Lecture 2 Fall 2025 Study Notes

Lecture on Phosphate Derivatives and Glycerophospholipids

Overview of Phosphate Derivatives in Biochemistry

• Phosphate derivatives are integral in biochemistry.

Phosphoric Acid (H₃PO₄)

• Described as a triprotic acid due to its three dissociable protons.

• At neutral pH, phosphoric acid exists as an equilibrium mixture of:

  • Dihydrogen phosphate (H₂PO₄⁻)

  • Hydrogen phosphate (HPO₄²⁻)

• The notation commonly used for inorganic phosphate is Pi.

Role of Phosphorylation

• Phosphorylation is the process by which phosphate groups are added to molecules, which:

  • Introduces negative charges.

  • Increases the water solubility of molecules, such as:

  • Phospholipids

  • DNA and RNA

  • Various proteins

• The pKa value of phosphoric acid is approximately 7.2.

Reaction of Phosphoric Acid with Alcohols and Acids

• Phosphoric acid can react to form two main types of derivatives:

  • Phosphate Esters:

  • Formed when phosphoric acid reacts with alcohols.

  • General representation:
    $R + H<em>2O ightarrow R-PO</em>4 + H_2O$

  • Phosphoanhydrides (Mixed Anhydrides):

  • Formed when phosphoric acid reacts with acids.

  • General representation:
    $R-C + H<em>2O ightarrow R-PO</em>4 + H_2O$

Glycerophospholipids (Phosphoglycerides)

• Major constituents of biological membranes.

• Structure consists of:

  • Glycerol molecule:

  • Carbons 1 and 2 are esterified to two fatty acids (tail).

  • A polar or charged group (denoted as X) is attached through a phosphodiester linkage to the third carbon (head).

• Amphipathic Nature:

  • Glycerophospholipids are amphipathic molecules, combining hydrophilic (head) and hydrophobic (tail) properties.

  • This amphipathic property differentiates them from triacylglycerols (TAGs) and enables the formation of lipid bilayers crucial in cellular membranes.

Major Classes of Glycerophospholipids

• Different types of glycerophospholipids exist based on the head-group substituent (X-OH). The following are examples:

  1. Phosphatidic Acid

  2. Phosphatidylcholine (Lecithin)

  3. Phosphatidylethanolamine

  4. Phosphatidylserine

  5. Phosphatidylglycerol

• The table listing the glycerophospholipid classes includes:

  • Head-group substituent (X)

  • Formula of X

  • Net charge at pH 7: Commonly ranges from -2 to -1 (due to partial ionization of phosphates).

Phosphatidylcholine (Lecithin)

• Represents a class of lipids rather than a specific singular molecule.

• Contains different fatty acids attached at positions R1 and R2, leading to various phosphatidylcholine molecules.

Hydrolysis of Phosphatidylcholine

• Complete hydrolysis of phosphatidylcholine results in fractional yields of:

  • Glycerol

  • Fatty acids

  • Phosphate

  • Choline

• Required understanding of the molar ratios in the context of glycerophospholipid breakdown.

Comparison Between TAGs and Glycerophospholipids

• TAGs are hydrophobic while glycerophospholipids are amphipathic.

• Importance of hydrophobicity in TAGs for their biological functions.

• How amphipathicity in glycerophospholipids facilitates membrane formation.

Lipid Aggregation in Aqueous Environments

• Lipids can spontaneously aggregate in water:

  • Fatty acids often aggregate into spherical micelles, typically measuring from ~3 nm to several hundred nm in diameter.

  • Glycerophospholipids aggregate into bilayers due to the size of their hydrophobic tails, forming liposomes or vesicles of up to 1 micron or larger, identifying lipid bilayers as essential in cellular structure.

Common Functional Groups in Biomolecules

• Various functional groups enhance the diversity of biomolecules. Examples include:

  • Methyl: $R-C-H$

  • Ester: $R₁-C-O-R²$

  • Phosphoryl: $R-O-P-OH$

  • Disulfide: $R<em>1-S-S-R</em>2$

Lipid Analysis Techniques

• Lipids can be separated based on polarity using:

  • Silica gel columns for chromatography or thin-layer chromatography (TLC).

  • Progressively polar lipids will elute as the solvents change in polarity.

• Two-phase extraction is used for lipid purification and analysis.

Trans-esterification and Identification

• Trans-esterification helps in the identification of fatty acids through mass spectrometry, distinguishing them based on chain length and saturation degree.

Carbohydrates (Sugars)

• Carbohydrates represent the most abundant biomolecules on Earth, playing crucial roles in energy metabolism and as components of nucleic acids.

Classification of Sugars

• Sugars, also referred to as saccharides, are classified based on their structure:

  • Monosaccharides: Simple sugars (e.g., glucose).

  • Oligosaccharides: Short chains of monosaccharides (e.g., disaccharides like sucrose).

  • Polysaccharides: Large polymers of sugar units (e.g., glycogen, cellulose).

Structure of Monosaccharides

• Composed of a carbonyl group and multiple hydroxyl groups, leading to classifications as:

  • Aldoses: Polyhydroxy-aldehydes

  • Ketoses: Polyhydroxy-ketones

  • General formula: $(CH₂O)_{n}$

• The simplest monosaccharides have three carbons (trioses), and other groups include tetroses, pentoses, hexoses, and heptoses, with hexoses being the most common.

Representation of Sugar Structures

• Fischer Projection Formula: Used to illustrate sugar structures, where

  • Vertical bonds indicate projection behind the plane.

  • Horizontal bonds indicate projection out of the plane.

• Perspective Formula: Another method indicated by solid wedge-shaped bonds representing forward projection and dashed bonds indicating projection away from the viewer.

Chirality in Monosaccharides

• Monosaccharides typically contain chiral carbon atoms, leading to the existence of optically active isomers:

  • Dihydroxyacetone, D-glyceraldehyde, and their enantiomers, the D and L-glyceraldehydes.

Enantiomers

• Defined as mirror images differing at every chiral center.

• They have identical chemical properties but differ in optical activity, affecting how polarized light passes through.

Diastereomers

• Similar to enantiomers but differ in configurations at some chiral centers; thus, they have different chemical properties.

D and L Designations

• Sugars with more than three carbons are classified as D or L based on their configuration at the chiral carbon farthest from the carbonyl group. Most naturally occurring sugars are D-sugars.