Lipids Flashcards
Functions of lipids
Fuel/energy storage
Biological membranes
Intracellular messengers, cofactors and vitamins
PTMs
Lipid classes?
8 main, e.g. sterols, FAs etc
Why is glucose converted to fats for storage? Lipids vs glycogen stores?
We have almost an almost unlimited capacity for fat storage, unlike glycogen which is finite
Lipids - 80% stored energy, supply for about 12 weeks
Glycogen - 2500kJ, about 12 hours
FAs vs triglycerides?
FAs amphipathic, forming micelles (not great)
Esterified to TAGs - highly reduced, hydrophobic (unlike glycogen)
Synthesis of FAs
Begins with acCoA – malonyl CoA via ACC in cytosol
AcCoA first transferred to cytosol with citrate shuttle
FAS (FA synthase) builds up malonyl CoA into FA chain until it is 16C long = palmitate
Citrate shuttle
Returns oxaloacetate to mitochondrial matrix by conversion to malate with NADH and H
Oxaloacetate + acCoA in matrix = citrate
Critrate exported out and cleaved in cytosol to leave oxaloacetate and acCoA
Uses 2ATP
Types of FAS?
FASI - vertebrates and fungi, single polypeptide chain with multiple active sites forming homodimer
Chain is built up within the complex, sheltering hydrophobic intermediates from cytosol
More efficient - concentrated intermediates in complex passed between sites
Coordinated regulation of only one enzyme
FASII - plants and bacteria, separate enzymes
Diversion of FA intermediates allowed if needed in other pathways
FA synthesis end product and consumption?
Uses 1 ATP for malonyl-CoA production, and 2NADPH = energy expensive
Palmitate main product, extended by elongase enzymes in ER or desaturated (double bonded) by desaturase enzymes in ER
Essential FAs?
Those mammals cannot produce e.g. linoleate, a-linoleate, needed from diet
FA synthesis regulation?
Max rate when metabolites and ATP are abundant
This is because when glycogen stores are saturated, excess glucose is converted to FA
ACC is main regulatory enzyme, controlled allosterically, by covalent mods, or by course control
Control of ACC
Hormones - global control
Insulin (stimulates) through protein phosphatase 2A activation for dephos of ACC
Glucagon and adrenaline inhibit via PKA activation = phosphorylation = inactive ACC. They also suppress protein phosphatase 2A
Local control
Low ATP/high AMP = AMPK = phosphorylation
Binding of citrate (allosteric) promotes polymerisation to active filaments, even if phosphorylated globally
Binding of palmitoyl-CoA does opposite (dissociation to inactive dimers) through feedback inhibition
Regulation points of phospholipids/TAG synthesis?
GPAT, PAP, DGAT
HMG-CoA reductase
Fate of FAs?
Phospholipids or TAGs depending on physiological needs e.g. fed state, growth requirements
Synthesis of phospholipids/TAG
Common pathway from G3P, derived from DHAP in glycolysis
FAs esterified to CoA to make acyl-CoA with acyl-CoA synthase
Acyl chains transferred to -OH glycerol by acyl transferases eg GPAT
Esterification of 2 acyl chians to G3P = intermediate phosphatic acid for further processing to either phospholipids/TAG
Difference between acyl and acetyl?
Acyl = chain tail region of FA Acetyl = 2c addition by malonyl-CoA earlier
TAG production
PAP (phosphatid acid phosphatase) removes phosphate from phosphatic acid - diacylglycerol
DGAT adds 3rd acyl chain = TAG
Occurs on cytosolic face of ER and mc
TAG storage
Stored in dynamic lipid droplets in all cells, budding off from ER
Coated with perilipins
Associated with lipase enzymes for TAG breakdowns, lipase regulatory proteins and components for TAG synthesis
(if from adipose tissue)
Alternative storage if liver-synthesised; as lipoproteins in circulation
Mammary glands - secreted into milk
Brown adipose tissue?
Hibernating animals/newborns
Generates heat through oxidation of FAs in lipid droplets by mc that express thermogenin so no ATP is made
Regulation of TAG synthesis
GPAT - first acyl transferase, fine control, inhibited by phosphorylation through PKA (glucagon/adrenaline): increasing FA oxidation or AMPK (low ATP:AMP): decreasing glycogen and TAG synthesis
PAP - branchpoint of TAG vs phospholipids
Compartmentation; movement to ER membrane = active, contacting PA stimulted by high FA in cytoplasm (feedforward to prevent FA toxic build up)
Role of cholesterol?
Fluidity of membrane
Steroid hormone precursor
Cardiovascular disease
Diet and synthesises in liver (not in plants)
Synthesis of cholesterol?
2 AcCoA + AcCoA = HMG-CoA on ER cytosolic leaflet
HMG-CoA reductase catalyses = mevalonate
Several steps = cholesterol
Fine control regulation of HMG-CoA reductase
Phosphorylation, short term
PKA, AMPK = inactive
Protein phosphatase = active
Responds to energy levels - low = inactive because process is energy expensive
Course control of HMG-CoA reductase
TF family SREBP
High cholesterol = SREBP on ER membrane, inactive
Low = cleavage of TF domain, migration to nucleus
Sensed by SACP and Insig, which bind sterols and oxysterol when their conc is high = binding to SREBP = retention at ER
Insig degraded when not binding sterols, SCAP and SREBP binding = golgi secretion where TF domain is cleaved
Role of TAGs
Store of metabolic fuel Membrane expansion Sterol provision Toxicity protection - sequestration of unesterified amphipathic FAs to prevent unwanted membrane interactions Signalling
Adipocytes?
