10.1 Storage Lipids · Biochemistry 2027

Lipids defined, and the structure of fatty acids

Lipids are a collective term for biomolecules with low water solubility and good solubility in organic solvents, not one structural family
A fatty acid is a carboxylic acid with a carboxyl group and a long hydrocarbon chain, R-COOH
The carboxyl group provides polarity and ionisation while the hydrocarbon chain is non-polar
Saturated fatty acids have no C=C and unsaturated ones have at least one
In 18:1(delta-9) there are 18 carbons and one double bond between C9 and C10, counting the carboxyl carbon as C1
The omega-9 or n-9 designation counts from the methyl end to the first double bond
Oleic acid is 18:1(delta-9, cis) and omega-9, and linoleic acid is 18:2(delta-9,12, cis,cis) and omega-6
Humans cannot synthesise linoleic acid (omega-6) or alpha-linolenic acid (omega-3), which are essential dietary fatty acids
Each additional C=C removes two hydrogens, so stearic acid is C18H36O2 and oleic acid is C18H34O2
cis and trans forms are geometric isomers with identical atom counts, formula and molar mass

Figure 10.1-1Overview of lipid classes and their biological roles

Melting point and chain packing

Melting point depends on chain length, the number, position and configuration of double bonds, and mixture composition
At equal carbon number a cis double bond disrupts packing and tends to lower the melting point
Pure oleic acid (18:1 cis) melts far lower than elaidic acid (18:1 trans)
Longer chains increase dispersion forces, pack more tightly and raise the melting point
Edible oils and animal fats are mixtures of many triacylglycerols, so pure fatty acid values cannot be assumed

Figure 10.1-2Study flow of unit 10.1 on storage lipids

Figure 10.1-3Bond structures of five representative fatty acids

Figure 10.1-4Counting direction for delta and omega notation

Figure 10.1-5Chain geometry of cis and trans double bonds

Figure 10.1-6Melting points of representative pure fatty acids

Triacylglycerols are fatty acid esters of glycerol

Glycerol is a trihydric alcohol with three hydroxyl groups
A triacylglycerol is a neutral lipid in which all three hydroxyls form ester bonds with fatty acid carboxyl groups
The teaching equation is glycerol plus three fatty acids in equilibrium with triacylglycerol plus three waters
Cells use activated precursors such as fatty acyl-CoA rather than simple mixing
Three palmitate residues give tripalmitin and three oleate residues give triolein
Lacking a phosphate-containing polar head, triacylglycerols form neutral lipid droplets in water rather than bilayers
A lipid droplet has a monolayer surface around a neutral lipid core
Lipases hydrolyse the ester bonds stepwise to diacylglycerol, monoacylglycerol and free fatty acids
Complete hydrolysis under strong alkali gives fatty acid salts, the saponification reaction
Triacylglycerol plus three NaOH gives glycerol plus three carboxylate salts in the ideal case
Example: tripalmitin molar mass is about 807.30 g/mol from 92.09 plus three times 256.42 minus three times 18.015

Figure 10.1-7Ester formation between glycerol and fatty acids

Figure 10.1-8Fatty acyl groups at the three positions of glycerol

Figure 10.1-9Enzymatic hydrolysis of a triacylglycerol and alkaline saponification

Figure 10.1-10Monolayer surface and neutral lipid core of a lipid droplet

Figure 10.1-11Functional difference between triacylglycerol and membrane phospholipid

Triacylglycerols store energy and provide insulation

Fatty acid chains are more reduced than carbohydrate and give more energy per gram on complete oxidation
Nutrition labelling uses about 9 kcal per gram for fat and about 4 for carbohydrate and protein
These are nutritional averages for metabolisable energy, not identical heats of combustion for every lipid molecule
Triacylglycerols carry almost no water, so the same energy is stored at lower mass
100 g of fat is about 900 kcal or 3,766 kJ, while 100 g of carbohydrate is about 400 kcal or 1,674 kJ
Adipose tissue reduces heat loss and cushions and protects organs
Insulation depends on fat thickness, blood flow and the state of skin and muscle
Adipose tissue secretes hormones and signals and is metabolically active, not a passive store
On mobilisation triacylglycerols are hydrolysed and fatty acids undergo beta oxidation to acetyl-CoA
Example: 200 g of fat at 9 kcal per gram gives 1,800 kcal or about 7,531 kJ, with use depending on digestion and metabolism

