membrane models and biosynthesis

1. Membranes

2. Membranes
• Complex lipids form the membranes around cells
and small structures within cells.
• In aqueous solution, complex lipids spontaneously
form into a lipid bilayer, with a back-to-back
arrangement of lipid monolayers.
– Polar (hydrophilic) head groups are in contact with the
aqueous environment.
– Nonpolar (hydrophobic) tails are buried within the
bilayer
– The arrangement of hydrocarbon tails in the interior
can be rigid (if rich in saturated fatty acids) or fluid (if
rich in unsaturated fatty acids).

3. • Reaction will take place
spontaneously if the
change in delta G is
negative or exergonic.
• In the case of the
membrane the
components must be
arranged in such a way
to minimize energetic
costs

4. • Extensive hydrogen bonding
in water molecules explains
the hydrophobic effect seen
in membrane
• The aggregation of water
around a single fatty acid
chain causes a decrease in
entropy
• Therefore layers of polar or
nonpolar motifs will cluster
together, not always in a
laminar fashion.

5. • Compartmentalization
• Create gradients
• Enzyme activity
• Signal transduction
• Selective passage
Purpose

6. Composition: Lipids
Phospholipids
Sphingolipids
Sterols

7. Composition: Proteins
• Integral/transmembra
ne
• Peripheral – can be
lipid anchored

8. Composition:
Carbohydrates
On external surface: glycoproteins & glycolipids

9. Figure 1.5 Early membrane models

10. Fluid Mosaic Model
-Singer & Nicolson (1972) describe integral proteins, lateral
diffusion of lipids and limited transverse diffusion
Figure 1.6 The freeze-fracture
technique reveals “bumps” in the
membrane interior.

11. Figure 1.7 The Fluid Mosaic Model proposed
by Singer and Nicolson
• Lipids form a fluid bilayer
• Bulk of the lipids form the
bilayer
• Lipids provide the solvent
for the proteins
• Most proteins are
embedded and globular
• It is a mosaic in that
proteins are scattered
across it or on its surface
• Both integral and
peripheral proteins exist
• Lipids and proteins are
amphipathic

12. Membrane Proteins are α- helical,
globular and membrane spanning
• Circular Dichroism: involves circular polarized light. It is
present in absorption bans of chiral molecules. When
circularly polarized light passes through an absorbing
optically active medium, the speeds between right and left
polarizations differ, as well as the wavelengths and the
extent at which they’re absorbed. Alpha helices and beta
sheets are optically active and have spectral signatures
unique the them.
• X-ray diffraction: is a tool used for identifying the atomic
and molecular structure of a crystal. An X-rays beam will
diffract once it hits the crystal and produce a 3-D picture,
representing the density of the electrons within the
crystals.

13. • Need 40-50 lipid
molecules form
single layer around
protein
• Lipids that
surround protein
are called the
annulus
• Protein-protein
interaction

14. Paradigm shift: Lipid Rafts
-Certain domains within the bilayer are not as 'fluid' and are
enriched with proteins (some are anchoring), cholesterol and
sphingolipids
-Signaling domains called “liquid ordered” microdomains

16. 3° dimension
• Asymmetry of
membrane
• Interacts with
substances at the
border
• Cytoskeleton (actin-
based)

17. 17
Lipid Biosynthesis
keystone concepts:
• Biosynthesis of fatty acids does not proceed as a simple reversal of fatty acid
oxidation
• These reactions are under tight control because the process is energetically
expensive
• Fatty acid synthesis and oxidation are coordinated and regulated together
• Synthesis of storage and membrane lipids from fatty acids is determined by the
metabolic needs of the organism
• Cholesterol is synthesized from acetyl CoA and has several fates
• Cholesterol and other lipids are transported through the blood as lipoproteins

18. Fatty Acid Biosynthesis
While degradation of fatty acids takes place in
mitochondria, the majority of fatty acid synthesis
takes place in the cytosol.
These two pathways have in common that they both
involve acetyl CoA.
– Acetyl CoA is the end product of each spiral of
b-oxidation.
– Fatty acids are synthesized two carbon atoms at a time
– The source of these two carbons is the acetyl group of
acetyl CoA.
The key to fatty acid synthesis is a multienzyme
complex called acyl carrier protein, ACP-SH.

19. 19
comparison to b-oxidation
• Different pathway
• Different enzymes
• Different parts of the cell
– b-oxidation is in the mitochondria
– Fatty acid synthesis is in the cytosol

21. Fatty Acid Biosynthesis
• Synthesis takes place in the cytosol
• Intermediates covalently linked to acyl carrier protein
• Activation of each acetyl CoA.
• acetyl CoA + CO2  Malonyl CoA
• Four-step repeating cycle, extension by 2-carbons /
cycle
– Condensation
– Reduction
– Dehydration
– reduction

22. 22
First Activation: Irreversible formation of malonyl-
CoA by acetyl-CoA carboxylase

23. Reaction catalyzed by acetyl-CoA
carboxylase

24. Malonyl CoA
• Malonyl CoA is synthesized by the action of
acetylCoA carboxylase.
• Biotin is a required cofactor.
• This is an irreversible reaction.
• Acetyl CoA carboxylation is a rate-limiting step
of FA biosynthesis.
• AcetylCoA carboxylase is under allosteric
regulation. Palmitate is a negative effector.

