid sigmoid binding curve for oxygen * need ↳ the R a sigmoid only way multisubunit conformational ↳ no pichip to small /"S shaped" curve have get this is to protein that indergoes to change differences . in Allows /"cooperative" a a "sensitivity" concentration . 44 44 Principle 6 (2 of 3) Interactions between ligands and proteins may be regulated. 45 45 15 10/2/2023 causes Allosteric Proteins · hemoglobin - O is an modulater 02 affecting 02 -amore I other activating humotropic • allosteric protein (e.g., hemoglobin) = binding of a ligand to one site affects the binding properties of another site on the same protein – modulators = ligands that bind to an allosteric protein to induce a conformational change – homotropic = normal ligand and modulator are identical – heterotropic = modulator is a molecule other than the normal ligand rotipottncantareE · I -> type of allosteric > types of protein modulater (CO2) I M thems -> home On 46 · -> 46 = Hetero = CO2 activating protein modulater = Inhibiter Carbon Monoxide Can Bind to Hemoglobin • CO has ~250-fold greater affinity for hemoglobin than does O2 so to ball- ↳ still nave sigmoid HgD shape 47 47 Cooperative Ligand Binding Can Be Described Quantitatively • for a protein with n binding sites, the equilibrium becomes: P + nL ⇌ PLn (5-12) Hemo : tems+ 42 = PLu 48 48 16 10/2/2023 The Ka and Y for Cooperative Ligand Binding • the expression for the association constant becomes: (5-13) ↳ association • the expression for Y is: (5-14) 49 49 The Hill Equation (5-14) • rearranging, then taking the log of both sides, yields: - (5-15) dent hud to know now its derived the Hill equation: (5-16) 50 50 Hill Plots and Hill Coefficients What a hill plot is & What It Hells you using the hill equation -> • Hill plot = plot of log [Y/(1 − Y)] versus log [L] • Hill coefficient = nH = slope of a Hill plot – If nH = 1, ligand binding is not cooperative binding is independent – nH > 1 indicates positive cooperativity helps Activating– nH < 1 indicates negative cooperativity -> Inhibiter binding I binding stops any situation where binding does not activate -> -> ↳ * UH bi at It Will = never= would the same require time of # on binding binding sites all proteins af all sites , I does not binding binding impact another chother another ... to be binded interest * 51 51 17 10/2/2023 Adapting the Hill Equation to the Binding of O2 to Hemoglobin (5-17) • Hill coefficient = nH = slope of a Hill plot – If nH = 1, ligand binding is not cooperative – nH > 1 indicates positive cooperativity – nH < 1 indicates negative cooperativity 52 52 Hill Plots for Myoglobin and Hemoglobin R * -> either know axis & Slope Interpretation binds Oz or it dusent -> the transition state shows I 53 53 Principle 5 (4 of 4) In a multisubunit protein, a conformational change in one subunit often affects the conformation of other subunits. 54 54 18 10/2/2023 Two Models Suggest Mechanisms for Cooperative Binding · both models make sense but neither are proven RRRR -ATT • MWC model = concerted model – all subunits in the same conformation "Functionally Identical" – ligand binds more tightly to the R state = -> - all same conformation & exist In the the same time subults transition • sequential model – each subunit can be in either conformation – equilibrium is altered as additional ligands are bound, progressively favoring the R state TTTT = TTTR Y TTRRI TRRRIRRRR 55 55 Concerted and Sequential Models Low affinity or inactive High affinity or active MWC model (concerted model) Sequential model 56 56 * Chot14 bioz) Red Nitebock( Hemoglobin Also Transports H+ and CO2 • hemoglobin carries two end products of cellular respiration: H+ and CO2 • carbonic anhydrase catalyzes the hydration of CO2 to bicarbonate: high (O2 & Law pH = Favor Tstate CO2 + H2O ⇌ H+ + HCO3– a stabolizes the T state lowers pH 57 57 19 10/2/2023 Principle 6 (3 of 3) Interactions between ligands and proteins may be regulated. 