Biochemistry Study Guide: Enzyme Kinetics, Carbohydrates, Lipids, Membranes, and Signal Transduction

Enzyme Kinetics and Inhibition

Types of Enzyme Inhibition

Enzyme inhibition refers to the decrease in enzyme activity due to the interaction with specific molecules called inhibitors. Understanding the types of inhibition is crucial for interpreting enzyme kinetics and designing drugs.

• Competitive Inhibition: The inhibitor competes with the substrate for binding to the active site. It increases the apparent $K_m$ but does not affect $V_{max}$.

• Noncompetitive Inhibition: The inhibitor binds to an allosteric site, not the active site, and can bind to both the free enzyme and the ES complex. It decreases $V_{max}$ but does not affect $K_m$.

• Uncompetitive Inhibition: The inhibitor binds only to the ES complex, forming an ESI complex. Both $K_m$ and $V_{max}$ decrease.

• Irreversible Inhibition: The inhibitor covalently modifies the enzyme, permanently inactivating it.

Example: Methanol poisoning is treated by administering ethanol, which acts as a competitive inhibitor for alcohol dehydrogenase.

Enzyme Kinetic Parameters

Enzyme kinetics are often analyzed using the Michaelis-Menten equation and Lineweaver-Burk plots.

• $V_{max}$: The maximum velocity of the reaction when the enzyme is saturated with substrate.

• $K_m$: The substrate concentration at which the reaction rate is half of $V_{max}$.

• Michaelis-Menten Equation:

$V_0 = \frac{V_{max}[S]}{K_m + [S]}$

• Lineweaver-Burk Plot: A double reciprocal plot used to determine $K_m$ and $V_{max}$.

Example: In the absence of inhibitor, $V_{max}$ and $K_m$ can be estimated from the y-intercept and x-intercept of the Lineweaver-Burk plot, respectively.

FRAP (Fluorescence Recovery After Photobleaching)

FRAP is a technique used to study the lateral diffusion of molecules within membranes and membrane fluidity.

• Application: Used to measure protein and lipid mobility in biological membranes.

Carbohydrates: Structure and Function

Monosaccharides and Disaccharides

Carbohydrates are classified based on the number of sugar units and the type of carbonyl group present.

• Aldose: Monosaccharide with an aldehyde group (e.g., glucose).

• Ketose: Monosaccharide with a ketone group (e.g., fructose).

• Reducing Sugar: A sugar that can act as a reducing agent due to a free anomeric carbon.

• Glycosidic Bond: The covalent bond formed between two monosaccharides.

Example: Trehalose is a disaccharide with two anomeric carbons and a specific glycosidic bond. It is not a reducing sugar.

Polysaccharides

Polysaccharides are long chains of monosaccharide units and serve structural and storage functions in plants and animals.

• Cellulose: Structural material in plants; composed of β(1→4) linked glucose units.

• Starch: Energy storage material in plants; composed of α(1→4) and α(1→6) linked glucose units.

• Glycogen: Energy storage material in animals; highly branched α(1→4) and α(1→6) linked glucose units.

Example: Cellulose provides rigidity to plant cell walls, while starch is used for energy storage.

Lipids and Membranes

Fatty Acids and Triglycerides

Lipids are hydrophobic molecules that include fatty acids, triglycerides, phospholipids, and sterols.

• Fatty Acid: Long hydrocarbon chain with a carboxylic acid group.

• Triglyceride: Ester of glycerol and three fatty acids; main energy storage lipid.

• Phospholipid: Major component of cell membranes; contains a phosphate group.

• Cholesterol: Steroid that modulates membrane fluidity.

Example: Triglycerides with unsaturated fatty acids are liquid at room temperature (oils), while those with saturated fatty acids are solid (fats).

Membrane Fluidity

Membrane fluidity is regulated by lipid composition and temperature.

• Cholesterol: Increases membrane fluidity at low temperatures and decreases it at high temperatures.

• Fatty Acid Length and Saturation: Longer and more saturated fatty acids decrease fluidity.

Example: The presence of cholesterol and unsaturated fatty acids increases membrane fluidity, which is essential for proper cell function.

Hemoglobin and Oxygen Transport

Allosteric Regulation and Oxygen Binding

Hemoglobin is a tetrameric protein that transports oxygen in the blood. Its oxygen affinity is regulated by several factors.

• Allosteric Effect: Binding of oxygen to one subunit increases the affinity of the remaining subunits (cooperativity).

• 2,3-Bisphosphoglycerate (2,3-BPG): Decreases hemoglobin's affinity for oxygen, facilitating oxygen release in tissues.

• Bohr Effect: Increased CO2 and decreased pH lower hemoglobin's oxygen affinity.

Example: In the presence of 2,3-BPG, hemoglobin releases oxygen more readily to tissues.

Signal Transduction and Second Messengers

Signal Transduction Cascades

Signal transduction involves the transmission of molecular signals from a cell's exterior to its interior, often amplifying the signal through cascades.

• Second Messengers: Small molecules such as cAMP, Ca2+, and IP3 that relay signals inside the cell.

• GTPase Activity: G-proteins hydrolyze GTP to GDP, acting as molecular switches.

• Phosphoinositide Cascade: Involves phospholipase C, which generates IP3 and DAG from membrane phospholipids.

Example: The signal can be amplified by generating many second messengers, and terminated by dephosphorylation or degradation of the messenger.

Tables

Comparison of Enzyme Inhibition Types

Type	Binding Site	Effect on $K_m$	Effect on $V_{max}$
Competitive	Active site	Increases	No change
Noncompetitive	Allosteric site	No change	Decreases
Uncompetitive	ES complex	Decreases	Decreases
Irreversible	Covalent modification	N/A	Decreases

Carbohydrate Classification

Type	Functional Group	Example
Aldose	Aldehyde	Glucose
Ketose	Ketone	Fructose

Additional info:

• Some questions referenced the calculation of $riangle G$ for ATP hydrolysis and the Na-K pump. The standard free energy change for ATP hydrolysis is $-30.5$ kJ/mol.

• Signal transduction cascades can be terminated by phosphatases, receptor desensitization, or degradation of second messengers.

• Enzyme catalysis often involves a catalytic triad, oxyanion hole, and transition state stabilization.