10.3 Lipids as Signals, Cofactors and Pigments

Functional lipid classes and the basis of signalling

Phospholipids and sphingolipids are both structural components and signalling precursors, so structure versus signal is not a clean division
A small amount of PIP2 in the inner leaflet is converted into two second messengers on receptor stimulation
Cholesterol modulates membrane properties and is also the starting material for steroid hormones
A signal molecule acts according to where it is made, how far it travels and which receptor it meets
Intracellular, autocrine, paracrine and endocrine actions are distinguished
One molecule need not act in only one way
The opening question, how water-insoluble lipids act quickly inside cells or travel in blood, is answered by their amphipathic structure, carriers, and enzyme and receptor locations

Figure 10.3-1Overview of the signalling, redox, pigment and biosynthetic-carrier roles of bioactive lipids

Phosphoinositides and intracellular second messengers

PI is a membrane lipid with two acyl chains on glycerol and an inositol ring attached through phosphate
Selective phosphorylation of inositol hydroxyls produces several phosphoinositides
PI(4,5)P2 carries extra phosphates at positions 4 and 5, while PI(3,4,5)P3 adds one at position 3
On receptor activation PLC hydrolyses the phosphodiester bond of PI(4,5)P2 to give DAG and IP3
DAG stays in the membrane through its hydrophobic chains while polar IP3 enters the cytosol and binds ER IP3 receptors
Opening of the receptor channel releases stored Ca2+ from the ER
IP3 does not become Ca2+, it triggers release of stored Ca2+
DAG recruits and activates some PKC isoforms, and some also need Ca2+, so the two signals act together
The representative equation is PI(4,5)P2 plus water giving DAG plus IP3 through PLC
PI3K instead phosphorylates PI(4,5)P2 to PI(3,4,5)P3 and recruits PH-domain proteins, a different mechanism from PLC

Figure 10.3-2Conceptual comparison of the range of signal action; distances are not quantitative

Signalling by sphingosine derivatives

Sphingomyelinase cleaves sphingomyelin to produce ceramide
Ceramide links membrane structure to stress responses, cell growth and death pathways
Ceramide increase does not automatically mean apoptosis, since cell type, molecular species and compartment matter
Ceramidase removes the fatty acid to give sphingosine, and sphingosine kinase phosphorylates it to S1P
S1P regulates intracellular proteins and also acts extracellularly at S1PR1 to S1PR5 receptors
The pairing ceramide for death and S1P for survival simplifies research trends and is not a universal law
A single concentration cannot fix cell fate, and chain length, double bonds and compartment also matter

Figure 10.3-3PLC cleavage of PI(4,5)P2 and the two branches through IP3, Ca2+ and DAG-PKC

Figure 10.3-4Two fates of PI(4,5)P2: PLC hydrolysis and PI3K phosphorylation

Figure 10.3-5Simplified conversion from sphingomyelin through ceramide and sphingosine to S1P

Eicosanoids: lipid signals to nearby cells

Eicosanoids are bioactive lipids derived mainly from 20-carbon polyunsaturated fatty acids
The main precursor is arachidonic acid (20:4, n-6) esterified at the sn-2 position of membrane phospholipids
PLA2 releases arachidonate, after which several enzyme routes can act
Not all eicosanoids come from arachidonic acid
The COX route forms precursors of prostaglandins and thromboxanes
The LOX route forms leukotrienes, lipoxins and some oxygenated metabolites, while CYP enzymes form epoxides and related products
The same precursor gives different product profiles depending on which enzymes a cell expresses
Most are made rapidly on stimulation and inactivated quickly after receptor binding, so they act locally
This differs from steroid hormones that circulate and act at steady concentrations over long periods
Aspirin and ibuprofen both inhibit COX but differ in mechanism and duration, and COX inhibition does not remove every inflammatory signal

Figure 10.3-6Representative products of the COX, LOX and CYP routes from arachidonic acid

Steroid hormones carry information between tissues

Steroid hormones derive from the fused four-ring skeleton of sterols
The pathway starts as cholesterol forms pregnenolone and proceeds through many oxidation, reduction and hydroxylation enzymes
Tissue-specific reactions produce glucocorticoids, mineralocorticoids, androgens and estrogens
They are poorly soluble in water and travel largely bound to plasma proteins, while the free fraction and its equilibrium determine action
Many bind intracellular nuclear receptors that regulate transcription through DNA control regions
Some receptors move to the nucleus after hormone binding while others already reside there
Rapid non-genomic effects also exist
Cortisol, thyroid hormone from tyrosine and insulin as a peptide all act through blood, but differ in origin, synthesis and receptor mechanism

Figure 10.3-7Steroid hormone families derived from cholesterol, simplified at family level

Figure 10.3-8Main chemical classes of plant volatile signals and their ecological relevance

Volatile signals of vascular plants: chemical ecology

Plants cannot move away but release chemicals that interact with pollinators, herbivores, their enemies and neighbouring plants
Thousands of volatile organic compounds are known across the plant kingdom
Main classes are terpenoids from isoprene units, fatty-acid-derived green leaf volatiles and other aromatic or aliphatic compounds
Wounded leaves rapidly make and release C6 aldehydes and alcohols from linoleic and linolenic acid
Some volatiles attract pollinators or the enemies of herbivores and contribute to indirect defence
Certain compounds prime the defence responses of neighbouring plants
Not every released odour is read as a message, and signal versus cue is judged by evolutionary function and receiver response
Effects depend on compound, concentration, mixture, wind, distance and species, so a stronger odour does not prove stronger defence