26. 26
Fatty Acid Synthase complex
• Multienzyme
Complex with 7
different active sites
• 4 repeated steps
include:
Condensation,
Reduction,
Dehydration, and
Reduction (NADPH
electron carrier)
• Saturated acyl group
produced is the
substrate for
additional rounds of
the pathway

27. Fatty Acid Biosynthesis
The biosynthesis of
fatty acids.
– ACP has a side
chain that carries
the growing fatty
acid
– ACP rotates
counterclockwise,
and its side chain
sweeps over the
multienzyme
system (empty
spheres).

28. Fatty Acid Synthase (FAS)
• FAS is a polypeptide chain with multiple domains, each
with distinct enzyme activities required for fatty acid
biosynthesis.
• ACP: Recall that CoA is used as an activator for β-oxidation.
For fatty acid biosynthesis, the activator is a protein called
the acyl carrier protein (ACP). It is part of the FAS complex.
The acyl groups get anchored to the CoA group of ACP by a
thioester linkage
• Condensing enzyme/β-ketoacyl synthase (KS). Also part of
FAS, has a cysteine SH that participates in thioester linkage
with the carboxylate group of the fatty acid.
• During FA biosynthesis, the growing FA chain alternates
between K-SH and ACP-SH

29. 29
saturated acyl group is the substrate for additional
rounds of the pathway
•Reducing agent is NADPH

30. Stepwise reaction
1. The acetyl group gets transferred from CoA to ACP
by malonyl/acetyl-CoA-ACP transferase.
2. The acetyl (acyl) group next gets transferred to the
β-ketoacyl-ACP synthase (KS) of FAS complex.
3. Next, the malonyl group gets transferred from CoA
to ACP by malonyl/acetyl CoA ACP transferase.
• This results in both arms of FAS occupied
4. The COO group of malonyl ACP is removed as CO2,
the acetyl group gets transferred to the alpha carbon of
malonyl ACP. This results in acetoacetyl-ACP

31. 31
Overview of
FAS

32. Repeat cycles for elongation
• The result of the first cycle of fatty acid biosynthesis is a four carbon
chain associated to the ACP arm.
• This chain gets transferred to the KS.
• A new malonyl CoA is introduced on the ACP arm.
• The reactions proceed as before. For each cycle the acyl group
transferred to the malonyl CoA is 2-carbons longer the previous
cycle.
• At the end of 7 cycles a 16 carbon chain is attached to the
ACP arm (palmitoyl ACP).
• The C16 unit is hydrolyzed from ACP yielding free palmitate
Net reaction: Acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+ 
Palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6H2O

33. Chem 674
De Novo to Palmitate:

34. Fatty Acid Biosynthesis
– Higher fatty acids, for example C18 (stearic acid), are
obtained by addition of one or more additional C2
fragments by a different enzyme system.
– Unsaturated fatty acids are synthesized from
saturated fatty acids by enzyme-catalyzed oxidation
at the appropriate point on the hydrocarbon chain.

35. 35
long chain saturated FA’s
are made from palmitate
• In the sER and mitochondria
• CoA is the acyl carrier
• Similar mechanism to FAS

36. 36
desaturation of FA’s requires a
mixed-function oxidase
• Mammalian liver cells desaturate fatty acids on sER
• Mammals can only make ω9 or higher fatty acids
• Plants can make ω6 and ω3 fatty acids in their sER and chloroplasts

37. Cholesterol
All carbon atoms of cholesterol and of all
steroids synthesized from it are derived from
the two-carbon acetyl group of acetyl CoA.
• Synthesis starts with reaction of three molecules
of acetyl CoA to form the six-carbon compound
3-hydroxy-3-methylglutaryl CoA (HMG-CoA).
• The enzyme HMG-CoA reductase then catalyzes
the reduction of the thioester group to a primary
alcohol.

38. 38
3 acetates condense
mevalonate
to isoprene
conversion
6 isoprenes polymerize
cyclization
Cholesterol biosynthesis

39. 39
Fates of cholesterol
• Synthesis in the liver
• Exported as: bile acids,
cholesteryl esters
• Needed for membrane
synthesis, hormone
precursors, Vitamin D
• Insoluble in water
• Cholesteryl esters (CE’s) are
transported in lipoprotein
particles or stored in the
liver.