GOB Chemistry Study Guide: Amino Acids, Enzymes, Carbohydrates, and Polysaccharides

Amino Acids and Stereochemistry

Physical Properties and Stereoisomers

Amino acids are organic compounds that contain both an amino group and a carboxyl group. Many amino acids are chiral, meaning they exist as enantiomers (mirror-image isomers). The physical properties of enantiomers are identical except for their interaction with plane-polarized light and reactions in chiral environments.

• Melting Point (m.p.): Both enantiomers have the same melting point.

• Optical Activity: Enantiomers rotate plane-polarized light in equal magnitude but opposite directions.

• pKa Values: Both enantiomers have identical pKa values for their functional groups.

• Example: L-malic acid and D-malic acid have the same melting point and pKa values, but rotate light in opposite directions.

Fischer Projections and Isomer Relationships

Fischer projections are a two-dimensional representation of three-dimensional molecules, commonly used for carbohydrates and amino acids. The relationship between two molecules in Fischer projection can be classified as:

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

• Anomers: Isomers differing at the anomeric carbon (in sugars).

• Example: D-glucose and L-glucose are enantiomers.

Amino Acids: Structure and Properties

Classification and Side Chains

Amino acids are classified based on the properties of their side chains:

• Aromatic Side Chains: Amino acids such as phenylalanine, tyrosine, and tryptophan contain aromatic rings.

• Hydrophobic Interactions: Amino acids like valine, leucine, isoleucine, and phenylalanine have nonpolar side chains that participate in hydrophobic interactions.

• Special Amino Acids: Proline is unique due to its cyclic structure.

Fischer Projections of Amino Acids

Fischer projections help determine the configuration (D or L) of amino acids. The D/L designation is based on the position of the amino group relative to the reference carbon.

• D-Amino Acid: Amino group on the right in Fischer projection.

• L-Amino Acid: Amino group on the left in Fischer projection.

Enzymes: Function and Catalysis

Enzyme Types and Mechanisms

Enzymes are biological catalysts that speed up chemical reactions. They are classified based on the type of reaction they catalyze:

• Ligase: Joins two molecules together.

• Isomerase: Converts a molecule into its isomer.

• Oxidoreductase: Catalyzes oxidation-reduction reactions.

• Transferase: Transfers functional groups between molecules.

• Hydrolase: Catalyzes hydrolysis reactions.

Enzyme Models and Catalysis

The induced-fit model suggests that enzyme shape changes upon substrate binding, enhancing specificity and catalysis.

• Lock-and-Key Model: Substrate fits exactly into the active site.

• Induced-Fit Model: Enzyme changes shape to accommodate substrate.

Enzymes increase reaction rates by lowering activation energy, not by increasing reactant concentration or temperature.

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors:

• Competitive Inhibition: Inhibitor competes with substrate for active site.

• Uncompetitive Inhibition: Inhibitor binds only to the enzyme-substrate complex.

• Irreversible Inhibition: Inhibitor permanently inactivates the enzyme.

Carbohydrates: Structure and Isomerism

Monosaccharides and Fischer Projections

Monosaccharides are simple sugars, classified by the number of carbons and the position of the carbonyl group:

• Aldose: Carbonyl group at the end (e.g., glucose).

• Ketose: Carbonyl group within the chain (e.g., fructose).

• D/L Designation: Determined by the configuration at the highest-numbered chiral carbon.

Reducing and Non-Reducing Sugars

A reducing sugar has a free anomeric carbon that can act as a reducing agent. Examples include glucose and maltose. Non-reducing sugars, such as sucrose, have no free anomeric carbon.

Glycosidic Bonds

Glycosidic bonds link monosaccharides in disaccharides and polysaccharides. The type of bond is determined by the carbons involved and their configuration:

• α-1,4 Glycosidic Bond: Connects the anomeric carbon of one sugar to the 4th carbon of another in alpha configuration.

• β-1,4 Glycosidic Bond: Similar linkage but in beta configuration.

• Example: Lactose contains a β-1,4 glycosidic bond.

Polysaccharides: Structure and Function

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. They serve structural and energy storage roles in living organisms.

• Cellulose: Structural polysaccharide in plants; β-1,4 linkages.

• Starch: Energy storage in plants; α-1,4 and α-1,6 linkages.

• Glycogen: Energy storage in animals; highly branched α-1,4 and α-1,6 linkages.

Table: Comparison of Polysaccharides

Polysaccharide	Linkage Type	Function	Source
Cellulose	β-1,4	Structural	Plants
Starch	α-1,4, α-1,6	Energy Storage	Plants
Glycogen	α-1,4, α-1,6	Energy Storage	Animals

Additional Topics

Protein Structure

Proteins have four levels of structure:

• Primary: Sequence of amino acids.

• Secondary: Local folding (α-helix, β-sheet).

• Tertiary: Overall 3D shape.

• Quaternary: Association of multiple polypeptide chains.

Enzyme Regulation and Zymogens

Enzyme activity is regulated by various mechanisms, including allosteric control, feedback inhibition, and zymogen activation. Zymogens are inactive enzyme precursors activated by specific cleavage.

Carbohydrate Reduction

Reduction of monosaccharides can produce sugar alcohols (e.g., sorbitol from glucose) or carboxylic acids at specific carbons.

• Example: Reduction of glucose at the C1 carbon yields sorbitol.

Key Equations

• General Enzyme Reaction Rate:

$\text{Rate} = k[\text{Enzyme}][\text{Substrate}]$

• Michaelis-Menten Equation:

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

• Fischer Projection D/L Assignment:

$\text{D/L is determined by the configuration at the highest-numbered chiral carbon}$

Additional info: Academic context and explanations have been expanded for clarity and completeness. Table entries and some examples have been inferred based on standard GOB Chemistry curriculum.