Amino Acids, Isomerism, and Lipids: Biochemistry Study Notes

Amino Acids and Isomerism

Isomerism in Amino Acids

Amino acids can exist as stereoisomers due to the presence of a chiral (asymmetric) carbon atom. The two main types of isomers are L-isomers and D-isomers, which differ in the spatial arrangement of their substituents.

• Chiral Center: The alpha carbon in amino acids is typically chiral, except in glycine.

• L-isomer: In biological systems, amino acids are almost exclusively found in the L-form. In Fischer projections, the amino group is on the left for L-isomers.

• D-isomer: The amino group is on the right in Fischer projections for D-isomers. Rare in nature.

• Wedge and Dash Notation: Wedges indicate bonds coming out of the plane toward the viewer; dashes indicate bonds going behind the plane.

Example: The structure shown in the image is an L-isomer of an amino acid, as indicated by the position of the amino group in the Fischer projection.

Structure of Amino Acids

General Structure

Amino acids are the building blocks of proteins and have a central alpha carbon attached to four different groups:

• Amino group (–NH2)

• Carboxyl group (–COOH)

• Hydrogen atom

• R group (side chain) – varies among different amino acids

Negatively Charged (Acidic) Amino Acids

Some amino acids have side chains that are negatively charged at physiological pH, making them acidic. The two main acidic amino acids are aspartic acid and glutamic acid.

• Aspartic Acid (Asp, D): Has a carboxyl group in its side chain, which is deprotonated and negatively charged at neutral pH.

• Glutamic Acid (Glu, E): Similar to aspartic acid but with an extra methylene group in the side chain.

Example: The hand-drawn structure in the image is aspartic acid:

• Central alpha carbon

• Amino group (–NH2)

• Carboxyl group (–COOH)

• Side chain: –CH2COOH

Lipids and Their Functions

Types of Lipids

Lipids are a diverse group of biomolecules that are insoluble in water but soluble in nonpolar solvents. They play key roles in energy storage, membrane structure, and signaling.

• Triglycerides: Main form of stored energy in animals; composed of glycerol and three fatty acids.

• Phospholipids: Major component of cell membranes; contain a phosphate group.

• Cholesterol: Steroid important for membrane fluidity and as a precursor for steroid hormones.

Functions of Lipids in the Small Intestine

• Triglycerides: The most abundant lipid in the small intestine, serving as a major energy source.

Structure of Fatty Acids

Fatty acids are carboxylic acids with long hydrocarbon chains. The carboxyl group is at one end, and the hydrocarbon tail can be saturated or unsaturated.

• Carboxyl group: –COOH at one end of the molecule.

• Alpha carbon: The first carbon after the carboxyl group.

• Beta carbon: The second carbon after the carboxyl group.

• Omega carbon: The last carbon in the chain.

Lipid Antigens and Blood Types

Certain lipids are involved in determining blood types and immune responses.

• Glycolipids: Lipids with carbohydrate groups attached; important for cell recognition and blood group antigens.

• Phospholipids: Not directly involved in blood type antigens.

• Cholesterol: Not involved in blood type antigens.

• Fiber: Not a lipid; does not play a role in blood type antigens.

Summary Table: Key Features of Amino Acids and Lipids

Feature	Amino Acids	Lipids
Basic Structure	Alpha carbon, amino group, carboxyl group, R group	Glycerol backbone, fatty acid chains (triglycerides); phosphate group (phospholipids)
Isomerism	L- and D- forms (chirality)	Cis/trans (unsaturated fatty acids)
Function	Protein building, enzyme catalysis	Energy storage, membrane structure, signaling
Examples	Aspartic acid, glutamic acid	Triglycerides, phospholipids, cholesterol

Key Equations

• General formula for an amino acid:

$\text{H}_2\text{N}–\text{CHR}–\text{COOH}$

• General formula for a fatty acid:

$\text{CH}_3–(\text{CH}_2)_n–\text{COOH}$

Additional info:

• Wedge and dash notation is used in stereochemistry to indicate the three-dimensional orientation of bonds.

• Glycolipids are responsible for blood group antigens (e.g., ABO system).