Lecture 19: Enzyme Regulation

Pathways and Regulation

• A metabolic pathway is a linked series of biochemical reactions moving towards a specific end, where the product of one reaction serves as the substrate for the next, ensuring a continuous flow of biochemical transformations.

• The reaction product of one enzyme becomes the substrate of the next, facilitating the sequential progression of the pathway.

• These pathways need to respond to external conditions. Example: $E. coli$ lac operon should not be active without lactose, demonstrating the importance of environmental cues in regulating metabolic processes.

  • Metabolic pathways involve a series of enzymatic reactions, each catalyzing a specific step to convert a starting molecule into an end product.

• Regulation often happens through gene expression, influencing the synthesis of enzymes involved in the pathway.

  • Regulation of metabolic pathways can occur at multiple levels, including transcriptional control of gene expression, allosteric regulation of enzyme activity, and post-translational modifications of enzymes.

Regulatory Enzymes

• Certain steps in a metabolic pathway are more rate-limiting or irreversible, acting as control points for the entire pathway.

• Enzymes managing these steps are regulatory enzymes, exerting significant influence over the flux through the pathway.

• The system exerts control by increasing/decreasing the activity of these key enzymes, allowing for fine-tuning of metabolic flow in response to cellular needs.

• Example: Phosphofructokinase (PFK) is a regulatory enzyme in glycolysis, catalyzing the committed step in glucose metabolism. PFK is inhibited by ATP and promoted by AMP, reflecting the energy status of the cell. Citrate inhibits PFK, indicating an abundance of downstream metabolites in the pathway.

  • Regulatory enzymes often catalyze reactions that are far from equilibrium, making them sensitive to changes in substrate and product concentrations. They are also subject to allosteric regulation by metabolites or cofactors that bind to sites distinct from the active site, inducing conformational changes that alter enzyme activity.

Methods of Control

• Regulation of enzyme availability: Balancing the rate of production with the rate of degradation ensures that enzyme levels are appropriate for the metabolic demands of the cell.

  • Control of gene expression: Regulating the transcription and translation of enzyme-encoding genes determines the amount of enzyme synthesized by the cell.

  • Control of protein degradation: Targeting enzymes for degradation through pathways such as the ubiquitin-proteasome system modulates enzyme levels and activity.

• Regulation of catalytic activity: Modification of protein structure leading to modification of protein activity enables rapid and reversible control of enzyme function.

  • Covalent modification: Adding or removing chemical groups, such as phosphate, methyl, or acetyl groups, can alter enzyme activity by inducing conformational changes or affecting substrate binding.

  • Non-covalent modification: Binding of regulatory molecules, such as metabolites, cofactors, or other proteins, can modulate enzyme activity through allosteric mechanisms.

Control of Gene Expression

• Constitutive enzymes: Enzymes consistently present in the organism in constant amounts regardless of metabolic state, ensuring that certain essential metabolic functions are always active. Example: Glycolytic enzymes (activity may be reduced, but proteins are not downregulated).

  • Constitutive expression of certain enzymes ensures that essential metabolic reactions can occur at all times, regardless of environmental conditions.

• Inducible enzymes: Enzymes that are not present in the cell until a specific environmental signal is triggered, allowing cells to respond to changing conditions by synthesizing enzymes only when they are needed. Could be the presence of a substrate. Example: COX-2 in macrophages (produces inflammatory prostaglandins only when needed).

  • Inducible enzymes are often regulated by transcription factors that bind to specific DNA sequences in the promoter region of the gene, enhancing or repressing transcription in response to environmental signals.

• Repressible enzymes: Enzymes consistently present unless a specific condition is triggered, preventing the overproduction of enzymes when the metabolic pathway is no longer needed. Example: Enzymes of cholesterol biosynthesis (sterol accumulation inhibits the pathway).

  • Repressible enzymes are often regulated by feedback inhibition, in which the end product of the pathway inhibits the activity of an enzyme involved in an earlier step, reducing the synthesis of downstream metabolites.