Main storage of lipid droplets
Produce hormones e.g. lectin to regulate appetite/hunger in response to lipid droplet size
Why are TAGs good energy stores?
Carry more energy per carbon (more oxidation allowed), and non-polar so bind less water e.g. than glycogen
Good for long term - released more slowly
TAG breakdown
By lipases
TAG - DAG - MAG - glycerol (can enter glycolysis for some energy)
FAs released = beta oxidation for majority of energy
Lipases in TAG breakdown?
Hormone sensitive lipases (HSL); first two
Adipose triglyceride (ATGL) for first step
Monoacylglycerol (ATGL) for last step
Regulation of TAG breakdown
Hormone-triggered
Glucagon/ adrenaline
bind GPCR = adenylyl cyclase activated = PKA activation = first HSL and perilipin (recruits HSL) phosphorylated for activation.
Perilipin in breakdown?
Coats the lipid droplets - conformational change from phosphorylation allows HSL to enter, increasing lipolysis by over 50 fold (HSL alone = 2/3 fold)
ATGL in lipolysis? Regulation?
HSL knock-out mice show this is key factor, main element acting in first step
Found in adipose tissue: transcription inhibited by insulin, induced by fasting
Fine control - activated by regulatory protein CGI-58 and phosphorylation by AMPK
Coordinated regulation of lipolysis?
Glucagon - perilipin phosph, CGI dissociates to form CGI-58
Recruits ATGL to surface for step 1
HSL recruitment and phosph = step 2
MGL = final stage
Inhibition of lipolysis?
Insulin, fine control
Phosphatase enzymes reverse phos
Phosphodiesterase (PDE) blocks cAMP activation of PKA
Processing of dietary FAs?
Emulsification in small intestine by bile salts = micelles
TAGs partly digested in small intestine = MAGs
MAGS – intestinal mucosa epithelial cells – TAGs
Form chylomicrons with cholesterols, secreted to lymph – blood – tissues
Chylomicrons
Packaging of hydrophobic TAGs for transport
Core = lipids
Surface = phospholipids/apolipoprotins for targeting
Apolipoprotein C-II
ApoCII binds lipoprotein lipase on capillary surfaces in muscle/adipose
Lipoprotein lipase
Extracellular, hydrolyses TAG = FAs and glycerol
Activated by ApoCII binding
Secreted by target cells
Lipoprotein classes
Chylomicrons - least dense, most TAG. Dietary lipid from intestine
VLDL
TAG and cholesterol ester
Endogenous lipid from liver
LDL
Cholesterol and chol esters
(VLDLs minus TAG)
Uses ApoB-100
HDL
Protein
Conversion of cholsterol in LDL/VLDL to esters
Various apolipoproteins
Interconversion of lipoproteins?
High dietary FAs = chylomicron remnants into liver = VLDLs, broken down by lipase = free FAs
VLDL remnants = LDL, cholesterol enters liver etc
LPL (lipoprotein lipase) regulation
Course control as extracellular
Inversely regulated in response to TAG needs in adipose (storage) or muscle (energy)
Fasting – muscle isoform upregulated — breakdown of FAs by oxidation
and vice versa
Expression controls where lipoproteins are used
LDL metabolism and regulation
Different mechanism - bind a ApoB-100 to cell-surface receptor
Taken up in clathrin-coated pits, enter endosomes
Receptor back to cell surface
LDL digested in lysosomes
LDL receptor regulation transcriptionally with SREBP
FA transport?
Longer chains - shorter ones can diffuse
FAT/CD36 = receptor
FATP1-6 = transporter protein
FABPpm = binding protein
Regulation of CD36?
Stored in vesicles (compartmentation) - released to membrane by insulin/AMPK
Carnitine shuttle?
Transport of FAs into mitochondria, as inner mc membrane is impermeable to >12c FAs
acyl-CoA – acyl transferred to carnitine by acyltransferase 1 in outer membrane,
Transporter in inner membrane exchanged this for carnitine
acyltransferase 2 in matrix returns acyl group to CoA
FA oxidation in mc matrix
FA chain oxidised to give 2c acetyl-CoA, NADH and FADH2 (sequential, 4 reactions in each)
acCoA oxidised to CO2 in CAC, also giving NADH and FADH2
ATP generated from ADH and FADH2 in electron transport chain
Alternative FA oxidation?
Long or branched FA breakdown happens in peroxisomes
FA chains with odd carbon numbers have added 3 reactions at the end
Regulation of FA oxidation
Derivation of FA from TAGs
Uptake of FA - proteins like CD38, FATP1-6, FABPpm
Carnitine shuttle
Regulation of carnitine shuttle
Component 1, CAT1 - catalyses acyl group transfer to carnitine
Catalytic domain inhibited by malonyl-CoA (first intermediate in FA synthesis, to inversely regulate FA synthesis and oxidation)
Tied in to ACC, reglating FA synthesis - glucagon = ACC inactivation = less malonyl CoA = FA uptake and breakdown
