Office Hours Biochem
Overview of Enzyme Interactions and Inhibition
Tropic Factors
Tropic factors are compounds that bind at sites other than the active site of an enzyme.
Competitive Inhibition
Activator and Inhibitor Behavior:
At lower concentrations, a compound can act as an activator by stabilizing the enzyme-substrate complex.
At higher concentrations, it can become a competitive inhibitor by occupying the active site and preventing substrate access.
TPCK and Serine Proteases
TPCK:
Stands for Tosyl phenylalanylamide chloromethyl ketone
Acts as a substrate analog that specifically binds to the active site of serine proteases like chymotrypsin and trypsin.
Serine Proteases Mechanisms:
Both chymotrypsin and trypsin contain the same catalytic triad: serine, histidine, and aspartate.
The active site configuration determines substrate specificity:
Chymotrypsin: Binds large, hydrophobic residues (e.g., phenylalanine, methionine, tryptophan).
Trypsin: Has a specificity pocket that contains a negatively charged aspartate, allowing it to bind positively charged residues.
Reactive Groups in Inhibition:
TPCK's structure has portions targeting the active site residues in chymotrypsin and forming a covalent bond.
Specificity Pocket and Enzyme Discrimination
Discrimination between substrates (e.g., ATP vs GTP):
The enzyme recognizes unique features in the substrate via non-covalent interactions in the specificity pocket.
Irreversible Inhibitors
Four Types of Irreversible Inhibitors:
Group Specific Reagents: Modify all similar functional groups in proteins, not just active sites. Example: DIPF.
Substrate Analogs: Specific to active sites (e.g., TPCK).
Transition State Analogs: Compounds that mimic the transition state of the substrate, binding tightly and effectively acting as inhibitors.
Mechanism-Based Inhibitors: Mimic the substrate and exploit the catalytic mechanism, such as penicillin targeting bacterial enzymes.
DIPF Studies:
Modifies serines throughout but only reacts with one out of 28 serines, suggesting this serine is uniquely reactive.
Other modifications help infer if that serine is part of the active site when combined with TPCK results.
Hemoglobin and Oxygen Binding
Fetal vs Adult Hemoglobin:
Fetal hemoglobin has a different gamma subunit (serine instead of histidine) that reduces binding of 2,3-bisphosphoglycerate (2,3-BPG), enhancing oxygen affinity.
2,3-BPG stabilizes the T state of hemoglobin.
Hill Plot Analysis:
Using the Hill equation to analyze cooperative binding.
Slope of the Hill plot reflects the degree of cooperativity in oxygen binding (n value).
Myoglobin: N value = 1 (non-cooperative), Hemoglobin: N value can be up to 4.
Cooperativity in Allosteric Regulation
Homotropic Factors: Bind at the active site (example: substrates for ATCase).
Heterotropic Factors: Bind at locations other than the active site (e.g., ATP and CTP for ATCase), influencing enzyme activity indirectly.
T State Stabilization: Increased binding of heterotropic effectors stabilizes the T state, reducing affinity for oxygen in hemoglobin.
Enzyme Mechanics and Catalysis
Enzymatic activity can vary based on regulatory factors affecting structure and binding properties.
Stability of States: T state (tense) stabilizes under lower oxygen concentration conditions while R state (relaxed) favors higher oxygen concentrations.
Energetic Changes: The energy changes associated with breaking and forming bonds drive changes in enzyme conformation, leading to functional outcomes.
Practical Applications and Study Implications
Familiarity with how enzymes interact with substrates and inhibitors is crucial for understanding metabolic pathways and designing drugs.
A deep understanding of kinetics, inhibition, and regulatory mechanisms aids in both biochemical research and therapeutic interventions in disease states where enzyme regulation is disrupted.