CHEM 161 Exam 2 Study Guide: Enzymes, Hemoglobin, and Carbohydrates

Enzymes

General Properties and Function

Enzymes are biological catalysts that accelerate chemical reactions in living organisms by lowering the activation energy required. They are highly specific for their substrates and reactions.

• Specificity: Enzymes recognize specific substrates due to their unique active sites.

• Efficiency: Enzymes can increase reaction rates by factors of 106 or more.

• Regulation: Enzyme activity can be regulated by various mechanisms, allowing cells to control metabolic pathways.

• Reusability: Enzymes are not consumed in the reactions they catalyze.

• Optimal Conditions: Each enzyme has optimal temperature and pH for activity.

Classification of Enzymes

Enzymes are classified into six major classes based on the type of reaction they catalyze:

• Oxidoreductases: Catalyze oxidation-reduction reactions.

• Transferases: Transfer functional groups between molecules.

• Hydrolases: Catalyze hydrolysis reactions (breaking bonds with water).

• Lyases: Add or remove groups to form double bonds.

• Isomerases: Catalyze isomerization (rearrangement of atoms within a molecule).

• Ligases: Join two molecules together, often using ATP.

Cofactors, Coenzymes, Apoenzyme, and Holoenzyme

• Cofactor: A non-protein chemical compound required for enzyme activity (can be metal ions or organic molecules).

• Coenzyme: An organic cofactor, often derived from vitamins (e.g., NAD+, FAD).

• Apoenzyme: The protein part of an enzyme, without its cofactor/coenzyme; inactive.

• Holoenzyme: The complete, active enzyme with its cofactor/coenzyme bound.

Induced Fit Model

The induced fit model describes how enzyme active sites are flexible and mold around the substrate upon binding, enhancing specificity and catalytic efficiency.

• Key Point: Substrate binding induces a conformational change in the enzyme.

• Example: Hexokinase undergoes a significant shape change upon glucose binding.

Enzyme Kinetics and Inhibition

Initial Velocity and Michaelis-Menten Kinetics

Enzyme kinetics studies the rates of enzyme-catalyzed reactions. The initial velocity (v0) is measured before significant substrate depletion or product accumulation.

• Michaelis-Menten Equation: Describes the relationship between reaction rate and substrate concentration for many enzymes.

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

• Vmax: Maximum reaction velocity at saturating substrate concentration.

• Km: Substrate concentration at which the reaction rate is half of Vmax; indicates enzyme affinity for substrate.

• kcat: Turnover number; number of substrate molecules converted to product per enzyme per second.

Lineweaver-Burk Plot

The Lineweaver-Burk plot is a double reciprocal plot used to determine kinetic parameters.

$\frac{1}{v} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$

• y-intercept: $\frac{1}{V_{max}}$

• x-intercept: $-\frac{1}{K_m}$

• Slope: $\frac{K_m}{V_{max}}$

Types of Enzyme Inhibition

• Competitive Inhibition: Inhibitor binds to the active site; increases Km, Vmax unchanged.

• Noncompetitive Inhibition: Inhibitor binds to an allosteric site; Vmax decreases, Km unchanged.

• Uncompetitive Inhibition: Inhibitor binds only to the enzyme-substrate complex; both Vmax and Km decrease.

Inhibition type can be determined by analyzing changes in Km and Vmax on Lineweaver-Burk plots.

Catalysis

Principles of Enzymatic Catalysis

• Acid/Base Catalysis: Enzyme side chains donate or accept protons (e.g., histidine, aspartate, glutamate, lysine).

• Covalent Catalysis: Enzyme forms a transient covalent bond with the substrate (e.g., serine, cysteine, lysine).

• Metal Ion Catalysis: Metal ions stabilize charges or participate in redox reactions (e.g., Zn2+, Mg2+).

Chymotrypsin Mechanism

Chymotrypsin is a serine protease that uses acid/base and covalent catalysis to cleave peptide bonds.

• Key Residues: Ser195, His57, Asp102 (catalytic triad).

• Mechanism: Involves formation of a covalent acyl-enzyme intermediate and stabilization of transition states.

Allosteric Enzymes and Regulation

• Allosteric Enzymes: Exhibit sigmoidal (S-shaped) kinetics; activity modulated by allosteric effectors.

• Allosteric Modulators: Can be activators or inhibitors; bind at sites other than the active site.

