BIOC 384: Module 5 Review
Module 5 Review - Hemoglobin Gene Mutations
Hemoglobin Gene Mutations
The β-E6V mutation results in a substitution of Glu→Val in β-globin, introducing a hydrophobic patch on the protein’s surface.
In the deoxygenated state (T state), this causes intermolecular interactions and polymerization of HbbS, leading to the distortion of red blood cells into a sickle shape.
Sickle cell disease arises from a genetically inherited mutation in the β subunit (βS) of hemoglobin (E6V), resulting in large polymeric fibers formed through hydrophobic interactions, which in turn leads to defective O2 transport by erythrocytes.
Module 5 – Key Concepts
Sickle Cell Disease and Malaria Resistance
The βS mutation provides heterozygous βS individuals with resistance to malaria.
Red blood cells infected by the malarial parasite are removed by the spleen, thus protecting these individuals from severe complications of malaria.
The βS allele is most prevalent in regions where malaria is endemic, especially in sub-Saharan Africa.
The non-random distribution indicates an evolutionary advantage to carrying the mutation, even with its negative health impacts.
The Actin-Myosin Motor
Thick and Thin Filaments
Thick filaments consist of myosin protein molecules aligned tail-to-tail within the fibers.
The protein titin connects the ends of thick filaments to anchoring proteins located in the sarcomeres' Z disks.
Each myosin complex in thick filaments comprises six polypeptide chains grouped into three structural areas: head, neck, and tail.
The head groups are responsible for ATP binding and hydrolysis, which regulates their attachment to actin.
Composition of Thin Filaments
Thin filaments are composed of polymerized actin proteins that provide binding sites for myosin, in addition to the troponin complex (TnC, TnT, and TnI), where TnC binds Ca2+.
Troponin and tropomyosin work together to facilitate myosin's binding to actin.
A model of the thin filament, built from cryo-EM and fluorescence energy resonance transfer (FRET) data, includes three essential protein complexes:
Actin polymer
Troponin complex
Tropomyosin helices
Muscle Contraction Cycle
Initiated by the Ca2+ release from the sarcoplasmic reticulum, which prompts myosin heads to bind to actin, resulting in Pi release that triggers a conformational change in myosin (the power stroke).
Steps of the contraction cycle:
Myosin–ADP–Pi binds to actin.
Pi release causes the power stroke.
ADP dissociates from the nucleotide site.
ATP binding causes myosin head release.
ATP hydrolysis resets the myosin head.
This process leads to the shortening of sarcomeres and production of contractile force.
Introduction to Enzymes
Characteristics of Enzymes
Enzymes are biological catalysts that speed up reactions without altering the overall Gibbs energy change (ΔG) or equilibrium constant (Keq) and remain unconsumed in the reaction.
Key properties of enzymes:
High affinity and specificity for substrates; active sites form pockets or clefts where substrates bind and catalysis occurs.
Binding induces structural changes resulting in numerous weak interactions (hydrogen bonds, ionic interactions, van der Waals) stabilizing the enzyme-substrate complex.
Activity is highly regulated to balance energy between catabolic and anabolic pathways, adapting cell behavior to environmental changes.
Rule of Threes: Information organized in groups of three enhances memorability and satisfaction.
Catalysis and Energy
The energy released from weak interactions stabilizes the transition state during catalysis, lowering activation energy.
A classic example of induced-fit substrate binding is hexokinase, phosphorylating glucose.
Substrate binding typically results in minor structural changes, with significant alterations near the active site.
Activation Energy (ΔG‡)
Defined as the difference between the energy of the reactant ground state and that of the transition state.
Enzymes lower ΔG‡ by providing optimal conditions for catalysis, stabilizing the transition state, thereby increasing reaction rates without affecting ΔG.
Cofactors Role in Enzymatic Reactions
Cofactors add reactive groups to enzymes assisting in catalysis, typically found as inorganic ions or organic coenzymes derived from vitamins.
Metal ion cofactors (e.g., Zn²⁺, Fe²⁺, Cu²⁺) play significant roles in redox reactions and stabilization of substrates, while organic coenzymes (e.g., NAD⁺, FAD) may attach non-covalently.
Enzyme Mechanisms
Mechanisms of Chemical Catalysis
The transition state stabilization is crucial in chemical catalysis and underpins the concept of transition state analogs as enzyme inhibitors.
Key mechanisms of catalytic activity include:
Acid–base catalysis (proton transfer)
Covalent catalysis (transient covalent bond formation)
Metal-ion catalysis (metal cofactors assist in charge stabilization)
Chymotrypsin Mechanism
Chymotrypsin employs both acid–base and covalent catalysis for peptide bond cleavage, requiring a tetrahedral intermediate resembling the transition state.
The active site includes a catalytic triad consisting of Ser195, His57, and Asp102 contributing to increasing the nucleophilicity of Ser195.
His57 functions as both a base/acid, and Asp102 stabilizes the protonated form of His57 to facilitate the reaction.
Process Overview:
Substrate binding: Active site, hydrophobic pocket engages substrate.
Nucleophilic attack: Ser195 attacks carbonyl carbon, forming the first tetrahedral intermediate stabilized by the oxyanion hole.
Proton addition: His57 donates proton to nitrogen in peptide bond, causing C-terminal fragment release.
Water enters: Deprotonated water activates nucleophile; His57 assists in this process.
Formation of second tetrahedral intermediate: Another attack by OH from water leads to stabilization required through the oxyanion hole.
Release of N-terminal fragment: His57 donates proton back to Ser195 regenerating the catalytic triad.
The specificity for peptide bond cleavage by chymotrypsin, trypsin, and elastase reflects variations in the size and chemistry of their respective specificity pockets.