Enzymes and Enzyme Inhibition

Types of Enzyme Inhibitors

Enzyme inhibitors are molecules that decrease or halt the activity of enzymes. They play a crucial role in regulating metabolic pathways.

• Competitive Inhibitors: These resemble the substrate and compete for binding at the enzyme's active site. For example, a competitive inhibitor of lipase will resemble a fatty acid.

• Noncompetitive Inhibitors: These bind to a site other than the active site, causing a change in enzyme shape and blocking activity.

Key Points:

• Competitive inhibitors resemble the substrate and bind to the active site.

• Noncompetitive inhibitors block the active site by binding elsewhere.

• Competitive inhibitors do not decrease the rate of reaction by binding elsewhere.

Example: In phenylketonuria (PKU), competitive inhibition of phenylalanine hydroxylase (PAH) leads to elevated levels of phenylalanine (Phe).

Enzyme-Catalyzed Reactions

Enzymes catalyze biochemical reactions by lowering activation energy.

• Endergonic reactions: Products have higher energy than reactants; energy is absorbed.

• Exergonic reactions: Products have lower energy than reactants; energy is released.

Equation:

Role of Cofactors and Coenzymes

Many enzymes require additional molecules for activity.

• Cofactors: Inorganic ions or organic molecules that assist enzyme function.

• Coenzymes: Organic cofactors, such as BH4 in the PAH-catalyzed reaction.

Metabolic Pathways

Catabolic and Anabolic Pathways

Metabolic pathways are classified based on their function and energy requirements.

• Catabolic pathways: Break down complex molecules and release energy.

• Anabolic pathways: Build complex molecules and require energy.

Membrane Transport

Types of Membrane Transport

Cells regulate the movement of substances across their membranes using various mechanisms.

• Simple Diffusion: Small, hydrophobic molecules cross the membrane easily without assistance.

• Facilitated Diffusion: Requires a carrier or channel protein for molecules like amino acids or ions.

• Active Transport: Moves substances against their concentration gradient using energy (often ATP).

Transport Protein Requirements

Osmosis and Tonicity

Osmosis is the movement of water across a semipermeable membrane. Tonicity describes the relative concentration of solutes in solutions separated by a membrane.

• Isotonic: Equal solute concentration; no net water movement.

• Hypotonic: Lower solute concentration outside; water enters the cell.

• Hypertonic: Higher solute concentration outside; water leaves the cell.

Phospholipid Bilayer Structure

The plasma membrane consists of a phospholipid bilayer.

• Heads: Hydrophilic, face outward toward water.

• Tails: Hydrophobic, face inward away from water.

Cell Signaling and Signal Transduction

Kinases and Phosphorylation

Kinases are enzymes that transfer phosphate groups to target molecules, often regulating activity.

• Protein kinases: Phosphorylate target proteins, including enzymes and transcription factors.

• Receptor tyrosine kinases: Activated by phosphorylation and dimerization.

Stages of Cell Signaling

• Reception: Signal molecule binds to receptor.

• Transduction: Signal is relayed and amplified inside the cell, often via phosphorylation cascades.

• Response: Cellular activity is altered (e.g., gene expression, metabolism).

Hormonal Regulation of Blood Sugar

• Epinephrine: Increases blood sugar by acting on liver cells.

• Glucagon: Elevates blood sugar by promoting glucose release from liver and muscle.

• Insulin: Allows glucose to enter muscle cells by facilitated diffusion.

Cell Cycle and Division

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• G1 phase: Cell growth

• S phase: DNA replication

• G2 phase: Preparation for mitosis

• Mitosis: Division of the nucleus

• Cytokinesis: Division of the cytoplasm

Cell Cycle Table

Regulation of the Cell Cycle

• Cyclin-dependent kinases (cdks): Active when bound to cyclin and phosphorylated; regulate progression through the cell cycle.

• Rb protein: Blocks progression from G1 to S phase unless phosphorylated.

• Sister chromatids: The two identical DNA molecules in a replicated chromosome.

Diabetes and Insulin Regulation

• Type 1 diabetes: Insulin deficiency

• Type 2 diabetes: Insulin resistance

• High blood sugar and high insulin may indicate either type

Additional info:

• PKU (Phenylketonuria) is a genetic disorder resulting in elevated phenylalanine due to defective PAH enzyme.

• Facilitated diffusion is passive and does not require energy, but does require a transport protein.

• Plasmolysis in plant cells indicates a hypertonic environment.