58 58 creat cen The Bohr Effect ↳ low • the structural effects of H+ and CO2 binding to affinity SAT hemoglobin favor the T state – the binding of H+ and CO2 is inversely related to the binding of O2 = * to , deliver Tstate and On to your tissethe .... PCOZ = * and pH = A ↳ Favors R State • Bohr effect = describes the effect of pH and [CO2] on the binding and release of O2 by hemoglobin ⑧+ HHB + O2 ⇌ HbO2 + H+ * HT, decrease PH Shift KAT , 59 59 The Effect of pH on O2 Binding to Hemoglobin saturations binding thate of • when [O2] is high, R State hemoglobin binds O2 and releases H+ -> • when [O2] is low, hemoglobin releases O2 and binds H+ ↳ ↑ PH PH * H binds to ACO2 4 Is the amino binding amino to one acids of the terminals 60 60 20 10/2/2023 Hemoglobin Binds CO2 • CO2 binding to hemoglobin is inversely related to binding of O2 – contributes to the Bohr effect by producing H+ -> when you bind you on Ht release 61 61 Oxygen Binding to Hemoglobin Is Regulated by 2,3-Bisphosphoglycerate (1 of 2) * - helps adapt IOW Oz in ↓ so • 2,3-bisphosphoglycerate (BPG): – example of heterotropic allosteric modulation – binds to a site distant from O2-binding site -middel of the pocket – greatly reduces the affinity of hemoglobin for oxygen to in to to high arbody the evelation alt increases decrease the high lungs BPG affitity so more to is 02 released tissue 62 62 Oxygen Binding to Hemoglobin Is Regulated by 2,3-Bisphosphoglycerate (2 of 2) • 2,3-bisphosphoglycerate (BPG): – example of heterotropic allosteric modulation – binds to a site distant from O2-binding site – greatly reduces the affinity of hemoglobin for oxygen HbBPG + O2 ⇌ HbO2 + BPG 63 63 21 10/2/2023 Effect of BPG on O2 Binding to Hemoglobin niberbolic • BPG increases at high altitudes are ↳sigmoid • hypoxia = lowered oxygenation of peripheral tissues – causes BPG increases 64 64 Binding of BPG to Deoxyhemoglobin • BPG binds to the cavity between the β subunits in the T state – cavity is lined with positively charged residues – BPG stabilizes the T state binds in pocket 65 65 Fetal Hemoglobin • fetus synthesizes α2γ2 hemoglobin -> – lower affinity for BPG than normal adult hemoglobin – higher affinity for O2 than normal adult hemoglobin - higher affinity fur low prevents Suite he CO2 66 66 22 10/2/2023 Sickle Cell Anemia Is a Molecular Disease of Hemoglobin • sickle cell anemia: – homozygous condition – single amino acid substitution (Glu6 to Val6) β chains produces a hydrophobic patch 67 67 Normal and Sickle Cell Hemoglobin • deoxygenated hemoglobin becomes insoluble and forms polymers that aggregate in tubular fibers • normal hemoglobin remains soluble upon deoxygenation 68 68 Sickle Cell Anemia • Physical exertion = weak, dizzy, short of breath – • • Heart murmurs and increased pulse Hemoglobin content in blood half the normal value (15-16 g /100 mL) Abnormally shaped cells block capillaries and interfere with normal organ function 69 69 23 10/2/2023 5.2 Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins 70 70 Principle 2 (3 of 4) A ligand binds a protein at a binding site that is complementary to the ligand in size, shape, charge, and hydrophobic or hydrophilic character. The interaction is specific: the protein can discriminate among the thousands of different molecules in its environment and selectively bind only one or a few types. A given protein may have separate binding sites for several different ligands. These specific molecular interactions are crucial in maintaining the high degree of order in a living system. 71 71 Immune Responses • immune response = coordinated set of interactions among many classes of proteins, molecules, and cell types – distinguishes molecular “self” from “nonself” and destroys “nonself” – eliminates viruses, bacteria, and other pathogens and molecules 72 72 24 10/2/2023 The Immune Response Includes a Specialized Array of Cells and Proteins • leukocytes = white blood cells, including macrophages and lymphocytes • The immune response consists of two complementary systems: – humoral immune system = directed at bacterial infections and extracellular viruses – cellular immune system = destroys infected host cells, parasites, and foreign tissues 73 73 The Humoral Immune Response • antibodies = immunoglobulins (Ig) = bind bacteria, viruses, or large molecules identified as foreign and target them for destruction – produced by B lymphocytes or B cells 74 74 The Cellular Immune Response • T lymphocytes = cytotoxic