Control of Protein Degradation - Ubiquitination

• The ubiquitin-proteasome system is the major ATP-dependent proteolytic pathway in eukaryotes, responsible for degrading misfolded, damaged, or regulatory proteins to maintain cellular homeostasis.

• Ubiquitin is a small, conserved protein with 76 amino acid residues, serving as a molecular tag that targets proteins for degradation.

• Ubiquitin is covalently attached to proteins by three enzymes (E1, E2, and E3): E1 activates ubiquitin through attachment to itself. E1 transfers its ubiquitin to E2, which then works with E3 to catalyze the addition of ubiquitin to the target protein. E3 recognizes the target protein, providing specificity to the ubiquitination process.

  • The ubiquitin-proteasome system plays a critical role in regulating protein turnover, signal transduction, cell cycle progression, and immune responses.

• Once initially ubiquitinated, additional cycles can occur, leading to a poly-ubiquitin tail, which induces proteolysis by the proteasome.

  • Polyubiquitination involves the sequential addition of ubiquitin molecules to lysine residues on the target protein, forming a chain that serves as a recognition signal for the proteasome.

Ubiquitination - Degrons

• Recognition of proteins for degradation is achieved by protein motifs called degrons, acting as signals that mediate the interaction between the target protein and the ubiquitin ligase.

• Degrons are tags that mark a protein for degradation. There are inherent degron tags (embedded within the protein sequence). Acquired degron tags (added post-translationally).

  • Degrons can be short amino acid sequences, structural motifs, or modified amino acid residues that are recognized by specific ubiquitin ligases.

Ubiquitination - Proteasome

• The target protein is introduced to the proteasome, which recognizes the poly-ubiquitination signal and degrades the protein into short peptides.

• The proteasome is large (dozens of subunits) and highly conserved across Eukarya, consisting of a catalytic core and regulatory particles that recognize and unfold ubiquitinated proteins.

  • The proteasome is a multi-subunit complex that degrades ubiquitinated proteins into small peptides, which are further degraded into amino acids by cytoplasmic peptidases.

Methods of Control (Revisited)

• Regulation of enzyme availability – Balancing rate of production with rate of degradation

  • Control of gene expression

  • Control of protein degradation

• Regulation of catalytic activity – Modification of protein structure → modification of protein activity

  • Covalent modification

  • Reversible and irreversible

  • Non-covalent modification

Irreversible Covalent Modification – Activation through Proteolysis

• A zymogen (proenzyme) is an inactive precursor of an enzyme, activated by proteolytic cleavage, ensuring that the enzyme is only active under specific conditions and in specific locations.

• Activation could involve a single peptide linkage being cut or an entire section of the protein being removed, triggering a conformational change that exposes the active site of the enzyme.

• Modifications grant access to the active site, allowing the enzyme to bind its substrate and catalyze the reaction.

• Can be autocatalytic or induced by another enzyme, providing a mechanism for signal amplification and coordinated activation of multiple enzymes.

• Zymogens are often indicated by the "pro-" prefix or "-gen" suffix. Example: pepsinogen (zymogen of pepsin), prothrombin (zymogen of thrombin).

  • Activation of zymogens by proteolytic cleavage is a common regulatory mechanism in processes such as digestion, blood coagulation, and apoptosis.

Zymogens

• Proteases are the most common type of protein that exists as zymogens (cell doesn’t want them active all the time), preventing uncontrolled proteolysis and tissue damage.

• Activation occurs in a specific context (e.g., coagulation involving prothrombin), ensuring that proteases are only active when and where they are needed.

  • Zymogen activation often involves a cascade of proteolytic events, in which one protease activates another, leading to rapid and localized activation of downstream enzymes.

Reversible Covalent Modifications – Phosphorylation

• Attachment of phosphoryl groups to proteins is a common form of regulatory modification, allowing for rapid and reversible control of protein function in response to various stimuli.

