4:Natural products

Lipids

Water-insoluble organic natural products from plant and animal cells

Main lipid classes

Polyketides (2C units) and Polyisoprenes (5C units)

Triglycerides

Glycerol esterified with 3 fatty acids

Hydrolysis of triglycerides

Produces glycerol and 3 fatty acids

Saturated fatty acids

No double bonds

Unsaturated fatty acids

Contain double bonds (usually cis)

Why saturated fats have high melting points

Tight packing and strong van der Waals forces

Why unsaturated fats are liquid

Cis double bonds disrupt packing

Animal fats

High saturation, solid at room temperature

Vegetable oils

More unsaturated, liquid at room temperature

Saponification

Hydrolysis of fats with alkali to form fatty acid salts (soap)

Micelle

Spherical structure with hydrophobic core and hydrophilic surface

How soaps clean

Trap grease in hydrophobic core of micelles

Hard water problem

Ca²⁺/Mg²⁺ form insoluble scum with soap

Waxes

Esters of long-chain fatty acids and alcohols

Function of waxes

Waterproofing and protection

Phospholipids

Lipids with phosphate-containing head group

Lecithin (phosphatidylcholine)

Common membrane phospholipid

Membrane structure

Lipid bilayer with hydrophobic tails inward

Membrane functions

Compartmentalisation, enzyme support, structural roles

Factors increasing membrane fluidity

Unsaturated fatty acids

Factors decreasing membrane fluidity

Cholesterol

Membrane dynamics

Lateral diffusion, rotation, flip-flop

Phase transition

Change from fluid to gel state in membranes

Prostaglandins

Hormone-like molecules derived from fatty acids

Precursor of prostaglandins

Arachidonic acid

Functions of prostaglandins

Inflammation, pain, blood pressure, clotting

Aspirin mechanism

Inhibits prostaglandin synthesis

Cyclised polyketides

Bioactive compounds used as drugs (e.g. antibiotics)

Source of polyketides

Bacteria, fungi, plants

Polyketide building block

Acetyl-CoA

Role of Acetyl-CoA

Reactive thiol ester enables chain growth

Acyl Carrier Protein (ACP)

Carries growing fatty acid chain

Unsaturated fatty acid formation (aerobic)

Desaturase enzymes

Unsaturated fatty acid formation (anaerobic)

Introduced during chain elongation

Isoprene unit

5-carbon building block (C₅H₈)

Isoprene rule

Terpenes built from multiples of C5 units

Monoterpenes

10 carbons

Sesquiterpenes

15 carbons

Diterpenes

20 carbons

Triterpenes

30 carbons

Uses of terpenes

Perfumes, flavourings, medicines

Steroids

Derived from triterpenes

Cholesterol function

Membrane component and hormone precursor

Examples of steroid hormones

Testosterone, estradiol, progesterone

Stages of polyisoprene biosynthesis

Mevalonate formation → IPP/DMAPP → coupling → cyclisation

IPP

Isopentenyl pyrophosphate (nucleophilic C5 unit)

DMAPP

Dimethylallyl pyrophosphate (electrophilic C5 unit)

Geranyl pyrophosphate

Precursor to monoterpenes

Farnesyl pyrophosphate

Precursor to sesquiterpenes and squalene

Squalene formation

Tail-to-tail coupling of two farnesyl units

Squalene role

Precursor to lanosterol and steroids

Lanosterol

Intermediate in cholesterol biosynthesis

Why unsaturated fats are healthier

More easily oxidised and metabolised

Risks of anabolic steroids

Liver damage, hormonal imbalance, tumours