T cells (TC cells) – recognition of infected cells or parasites involves Tcell receptors on the surface of TC cells • helper T cells (TH cells) = produce soluble signaling proteins called cytokines – interact with macrophages – stimulate the selective proliferation of TC and B cells that can bind to a particular antigen (clonal selection) • memory cells = permit a rapid response to pathogens previously encountered 75 75 25 10/2/2023 Vaccines • often consists of weakened or killed virus or isolated proteins from a viral or bacterial protein coat • “teaches” the immune system what the viral particles look like, stimulating the production of memory cells 76 76 Principle 1 (3 of 4) The functions of many proteins involve the reversible binding of other molecules. A molecule bound reversibly by a protein is called a ligand. A ligand may be any kind of molecule, including another protein. The transient nature of protein-ligand interactions is critical to life, allowing an organism to respond rapidly and reversibly to changing environmental and metabolic circumstances. 77 77 Antigens and Haptens • antigen = molecule or pathogen capable of eliciting an immune response – can be a virus, a bacterial cell wall, or an individual protein or other macromolecule – antibodies or T-cell receptors bind to an antigenic determinant or epitope within the antigen • haptens = small molecules that can elicit an immune response when covalently attached to large proteins 78 78 26 10/2/2023 Antibodies Have Two Identical Antigen-Binding Sites • immunoglobulin G (IgG) = major class of antibodies – one of the most abundant blood serum proteins – 4 polypeptide chains: 2 heavy chains and 2 light chains – cleavage with protease papain releases the basal fragment Fc and two Fab branches (each with a single antigen-binding site) – constant domains contain the immunoglobulin fold structural motif 79 79 The Structure of Immunoglobulin G 80 80 The Variable Domain of Immunoglobulin G • heavy and light chains each have a variable domain – variable domains associate to create the antigen-binding site – allows formation of an antigenantibody complex 81 81 27 10/2/2023 Classes of Immunoglobulins • 5 classes in vertebrates: – characterized by heavy chain: • α for IgA • δ for IgD • ε for IgE • γ for IgG • μ for IgM 82 82 Structure of Immunoglobulins • IgD and IgE = similar in structure to IgG • IgM = monomeric, membrane-bound form or in a secreted form that is a cross-linked pentamer of this basic structure • IgA = monomer, dimer, or trimer IgM pentamer 83 83 Phagocytosis of Antibody-Bound Viruses by Macrophages • When Fc receptors bind an antibody pathogen complex, macrophages engulf the complex 84 84 28 10/2/2023 Principle 2 (4 of 4) A ligand binds a protein at a binding site that is complementary to the ligand in size, shape, charge, and hydrophobic or hydrophilic character. The interaction is specific: the protein can discriminate among the thousands of different molecules in its environment and selectively bind only one or a few types. A given protein may have separate binding sites for several different ligands. These specific molecular interactions are crucial in maintaining the high degree of order in a living system. 85 85 Antibodies Bind Tightly and Specifically to Antigen • induced fit = conformational changes in the antibody and/or antigen allow the complementary groups to interact fully • Kd values as low as 10–10 M 86 86 The Antibody-Antigen Interaction Is the Basis for a Variety of Important Analytical Procedures • two types of antibody preparations are used: – polyclonal antibodies • produced by injecting a protein into an animal • contain a mixture of antibodies that recognize different parts of the protein – monoclonal antibodies • Synthesized by a population of identical B cells (a clone) grown in cell culture • homogeneous, all recognizing the same epitope. 