• A significant protion of the human proteome is able to be phosphorylated, highlighting the importance of phosphorylation as a regulatory mechanism.

• Phosphoryl groups are attached to specific residues by kinases, which transfer a phosphate group from ATP to the hydroxyl group of serine, threonine, or tyrosine residues.

• Phosphate groups are removed by phosphatases, which hydrolyze the phosphate ester bond, releasing inorganic phosphate and restoring the protein to its dephosphorylated state.

• Phosphorylation usually requires a hydroxyl group for attachment; serine is the most commonly phosphorylated residue, followed by threonine and tyrosine, and rarely histidine.

  • Protein phosphorylation is a dynamic process that is tightly regulated by kinases and phosphatases, allowing cells to rapidly respond to extracellular signals and adapt to changing conditions.

Impact of Phosphorylation

• Protein phosphorylation introduces a bulky, charged group into a region of the protein, changing its electrostatic properties, leading to conformational changes that affect protein-protein interactions, substrate binding, and catalytic activity.

• This leads to conformational changes that can either activate or deactivate the protein, depending on the specific protein and the site of phosphorylation.

• Can lead to an increase or decrease in catalytic efficiency, altering the rate at which the enzyme catalyzes its reaction.

• Example: Phosphorylation of the phosphorylase dimer disrupts electrostatic interactions, leading to activation of the enzyme and increased glycogen breakdown.

  • The effects of phosphorylation on protein function can be diverse, ranging from subtle changes in enzyme kinetics to dramatic alterations in protein localization and stability.

Active/Inactive States

• Having multiple stable protein conformations is metastability, allowing proteins to switch between different functional states in response to regulatory signals.

• Phosphorylation triggers a transition between active and inactive conformations (alters the free energy of each), providing a mechanism for rapidly switching protein function on or off.

  • The equilibrium between active and inactive conformations of a protein is determined by the relative rates of phosphorylation and dephosphorylation, which are influenced by the activity of kinases and phosphatases.

Phosphorylation - Consensus Sequence

• Residues that are phosphorylated occur in a common structural motif (consensus sequence) recognized by the protein kinase, ensuring that the kinase phosphorylates the correct target residues.

• Different sequences can be recognized by different kinases, allowing for specific phosphorylation of different target proteins by different kinases.

• A given protein can have multiple of these sequences, enabling it to be regulated by multiple kinases and phosphatases.

• Phosphorylation events can be sequential. Example: A certain residue can only be phosphorylated if a phosphoryl group is already present nearby, creating a hierarchical phosphorylation cascade.

  • The consensus sequence for phosphorylation often includes basic or acidic residues that flank the phosphorylation site, as well as hydrophobic residues that contribute to the overall structure of the motif.

Reversible Non-Covalent Modifications - Allostery

• Allosteric regulation of protein function involves the binding of an effector molecule at a site other than the active site, inducing conformational changes that alter the protein's activity.

• Allosteric regulation can be homotropic or heterotropic. Homotropic regulation is when the substrate regulates function, often resulting in cooperative substrate binding. Heterotropic regulation is when a different molecule than the substrate regulates function, allowing for integration of diverse regulatory signals.

  • Allosteric regulation is a common mechanism for feedback inhibition, in which the end product of a metabolic pathway inhibits an enzyme involved in an earlier step, preventing overproduction of the product.

• Can be positive or negative. Positive regulation increases activity/binding. Negative regulation decreases activity/binding.

  • Allosteric regulators can either enhance or inhibit enzyme activity by stabilizing different conformational states of the protein.

Complex Regulatory Patterns

• Many proteins have complex regulatory patterns, integrating multiple regulatory signals to fine-tune their activity in response to diverse cellular conditions.

• Glycogen phosphorylase uses both covalent and non-covalent modifications to create two levels of modular control, allowing for precise regulation of glycogen breakdown in response to energy demands and hormonal signals.