Figure 10.1-12Energy density of fat, carbohydrate and protein on a labelling basis

Figure 10.1-13Energy comparison at equal mass using labelling values

Figure 10.1-14Energy storage, insulation and cushioning roles of adipose tissue

Figure 10.1-15Outline from triacylglycerol mobilisation to fatty acid oxidation

Figure 10.1-16Hydrogen addition and cis-trans isomerisation possible during partial hydrogenation

Figure 10.1-17Comparison model of cis and trans fatty acids: shape and packing

Figure 10.1-18Teaching model of partial hydrogenation progress and relative indices; not real reaction data

Figure 10.1-19Chemical, physical and health summary of partially hardened oils

Partial hydrogenation can create trans fatty acids

Hydrogenation adds H2 across a C=C to give a single bond and lowers the degree of unsaturation
Under metal catalysis partial hydrogenation can be accompanied by cis to trans isomerisation
Partial hydrogenation can therefore increase saturation and create trans double bonds at the same time
Not every hydrogenation process produces trans fat
Partial hydrogenation has been used to adjust solidity and oxidative stability of edible oils
Evidence links high intake of industrially produced trans fat to cardiovascular disease
WHO promotes elimination of industrially produced trans fat from food supply chains
Ruminant foods also contain small amounts of naturally occurring trans fat
Heating cooking oil is not equivalent to partial hardening
Unsaturated fatty acids oxidise readily to peroxides and breakdown products that cause off-flavours
Oxidation depends on oxygen, temperature, light, antioxidants and fatty acid composition, so oxidative stability and nutritional safety are different concepts

Figure 10.1-20Typical ester linkage of a wax ester

Figure 10.1-21Plant surface wax layer and water droplet wettability

Figure 10.1-22Roles of beeswax, plant surface wax and copepod wax esters

Wax esters store energy and repel water

A wax ester is an ester of a long-chain fatty acid with a long-chain monohydric alcohol
Unlike a triacylglycerol with glycerol and three fatty acids, it has one fatty acid residue and one fatty alcohol residue
Long chains and a large non-polar portion make wax esters very poorly soluble in water
Beeswax is not a single substance but a mixture of wax esters, hydrocarbons and fatty acids
Waxes on leaves and fruit affect water loss, water entry and adhesion of contaminants
Beeswax is the building material of the comb
Some marine copepods use wax esters as their main storage lipid
So waxes serve neither energy storage alone nor waterproofing alone

Figure 10.1-23Calculation structure of the iodine value: a model proportional to C=C count

Quantitative measures of structure and function

The iodine value is the grams of iodine absorbed by 100 g of fat, an index of unsaturation
Assuming one C=C reacts with one I2, d double bonds correspond to d moles of I2
Theoretical iodine value is the number of C=C times the molar mass of I2 divided by lipid molar mass, times 100
Example: triolein at 885.43 g/mol with three C=C gives about 86.0 g I2 per 100 g
Real measurement uses defined reagents such as the Wijs method, and conjugated double bonds or side reactions cause deviations
The saponification value is the milligrams of KOH needed to saponify 1 g of lipid
Three ester bonds per triacylglycerol require three moles of KOH
The theoretical value is 3 times 56.11 times 1000 divided by the molar mass, in mg KOH per g
Example: tripalmitin at 807.30 g/mol gives 208.5 mg KOH per g
At equal ester count a larger molar mass means less KOH per gram

Figure 10.1-24Comparison activity for inferring structure from hypothetical lipid samples