21: lipid synthesis Flashcards
Do you know how to name fatty acids? and recognize structure?
count from the carboxyl carbon, unsaturations denoted with ∆. for common naming, saturated ate -ate and unsaturated are -oleic acid, eg palmitate and palmitoleic acid remember most fatty acids are even numbers.
Eicosanoids structure and function
20C polyunsaturated FA with derivatization
signaling, such as PGEs and thromboxanes
triacylglycerols structure and function
glycerol molecule with three fatty acids
energy storage
glycerophospholipids structure and function
glycerol molecule with two FA and a phosphate linked head group, can have a few different head group that define identity
membrane components
cholesterol structure and function
27C, multi cyclic, derived from isoprene units (from mevalonate)
membrane component and precursor for steroid hormones
Describe the steps of biosynthesis of FAs
occurs in cytoplasm
- synthesize malonyl-CoA by AcetylCoA carboxylase ACC
- long C chains of FA assembled in 4 step sequence by FA synthase
- acetate precursor shuttled into cytoplasm as citrate
- desaturation requires mixed-function oxidase activity
mechanism of ACC acetyl-coA carboxylase
21 Pt 1 slide 5
uses biotin carboxylase and ATP to carry CO2 (from HCO3-). then transcarboxylase combines acetyl-coA and CO2 to make malonyl-coA
1st, committed step in FA synthesis (highly regulated)
mechanism of FAS fatty acid synthase
21 pt1 slide 6
multi enzyme complex has long and short arm. Malonyl and acetyl precursors are activated by a thioester. decarboxylation condensation occurs and the acetyl group is added to the malonyl. reduction occurs using NADPH and then dehydration and then another reduction from NADPH. Then another malonyl is added and the cycle repeats.
Condensation, Reduction, Dehydration, Reduction
FAS involves getting rid of CO2, is this wasteful since we just worked so hard to incorporate CO2??
Nah. condensation of an acetyl group to and acyl chain is an endergonic process, so the CO2 is needed as a good leaving group to make the process favorable. CO2 loss during condensation results in an exergonic process
what is acyl carrier protein and how does it work?
ACP has a 4’ phosphopantethiene group which acts as a flexible arm. it has a thiol group which esterifies to malonyl groups and can transfer the growing product to the next enzyme active site.
how many carbons is palmitate?
16
what is required to make one molecule of palmitate? stoichiometry practice
ACC (7 cycles): needs 7 acetyl-coA + 7 CO2 + 7 ATP to produce 7 malonyl CoA
FAS (7 cycles): needs 1 acetyl-coA + 7 malonyl-coA + 14 NADPH + 14 H+ to produce one palmitate
NET: 8 acetyl-coA + 7 ATP + 14 NADPH + 14 H+ –> palmitate + 8 CoA + 7ADP + 7 Pi + 14 NADp + 6H2O
what are some homologues to FAS in other organisms
bacteria, plants: seven activities in seven separate polypeptides
yeast: seven activities in two separate peptides
vertebrates: seven activities in one large polypeptide
where in the cell does FA synthesis take place? plant and animal
animal cell: FA synthesis in cytosol, elongation and desaturation and modifications in ER and mitochondria (elongation)
plant cell: FA synthesis in chloroplast, elongation and desaturation and modifications in ER and mitochondria (elongation)
where does the acetyl-coA for FA synthesis come from?
nearly all from mitochondrial oxidation of pyruvate and amino acid catabolism. mitochondrial membrane is impermeable to acetyl-coA, so it must be made into citrate and go through citrate transporter to get out
how is FA synthesis regulated?
ACC is rate limiting step. palmitate (the ultimate product) inhibits ACC and citrate is and allosteric activator. ACC can also by phosphorylated. glucagon and epinephrine inhibit ACC. It is important to keep tight control bc it is so energetically costly
what happens if a FA longer than 16 C is needed?
fatty elongation system is in the smooth ER and mitochondria.
what happens if a FA needs a double bond?
needs mixed function oxidases, meaning 2 substrates are oxidized. desaturases have substrate specificity so that double bonds cannot be formed anywhere, some FAs have to be ingested bc the desaturases don’t make them. Desaturations require O2 and NADPH
21 pt 1 slide 17
how are Eicosanoids synthesized?
a phospholipid containing arachidonate is modified by cyclooxygenase (COX). the products are prostaglandins and thromboxanes, which are responsible for controlling fever, pain, blood clotting, and other responses.
why is COX a drug target? what are some ways it is targeted?
COX is responsible for producing prostaglandins and thromboxanes which are pain, fever, clotting, etc signals. less thromboxanes means less platelets aggregation and clotting. this prevent heart attacks. Aspirin has an acetyl group that acetylates a Ser residue on COX, inactivating it.
COX 1 vs COX 2
COX 1: synthesis of PGEs that regulate secretion of gastric mucin
COX 2: synthesis of PGEs that mediate inflammation, pain, and fever
inhibitors of COX 2 will avoid bothering your stomach!
what are leukotrienes and how are they synthesized?
inflammatory molecules responsible for causing contractions in smooth muscles linking bronchioles. basically a special type of Eicosanoids. They originate from arachidonate and require mixed function oxidases to make
compare TAGs and glycerophospholipids
Triacylglycerols have three FA chains, and function to store energy. glycerophospholipids have two FA chains and one phosphate attachment and function in membranes. Both are made from the same precursor, phosphatidic acid (branch point)
how is phosphatidic acid (and then glycerolphospholipids and triacylglycerols) synthesized?
21 pt 2 slide 3
DHAP or glycerol goes through acyl transferase twice. Uses ATP and CoA-SH and acyl group
Phosphatidic acid can then make glycerophospholipids by CTP reaction or it can make TAGs by being hydrolyzed and acylated
how is TAG synthesis regulated?
by hormones and diet. lack of insulin results in lower FA and TAG production. low carb diets create lots of ketone bodies (CAC intermediates are limiting) and less TAGs
what is the TAG cycle? what’s the point?
an apparently futile cycle that breakdown and resynthesizes TAG molecules between adipose tissue, blood, and liver. phospholipases cleave the ester bond (different lipase for each position). This cycle could exist for a faster source of energy in an emergency as it would skip the lipase step of TAG breakdown
what is glyceroneogenesis?
synthesis of glycerol 3-phosphate via a shortened version of gluconeogenesis. PEP carboxykinase is important for limiting the rate and regulating flux in adipose/blood/liver
full pathway 21 pt 2 slide 6
2 strategies for glycerophospholipid synthesis
both start with backbone synthesis (glycerol) and attachment of FA via ester or amide linkages (creating diacylglycerols). next is addition of head group, first strategy to activate the diacylglycerol with CDP and second strategy is too activate the head group with CDP. both result in the same glycerophospholipid. bacteria only use 1, eukaryotes use both.
21 pt 2 slide 7
function of plasmalogen, cardiolipin, phsophatidyl-X, sphingolipids, and phosphatidyl inositol
plasmalogen: ether analog of phosphatidylethanolamine, common in heart tissue but function not well understood. could increase membrane rigidity, signal, or be antioxidant
cardiolipin: inner mitochondrial membrane
phosphatidyl-X: membranes, X can be a few different head groups (PS, PE, PC)
sphingolipids: regulation of cell growth, differentiation, and function. has sphingosine backbone
phosphatidyl inositol: special phosphatidyl that can be cleaved to form IP3, and important signaling molecule
how is cholesterol synthesized
starts with acetate made into mevalonate. mevalonate converts to activated isoprene units, polymerization of isoprene units forms squalene. cyclization of squalene forms the steroid nucleus followed by additional modifications.