Figure 10.3-9Retinoid conversions of vitamin A and the activation pathway of vitamin D3

Vitamins A and D: from lipid precursors to active regulators

Vitamin A is a nutritional concept covering retinol, retinal, retinoic acid and related retinoids
Retinol oxidises to retinal and further oxidation gives retinoic acid
Retinoic acid regulates transcription and cell differentiation through the RAR and RXR nuclear receptors
Retinoic acid should not be assumed to convert easily back to retinol
Retinal serves as the 11-cis chromophore of rhodopsin in rod cells, and photon absorption initiates isomerisation
Transcriptional control by retinoic acid and light sensing by retinal use different mechanisms within the same vitamin A family
UVB acting on 7-dehydrocholesterol gives vitamin D3 through a previtamin intermediate
Both D2 and D3 need activation, mainly 25-hydroxylation in liver and 1-alpha-hydroxylation mainly in kidney
Active 1,25-dihydroxyvitamin D, calcitriol, acts through VDR to control calcium and phosphate homeostasis and intestinal absorption
Vitamin D is a secosteroid with one ring opened, and although A and D are both fat-soluble vitamins their active forms have distinct targets

Figure 10.3-10How tocopherol breaks the lipid peroxidation radical chain

Figure 10.3-11Reduced and epoxide forms of vitamin K and the link to gamma-carboxylation

Figure 10.3-12Representative locations of mitochondrial ubiquinone and photosynthetic lipid quinones

Vitamins E and K, and lipid quinone redox roles

Polyunsaturated chains are exposed to lipid peroxidation by reactive oxygen species
A lipid radical reacting with oxygen forms a peroxyl radical and the chain continues
Alpha-tocopherol acts as a lipid-soluble chain-breaking antioxidant, ROO plus tocopherol-OH giving ROOH plus tocopheroxyl radical
The tocopheroxyl radical is regenerated by other reductants or undergoes further reaction
Antioxidants are not themselves immune to oxidation, and giving up hydrogen is the essence of the protection
Vitamin K is a quinone derivative required for converting glutamate residues to gamma-carboxyglutamate
Gamma-glutamyl carboxylase uses reduced vitamin K, which is oxidised to the epoxide and regenerated by the VKOR cycle
Ubiquinone accepts electrons from complexes I and II, becomes ubiquinol and passes them on, following Q plus 2e- plus 2H+ in equilibrium with QH2
Its long hydrophobic isoprenoid chain allows movement within the membrane
Vitamin E, vitamin K and CoQ all take part in redox chemistry yet serve distinct roles in radical control, carboxylation and electron transport

Figure 10.3-13Dolichol phosphate in ER assembly, membrane flipping and transfer of an N-glycan precursor

Dolichol phosphate: a lipid carrier in protein glycosylation

Dolichol is a long polyisoprenoid alcohol built from many isoprene units
Dolichol phosphate sits in the ER membrane and temporarily binds and moves sugar precursors or oligosaccharides
It is central to N-linked glycosylation
Assembly begins on the cytosolic face using Dol-P, building a Dol-PP-linked oligosaccharide precursor
At a defined stage the precursor flips across the membrane and matures on the lumenal side
Oligosaccharyltransferase transfers the assembled glycan to a specific asparagine of a nascent protein
Monosaccharide carriers such as Dol-P-Man and Dol-P-Glc also participate
Saying dolichol activates sugars means the lipid-linked precursor enables membrane reactions and transfer, not that dolichol oxidises the sugar
Dolichol is a recycled carrier rather than a consumable, and membrane proteins and enzyme systems are needed for the flip

Figure 10.3-14Isolated double bonds compared with a conjugated pi system

Figure 10.3-15Polyketide synthase repeatedly linking acyl precursors and processing them into natural products

Lipid-soluble pigments and polyketides

In a conjugated pi system electrons are delocalised and longer conjugation tends to shift absorption to longer wavelengths
From E = h nu = hc over lambda, longer wavelength means lower photon energy
Carotenoids have a C40 terpenoid skeleton with a conjugated polyene and split into carotenes without oxygen and xanthophylls with oxygen
Carotenoids contribute to light harvesting and protection against excess light in photosynthetic complexes
Chlorophyll is a magnesium-centred tetrapyrrole pigment and is not a carotenoid
Anthocyanins are water-soluble flavonoids and differ in structure and solubility from lipid-soluble polyenes
Absorbance is log base ten of incident over transmitted intensity, and the Beer-Lambert relation holds under defined conditions
Mixture absorbance reflects several components, scattering and instrument conditions, so colour alone cannot fix identity or concentration
Polyketides are natural products assembled by polyketide synthases from small acyl precursors with varied ketoreduction, dehydration, cyclisation and glycosylation
Polyketide synthases are classified as multidomain type I, dissociated type II and chalcone-synthase-like type III, with erythromycin, tetracycline, doxorubicin and lovastatin as examples
Fatty acid synthesis tends toward a reduced hydrocarbon chain, while polyketide synthases combine selective processing of beta-keto groups to give very different functions

Figure 10.3-16Simulated COX and LOX analysis data; all values and trends are illustrative