• Covalent Modification: Enzyme activity regulated by reversible covalent attachment of groups (e.g., phosphorylation).

• Proteolytic Cleavage: Activation of enzymes by irreversible cleavage of zymogens (inactive precursors).

Hemoglobin and Myoglobin

Structure and Function

• Myoglobin: Monomeric protein with 8 α-helices; stores O2 in muscle.

• Hemoglobin: Tetramer (2α, 2β subunits), each with a heme group; transports O2 in blood.

• Heme Group: Porphyrin ring with central Fe2+ ion; binds O2 reversibly.

• O2 Binding: Myoglobin shows hyperbolic binding curve; hemoglobin shows sigmoidal curve due to cooperativity.

Hemoglobin: T and R States, Effectors

• Deoxy-Hb (T state): Tense, low O2 affinity; stabilized by salt bridges.

• Oxy-Hb (R state): Relaxed, high O2 affinity; stabilized by O2 binding.

• Effect of pH (Bohr Effect): Lower pH decreases O2 affinity.

• CO2: Increases H+ concentration, stabilizing T state.

• CO: Binds heme with higher affinity than O2, blocking O2 transport.

• BPG (2,3-bisphosphoglycerate): Stabilizes T state, decreases O2 affinity; adaptation to high altitude.

Hemoglobin Variants

• Fetal Hemoglobin (HbF): Higher O2 affinity due to different amino acid composition; facilitates O2 transfer from mother to fetus.

• Mutations: Certain amino acid changes can alter O2 binding (e.g., sickle cell anemia).

Monosaccharides

Classification and Structure

• By Functional Group: Aldoses (aldehyde group) and ketoses (ketone group).

• By Carbon Number: Trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), etc.

Fischer and Haworth Projections

• Fischer Projection: Linear representation of sugars.

• Haworth Projection: Cyclic (ring) form; pyranose (6-membered) or furanose (5-membered) rings.

• Examples: Glucose (pyranose), fructose (furanose), galactose, mannose, ribose.

D- and L- Isomers

• D-sugar: Hydroxyl group on the right at the chiral center farthest from the carbonyl in Fischer projection.

• L-sugar: Hydroxyl group on the left at the same position.

• Conversion: Mirror image of all chiral centers.

Stereoisomers and Nomenclature

• Number of Stereoisomers: $2^n$ where n = number of chiral centers.

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

• Epimers: Differ at only one chiral center.

• Anomers: Differ at the anomeric carbon (α or β form in cyclic sugars).

Hemiacetal and Hemiketal Formation

• Hemiacetal: Formed when an aldehyde reacts with an alcohol (aldoses).

• Hemiketal: Formed when a ketone reacts with an alcohol (ketoses).

Reactions of Sugars: Oxidation and Reducing Sugars

• Reducing Sugar: Has a free anomeric carbon that can reduce mild oxidizing agents (e.g., glucose, maltose).

• Non-reducing Sugar: Anomeric carbon involved in glycosidic bond (e.g., sucrose).

Disaccharides

Structure and Composition

• Maltose: Glucose + glucose (α1→4 linkage).

• Lactose: Galactose + glucose (β1→4 linkage).

• Sucrose: Glucose + fructose (α1→β2 linkage).

Reducing vs. Non-reducing Disaccharides

• Reducing Disaccharides: Have a free anomeric carbon (e.g., maltose, lactose).

• Non-reducing Disaccharides: Both anomeric carbons involved in glycosidic bond (e.g., sucrose).

Determining Structure and Reducing Nature

• Analyze the glycosidic linkage and the presence of a free anomeric carbon to determine if a disaccharide is reducing or non-reducing.

Polysaccharides

Major Types and Features

Name	Monosaccharide Units	Linkage	Characteristics
Amylose	Glucose	α1→4	Unbranched, helical
Amylopectin	Glucose	α1→4, α1→6 (branches)	Branched (every 24-30 residues)
Glycogen	Glucose	α1→4, α1→6 (branches)	Highly branched (every 8-12 residues)
Cellulose	Glucose	β1→4	Unbranched, straight, forms fibers
Chitin	N-acetylglucosamine	β1→4	Structural, in exoskeletons

Glycosaminoglycans (GAGs)

• Definition: Long, unbranched polysaccharides containing repeating disaccharide units, often sulfated.

• Examples: Hyaluronan, chondroitin sulfate, heparin.

• Properties: Highly negatively charged, attract water, provide lubrication and structural support in connective tissues.