21 pt2 slide 9-10
first step of cholesterol biosynthesis mechanism
formation of mevalonate
reactants: 3 acetyl-coA, 2 NADPH
products: mevalonate, 3 CoA-SH, 2 NADP+
enzyme: thiolase, HMG-coA synthase, HMG-coA reductase
Committed and rate limiting step is HMG-coA reductase. important intermediate HMG-CoA
21 pt 2 slide 11
seconds step of cholesterol biosynthesis mechanism
formation of isoprene units
reactants: mevalonate, 3 ATP
products: activated isoprenes, CO2, Pi
lots of enzymes, if it has mevalonate in the name its part of this step.
the 3-phospho-5-pyrophosphomevalonate intermediate has a CO2 and Pi group that set up a good way to couple when they leave. Plants and animals use this path, but chloroplasts and bacteria use mevalonate-independent version
third step of cholesterol biosynthesis mechanism
formation of linear squalene reactants: isoprene units, NADPH products: squalene, NADP+, PPi know the geranyl pyrophosphate and farnesyl pyrophosphate intermediates. 21 pt2 slide 13
fourth step of cholesterol biosynthesis mechanism
conversion of squalene to cholesterol
reactants: squalene, NADPH, O2
products: cholesterol, NADP+, H2O
requires mixed-function oxidase activity (cyclase in animals)
goes through squalene 2,3 epoxide intermediate
21 pt2 slide 14
what are the fates of cholesterol?
cholesterol is synthesized in the liver then can be used as precursor for steroid hormones or hydroxysterols or it can be exported. It is exported in the form of bile acids (less hydrophobic, can be in blood) or cholesteryl ester (more hydrophobic, packs into plasma lipoproteins in blood)
21 pt2 slide 15
what are plasma lipoproteins and different types?
combinations of phospholipids, cholesterols, cholesterol esters, and triacylglycerols within a monolayer, they have apolipproteins that cells recognize to receive contents. This is how these things are transported throughout the body.
least dense type is chylomicron (high TAG), VLDL is next, LDL is Low Density (high cholesterol), and HDL is High Density (little cholesterol)
what determines they type of lipoprotein?
it depends on what lipids they pick up and drop off, as they lose and gain contents they will convert between the types. like a VLDL particle can become an LDL particle if it picks up a lot of cholesterol
how are dietary lipids transported?
95% of dietary lipids are TAGs. In the stomach, TAGs are hydrolyzed to monoacylglycerides (MGs) and free FAs by lipoprotein lipases. in the intestinal tract, MGs and free FAs form TAGs which are packaged into chylomicrons which enter circulation
how are de novo lipids transported?
liver and adipose tissue make TAGs. liprproteins circulate until TAGs are taken up.
VLDL exports excess TAGs from chylomicron remnants or new TAGs from liver. increases free FA in circulation, increases heart diseases
LDL is part of specialized transport route requiring LDL receptor. shoes correlation to heart disease
HDL is initially cholesterol free, but picks them up as it roams. uses enzyme LCAT to make cholesterol esters to package and deliver back to liver.
21 pt2 slide 19
How are LDL particles transported into cells
specialized receptors (LDL receptors) are in the cell membrane and recognize the apolipoprotein in LDL (ApoB-100). They bind upon contact and induce endocytosis. once inside an endosome, the receptor can be recycled back to the surface. 21 pt2 slide 20
how is cholesterol synthesis regulated in nature?
regulated at HMG-CoA reductase at transcription level and posttransationally by hormones.
transcriptional: when cholesterol is high, it binds to SCAP/INSIG, SCAP binds SREBP in the ER. when cholesterol is low, does not bind SCAP/INSIG, SCAP chaperone SREBP to Golgi where it’s cleaved. piece of SREBP goes to nucleus, activates transcription of HMG-CoA reductase.
21 pt2 slide 22
how is cholesterol synthesis regulated by medicine?
medicine is designed as inhibitors of HMG-CoA reductase with a group that mimics mevalonate, binds, and fills the enzyme pocket.
how are steroid hormones synthesized from cholesterol?
humans derive ALL steroid hormones from cholesterol. this requires activity of a mixed function oxidase. I don’t think I need to know the intermediates and such, but its 21 pt 2 slide 24
