Información sobre las biomembranas y la señalización por el profesor Rebollo

2. Juan Alberto Rebollo Pérez
Docente de planta
jrebollo@utb.edu.co
Biólogo con énfasis en Biotecnología– U de Sucre
Doctoradoen Ciencias Biomédicas – U de Cartagena

3. CLASE 5: Biomembranas
Importancia biomédica. Dinámica de la membrana.
BIOQUÍMICA
Juan A. Rebollo Perez, Ph.D.
Facultad de Ciencias Básicas
2021

4. BIOMEDICAL
IMPORTANCE OF
MEMBRANES
define cell boundaries dynamic, highly fluid structures
lipid bilayer
associated proteins
closed compartments
selective permeabilities
acts as a barrier
differences inside and outside
molecular permeability
specific transporters
ion channels
exchanges material
exocytosis and endocytosis
gap junctions
cell–cell interactions
transmembrane signaling
specialized compartments shape organelles localize enzymes
energy transduction
excitation-response coupling
diseases
Normal cellular function critically depends on normal membranes.

5. MAINTENANCE OF A NORMAL INTRA- & EXTRACELLULARENVIRONMENT IS
FUNDAMENTAL TO LIFE
The Body’s Internal Water Is Compartmentalized
Intracellular Fluid (ICF) Extracellular Fluid (ECF)
Constitutes two-thirds of total body water and
provides a specialized environment for the cell to:
(1) make, store,and utilize energy;
(2) to repair itself;
(3) to replicate;and
(4) to perform cell-specific functions.
Contains about one-third of total body water and
is distributed between the plasma and interstitial
compartments.
The extracellularfluid is a delivery system.
It brings to the cells nutrients, oxygen, various ions
and trace minerals, and a variety of regulatory
molecules that coordinate the functions of widely
separatedcells.
Extracellular fluid removes CO2, waste products,
and toxic or detoxified materials from the
immediate cellular environment.

6. Membrane Dynamics

7. Acyl Groups in the Bilayer Interior Are Ordered to
Varying Degrees
In the transition from the Lo state to the Ld state, the general shape and dimensions of the bilayer
are maintained; what changes is the degree of motion (lateral and rotational) allowed to individual
lipid molecules.

8. Fatty Acid Composition of E. coli Cells Cultured at
Different Temperatures
Cells regulate their lipid composition to achieve a constant membrane
fluidity under various growth conditions

9. Transbilayer Movement of Lipids Requires Catalysis

10. Flippases and Floppases
Flippases: Catalyze translocation of the aminophospholipids from the extracellular to the cytosolic leaflet of the
plasma membrane.
Floppases: Move plasma membrane phospholipids and sterols from the cytosolic to the extracellular leaflet and,
like flippases, are ATPdependent
PE, PS
PE, PS
ATP ADP

11. Flippases (left) use ATP to move the aminophospholipids PS and, to a lesser extent, phosphatidylethanolamine (PE), from
the outer leaflet to the inner leaflet of the PM against a concentration gradient. Floppases (middle) use ATP to transport
substrates such as phosphatidylcholine (PC), sphingolipid (SL) and cholesterol against concentration gradients in the
opposite direction. Scramblases (right) are ATP independent and less substrate specific and facilitate the movement of lipids
along concentrationgradients.

12. Lipids and Proteins Diffuse Laterally in the Bilayer
fluorescence recovery after
photobleaching (FRAP)

13. Lipids and Proteins Diffuse Laterally in the Bilayer
Hop diffusion of individual lipid molecules
Restricted motion of the erythrocyte
chloridebicarbonate exchanger and glycophorin

14. Sphingolipids and Cholesterol Cluster Together in Membrane Rafts

16. MembraneCurvatureand Fusion Are Central to Many
Biological Processes

17. Membrane fusion during neurotransmitter release at a synapse.

18. The Glucose Transporter of Erythrocytes Mediates Passive Transport
Glucose enters the erythrocyte by passive transport via a specific glucose transporter called GLUT1, at a
rate about 50,000 times greater than it could cross the membrane unassisted.

19. Membrane topology of the glucose transporter GLUT1.

20. Problems:
1. Properties of Lipids and Lipid Bilayers. Lipid bilayers formed between two aqueous phases have this important
property: they form two-dimensional sheets, the edges of which close on each other and undergo self-sealing to form
vesicles (liposomes).
(a) What properties of lipids are responsible for this property of bilayers? Explain.
(b) What are the consequences of this propertyfor the structure of biological membranes?

21. Problems:
2. Flip-Flop Diffusion The inner leaflet (monolayer)of the human erythrocyte membrane consists predominantly of
phosphatidylethanolamine and phosphatidylserine. The outer leaflet consists predominantly of phosphatidylcholine
and sphingomyelin.Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this
sidedness is preserved at all times. How?

22. Problems:
3. Predicting Membrane Protein Topology from Sequence You have cloned the gene for a human erythrocyte
protein, which you suspect is a membrane protein. From the nucleotide sequence of the gene, you know the amino
acid sequence. From this sequence alone, how would you evaluate the possibility that the protein is an integral
protein?

23. Problems:
4. Use of the Helical Wheel Diagram Ahelical wheel is a two-dimensional representation of a helix, a view along its
central axis. Use the helical wheel diagram shown here to determine the distribution of amino acid residues in a
helical segment with the sequence
–Val–Asp–Arg–Val–Phe–Ser–Asn–Val–Cys–Thr–His–Leu–Lys–Thr–Leu–Gln–Asp–Lys–