87 87 29 10/2/2023 Western Blots • immunoblot = Western blot assay = uses antibodies to detect a protein 88 ↑ student 88 presentation 5.3 Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors 89 89 The Major Proteins of Muscle Are Myosin and Actin -> remember a herativ temporar • arranged in filaments that undergo transient interactions and slide past each other to bring about contraction 90 90 30 10/2/2023 or-ATP snolignushaun wher gua Myosin Amino term • myosin (Mr 520,000) – 2 heavy chains and 4 light chains – forms a fibrous, left-handed coiled coil domain (tail) and a large globular domain (head) Hydraying site . Right handedLet madeil 91 carbox term . 91 Thick Filaments • thick filaments = rodlike structures of aggregated myosin - > core of contractive mit 92 92 Actin a globular • actin: in thin fillamints made from fractin – monomeric G-actin (Mr 42,000) associates to form a long polymer called F-actin ↳ Filliment , * right handed G-actin = O ... they form together to make F actin 93 93 31 10/2/2023 Thin Filaments • thin filaments = F-actin along with the proteins troponin and tropomyosin – assemble as successive monomeric actin molecules add to one end – upon addition each monomer binds and hydrolyzes ATP -> It takes tip actin to id (not for ) motion 94 94 mosine th actin binds to head of myosin = myofibrils : muscle Abe it Components of Muscle • each actin monomer in the thin filament binds to one myosin head group 95 95 Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures • muscle fiber = large, single, elongated, multinuclear cell • each muscle fiber contains ~1,000 myofibrils, each consisting of thick and thin filaments and surrounded by sarcoplasmic reticulum -> receases control Cat to 96 96 32 10/2/2023 ~ simplest/smallest contractive cult The Structure of a Sarcomere • sarcomere = entire contractile unit – A band = stretches the length of the thick filament – I band = contains only thin filaments – Z disk = attachment site for thin filaments – M line = bisects the A band 97 zdisk intersects the centracted when - ↳ thin suand97 thick - 2 . Shrinks/HANow disn's also move together duser ↳ band I thich filaments we created by the association of many myosin * molecules * by Principle 1 (4 of 4) muscle centraction occurs thin sliding over thin . . . . The functions of many proteins involve the reversible binding of other molecules. A molecule bound reversibly by a protein is called a ligand. A ligand may be any kind of molecule, including another protein. The transient nature of protein-ligand interactions is critical to life, allowing an organism to respond rapidly and reversibly to changing environmental and metabolic circumstances. 98 98 Myosin Thick Filaments Slide along Actin Thin Filaments • myosin binds actin tightly when ATP is not bound to myosin ① -> ② -> • series of conformational changes due to binding, hydrolysis, and release of ATP and ADP causes muscle contraction infavorable ③ high energy state -> Pi= released . & glob reatactles down the Further change 99 99 33 10/2/2023 4 Steps -> 1. a. 1. unfavorable . ATP binds myosin; cleft in myosin molecule opens Disrupts actin-myosin interaction ATP is hydrolyzed; conformational change in protein to “high-energy” state a. b. Moves myosin head, changes orientation in relation to actin thin filament Myosin binds weakly to F-actin subunit closer to Z disk than what was just released 100 100 4 Steps Continued 3. Phosphate product of ATP hydrolysis is released a. Conformational change; myosin cleft closes (strengthening myosin-actin binding) 4. Conformation of myosin head returns to original resting state a. “Power stroke” b. Orientation pulls tail of myosin toward Z disk c. ADP released to complete the cycle 101 101 Tropomyosin regulates movement blocking myosin binding sites tropmyosin • tropomyosin = binds to the thin filament and blocks the myosin-binding sites by 102 102 34 10/2/2023 Troponin -> Bouncer • troponin = binds Ca2+ released from the sarcoplasmic reticulum, causes a conformational change, and exposes myosin-binding sites – subunit C binds Ca2+ – subunit I prevents binding of the myosin head to actin – subunit T links the troponin complex to tropomyosin cast then ti will troponin tropomyosin from tell reased be to stop blocking 103 103 Skeletal Muscle • • • Requires two types of molecular function: binding and catalysis Actin-myosin interaction is reversible and leaves participants unchanged Myosin is an actin-binding protein and an ATPase (enzyme) 104 104 35