Lipids

Lipids are a large and diverse group of naturally occurring organic

compounds that are related by their solubility in nonpolar organic

solvents (e.g. ether, chloroform, acetone & benzene) and general

insolubility in water. lipids are highly diverse in their structure and functions.

classification

Lipids can be classified according to their physical properties as

•Fats (solid at room temperature) or

•Oils (liquid at room temperature)

Fatty Acids

Long chain carboxylic acids which are usually unbranched they can be saturated or unsaturated.

Saturated fatty acid

Saturated fatty acids have no double bond in the carbon chains. Naturally occurring fatty acids usually have carbons of even numbers, but the ones with odd number of carbons also exist. Acids from C2 to C30 (or longer) are known, but the most common and most important fall in the range of C12 to C22.

Unsaturated Fatty Acid

Unsaturated fatty acids are fatty acids with one

or more double bonds. Usually, the double bonds are in cis or trans configuration, the double bonds could be single or multiple yielding polyunsaturated fatty acids interrupted by a methylene (-CH2-) group. Thus, generally, unsaturated fatty acids can be expressed by

•  Number of carbons,

•  Number of double bonds and

•  The position at which the first double bond from the omega position (= methyl group) appears. For example, an expression

• “18:2 omega-6 for linoleic acid” indicates “a fatty acid with 18 carbons, with 2 double bonds starting at the 6th carbon from the methyl end”.

Conjugated polyene fatty acids have conjugated double bonds as shown below. Although these fatty acids occur as “minor” components of some natural oils, but recently have attracted researchers’ attention because of their biological activities.

some commonly occurring fatty acids

Acylglycerols
Acylglycerols are fatty acid esters of glycerol. They can be mono, di, or tri acylglycerol depending on number of fatty acids esterified to glycerol.

In most eukaryotic cells, triacylglycerols form a separate phase of

microscopic, oily droplets in the aqueous cytosol, serving as

depots of metabolic fuel. In vertebrates, specialized cells called

adipocytes, or fat cells, store large amounts of triacylglycerols as

fat droplets. that nearly fill the cell

               Basic Structure of Triacylglycerols
Eicosanoids
These compounds, derived from eicosa- (20-carbon) polyenoic
fatty acids, comprise the prostanoids, leukotrienes (LTs), and
lipoxins (LXs). Prostanoids include prostaglandins (PGs),
prostacyclins(PGIs), and thromboxanes (TXs).

PHOSPHOLIPIDS
These are a class of lipids that are a major component of all cell
membranes as they can form lipid bilayers.
Most phospholipids contain a diglyceride, a phosphate group,
and a polar head group.

one exception to this rule is sphingomyelin, which is derived
from sphingosine instead of glycerol.
Phospholipids may be regarded as derivatives of phosphatidic
acid

 
Glycerophospholipid (phosphoglycerides) are common
constituents of cellular membranes. They have a glycerol
backbone. The hydroxyls at C1 & C2 of glycerol are esterified to
fatty acids.

the X group in the figure above can come from any of the below
polar head groups.



culled from Lehninger's Principle of Biochemistry 4th Edition

Ether linked lipids
Some phospholipids have one of the two acyl chain linked to
glycerol in an ether linkage rather than an ester linkage
The ether linkage could be saturated or unsaturated between C1
and C2 e.g. plasmalogens consider the two ether lipids below and
note the differences in ether linkage

Galactolipids and Sulpholipids

These are group of phospholipids abundant in the membranes of

plant cells. Contains one or two galatosyl residue linked to C-3 of

a 1, 2 diacyglycerol through a GLYCOSIDIC linkage

In sulpholipids a sulphonated glucose moeity serves as the

substituent on C-3 of a 1, 2 diacyl glycerol also through a

glycosidic linkage.

Sphingolipids

Sphingolipids are a second type of phospholipid found in cell

membranes, particularly nerve cells and brain tissues. They do

not contain glycerol, but retain the two alcohols with the middle

position occupied by an amine.

They are derived from sphingosine

the X- substituent on C1 could be any of the following substituents as shown below

Steroids

Steroids are oxidized derivatives of sterols; they have the sterol

nucleus but lack the alkyl chain attached to ring D derived from.

They serve a variety of functions physiologically- bile acids,

vitamin D, adrenocorticortical hormones, sex hormones.

Terpenoids

Terpenes include a large variety of compounds with isoprene (C5)

structure as the basic unit. Terpenes can be cyclic or acyclic, a

large range of structural variations. A few examples are given

below.

• Monoterpenes are based on two isoprene units and so have the molecular formula C₁₀H₁₆
• Other terpenes have multiples of C₅ units so C₁₅, C₂₀, C₂₅, C_{30  can exist}.