8.3

Historical Context:
- Walter Bradford Cannon (1915) described physiological responses:
- Animals react to threats with a general discharge of the sympathetic nervous system, preparing for action (fight or flight).
- Secretion of epinephrine (adrenaline) from the adrenal glands triggers pathways for muscular action.
Consequences of Unchecked Response:
- Without regulation, responses would be uncontrolled leading to possible physiological damage or inefficiency.
Functions of Regulation:
- Regulation controls the amount of product formation and the speed of reactions.
- Ensures coordination with physiological requirements.

Substrate Level Control:
- Reaction rate is proportional to substrate concentration.
Feedback Control:
- End products modulate enzyme activity.
Allosteric Control:
- Signal molecules bind at regulatory sites affecting enzyme activity.
Covalent Regulation:
- Enzyme activity can be reversibly altered by post-translational modifications or irreversibly altered.
Isozymes:
- Multiple forms of an enzyme present in an organism that perform the same functions.
Allozymes:
- Common enzymes showing high evolutionary conservation.
Controlling Amount of Enzyme:
- Typically regulated at the transcription level.

Enzymatic Reaction Rate Response:
- When substrate concentration S is much greater than Michaelis constant (Km), product formation rate increases until enzyme saturation or steady state is achieved.
- As product levels rise, release from enzyme declines leading to enzyme catalysis inhibition (diffusion limitation).
Example: Hexokinase (HK) mechanisms involving glucose and ATP conversion into glucose 6-phosphate (G6P).

Feedback (End Product) Control:

Can be activating or inhibiting.
End products control the accumulation of intermediates by acting on enzymes early in pathways, typically binding to them allosterically.

Consider enzymes labeled E1, E2, E3, and E4 producing products P0, P1, P2, P3, and P4 for pathway feedback questions.

Definition of Allostery:
- Involves binding at sites other than the catalytic site to regulate enzyme activity.
- Michaelis-Menten Kinetics: Allosteric enzymes do not follow classic kinetics but yield sigmoidal curves due to conformational changes between R (Relaxed) and T (Tense) states.
Types of Allostery:
- Heteroallostery (heterotropic) - involves the regulation of enzyme activity by binding of a ligand at a site other than the active site, resulting in a conformational change that affects the enzyme's functionality.
- Homoallostery (homotropic)- involves the regulation of enzyme activity by the binding of substrates at the active site, which can trigger conformational changes that enhance or inhibit the activity of the enzyme.
Mechanisms:
- Activators drive enzymes into the R-state, while inhibitors push towards the T-state.

Reaction velocity is affected by substrate concentrations and activators/inhibitors, demonstrating nonlinear kinetics in allosteric enzyme activity.

Reversible modification allows fine tuning of cell processes and metabolism within cells
- Phosphorylation and dephosphorylation
Irreversible modification - ultimate cellular regulation and basis for drug design
- Lipid attachment to proteins results in the protein being affixed to plasma membrane

Free energy for phosphorylation is large. ATP is typical phosphate donor and releases energy
- ATP and ADP/Pi resonance
Phosphate adds 2 negative charges to protein - affects electrostatic interactions
Phosphate group can form 3 or more hydrogen bonds
Phosphorylation can change protein conformation
ATP is typical phosphate group donor linking phosphorylation to metabolic regulation
Regulation can be modulated (fast/slow) to meet needs of the cell

Kinases require 1-2 MG2+ (or Mn2+) cofactors to catalyze group-transfer rxn
Ping-pong or double displacement is most common strategy
Dedicated kinases phosphorylate a single protein to closely related ones
Multifunctional kinases modify many different targets and can coordinate diverse processes and can amplify signal

Illustration of how end products like CTP and UTP are inhibitors while ATP serves as an activator in ATCase, impacting product ratios of pyrimidines to purines.

PALA: A potent ATCase inhibitor acting as transition state analog; inhibits pyrimidine biosynthesis leading to cell proliferation insight.
Clinical Implications: Resistance in cells upon continual exposure.

Discusses major forms of transcriptional regulation and introduces the significance of covalent modifications in enzyme activity through processes such as phosphorylation, acetylation.

Differentiation between types of enzymatic reactions by kinases and phosphatases (group transfer vs. hydrolysis).
True/False questions about substrate concentration impacts and enzyme regulation principles.

Definition: Zymogens are inactive precursors activated by hydrolysis.
Key examples include blood coagulation proteins and digestive enzymes from the pancreas.

Enteropeptidase activates trypsinogen to trypsin, after which trypsin activates other zymogens through autocatalysis.

Trypsin cleaves the bond between Arg 15 and Ile 16 but N-terminus remains attached due to disulfide bond between residues
Chymotrypsin autocatalyzes the removal of residue 14, 15 and 147 & 148

Importance of competitive inhibition through proteins like pancreatic secretory trypsin inhibitor (PSTI) to protect against autodigestion and maintain physiological functioning.

Genetic mutations leading to elastase activity unchecked in lung tissue cause diseases such as emphysema — tissue damage correlated to enzyme regulation failures.

Understanding enzyme regulation encompasses vast implications across metabolic regulation, disease states, and therapeutic targets, paving the way for advancements in medicinal chemistry.