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Biomembrane Synthesis and Trafficking - Coggle Diagram
Biomembrane Synthesis and Trafficking
How are the different membrane components synthesized?
And how are they trafficked?
Fatty Acids :fuelpump: ⚓️
HC (acyl) chain attached to a carboxyl group (—COOH)
14, 16 (P), 18 (O), or 20 carbon atoms
but! FAs on sphingolipids can be longer than those in PGs
up to
26
carbon atoms, and may bear other chemical modifications (e.g., hydroxylation)
saturated or unsaturated chains
saturated
no double bonds
often linear
cis
double bond
creates rigid kink in HC chain
unsaturated
components of PLs and sphingolipids
2C building block
acetate
, CH3COO-
intermediates
not soluble in aq. solution
esterified to large water-soluble molecule
coenzyme A (CoA)
saturated FAs containing 14 or 16 C atoms are made from acetyl-CoA
by two enzymes
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can introduce double bonds at specific positions to prod. unsaturated fatty acids
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can elongate to 18–24 C atoms
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free/unesterified FAs (those unlinked to a CoA)
transported by fatty-acid-binding proteins (
FABPs
)
contain a hydrophobic pocket lined by beta sheets
FAs fit into this pocket and interact non-covalently w/ protein
FABP levels high in
active muscles
use fatty acids for generation of ATP
adipocytes (fat-storing cells)
taking up fatty acids to be stored as triglycerides
or releasing fatty acids for use by other cells
Sphingolipids
Building block: sphingosine
Sphingosine prod. starts on the cytosolic face of ER
coupling of a palmitoyl group from palmitoyl CoA to serine
subsequent addition of a second fatty acyl group to form N-acyl sphingosine (ceramide)
in the Golgi: polar head group is added to ceramide
head group is
phosphorylcholine
sphingomyelin
head group monosaccharide or a more complex oligosaccharide
various glycosphingolipids
Function of ceramides:
backbone for sphingolipids
important signaling molecules that can influence many cellular processes (growth, endocytosis, apoptosis...)
can also take place in
mitochondria, but only a fraction
Cholesterol
principal sterol in animal cells
synthesized mainly in the
liver
First steps of cholesterol synthesis (
cytosol
)
conversion of three acetyl groups linked to CoA (acetyl CoA) to form a 6C molecule
beta-hydroxy-beta-methylglutaryl linked to CoA (HMG-CoA)
Following steps on
ER
membrane
conversion of HMG-CoA into mevalonate
key rate-controlling step
statins target reductase
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catalyzed by HMG-CoA reductase :two_women_holding_hands:, an ER integral membrane protein (organized in dimers)
water-soluble
catalytic
domain of HMG-CoA reductase extends into cytosol
but its eight transmembrane alpha helices firmly embed enzyme in ER membrane.
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Mevalonate is converted into isopentenyl pyrophosphate (IPP) :five:
has basic five-carbon isoprenoid structure
IPP can be converted into cholesterol and many other lipids
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lipid compositions of different organelle membranes vary considerably
Differential transport of lipids
secretory pathway
vesicles bud from ER and fuse with membranes in Golgi
other vesicles bud from the Golgi and fuse with PM
inter-organelle movement through other mechanisms
direct contact
between membranes that is mediated
by membrane-embedded
proteins
e.g.
ER-mitochondrion encounter structure (ERMES) complex
ER–mitochondrion tether
in mammalian cells, PtdSer can be formed by exchanging L-Ser for ethanolamine from PE or for choline from PC (at ER)
PtdSer is transported through the mitochondrion- associated membrane (MAM)
converted to PtdEtn by decarboxylation in the mitochondrial intermembrane space
small, soluble lipid-transfer proteins
Different sites of synthesis :<3: :battery: