Protein structure. Primary structure Amino acids: Polar (charged and uncharged) Non polar Unique side chains (Gly, Cys, Pro)

Transcription

1 Membrane Proteins

2 Membrane Proteins

3 Protein structure Primary structure Amino acids: Polar (charged and uncharged) Non polar Unique side chains (Gly, Cys, Pro) Secondary structure Tertiary structure Quaternary

4 * * * Polar amino acids Serine Threonine Glutamine Tyrosine Asparagine Non-polar amino acids Alanine Valine Leucine Isoleucine Methionine Phenylalanine Tryptophan Side chains have partial charge participate in reactions, associate with water Side chains have H and C atoms; hydrophobic Associate with lipid layer Amino acids with unique side chains Glycine (H side chain hydrophobic or hydrophilic Cysteine (contains SH moiety can form S-S bridge) Proline (hydrophobic side chain; can create kinks and disrupt 2 structure

5 Secondary structure: α-helix Opposite sides of helix may have contrasting properties -hydrophobic -hydrophilic

6 Secondary structure: β-sheet

7 Bonds which maintain protein structure

8 Tertiary structure Stabilized by non covalent bonds between diverse chains of protein

9 Some tertiary structures

10 Haemoglobin has a quaternary structure: 2 α-subunits and 2 β-subunits Quaternary structure enhances O 2 association

11

12

13 Proteins in membrane 1. Receptors: specificity, subtypes 2. Ion channels: specificity, selectivity importance of pore size, and charge 3. Ion pumps 4. Enzymes: mainly on intracellular face 5. GTP-binding proteins 6. Carrier molecules: very specific 7. Cell adhesion molecules (Glycoproteins)

14 MEMBRANE PROTEINS Communication Receptors Transport: Ion channels Pumps Carrier proteins Release: Synaptic plasma membrane proteins

15 Integral membrane protein Single/several hydrophobic domain e.g. glycophorin/channels+pumps Peripheral: anchored by glycolipid e.g. Receptor Anchored to lipid bilayer eg G-prot

16 Membranes as Barriers Hydrophobic interior of bilayer is a barrier to transport (size & charge) The membrane is impermeable to ions and large charged molecules and requires the aforementioned special membrane proteins to transport across

17 Membrane proteins - NOTES Transmembrane proteins Protein has hydrophilic and hydrophobic portions Hydrophilic will interact with the aqueous solutions on either surface Hydrophobic will be in contact with the hydrophobic interior of the bilayer Also called integral membrane proteins Peripheral membrane proteins Are attached to either surface of the bilayer Those attached to lipids are covalently linked Those that interact with other transmembrane proteins are attached by noncovalent interactions, such as: H-bonds, hydrophobic and hydrophilic interactions Membrane-spanning proteins Must have hydrophobic side chains in the area that spans the membrane Peptide backbone is polar Not happy in the hydrophobic interior

18 Membrane Pores When proteins span the membrane several times they usually form pores that allows molecules to move back and forth through the membrane Multiple α helices span membranes Hydrophilic on the inside of the channel Hydrophobic on the outer surface of the channel

19 α-helix Span Interior (Pore) Interior forces the peptide backbone to form α helix Non-polar R groups are on the outside of the helix Transmembrane proteins usually span the membrane once Receptors collect signal, pass on to the inside of cell

20 β Barrel Pore β barrels are made of β sheets that are curved into a cylinder The hydrophilic line the inner side and hydrophobic the outer surface Larger pore than α helix pore

21 Membrane Transport of Small Molecules Membranes present a barrier to the movement of most materials Transport proteins allow movement Transport proteins comprise 15-30% of membrane proteins. Up to 2/3 of a cell s metabolic energy can be used for transport

22 Tight Junctions restrict transport Proteins can diffuse within their own domains, but are prevented from entering the other domain by tight junctions (specialized cell junction)

23 Transport 1. Simple Diffusion 2. Carrier-mediated (a) Facilitated diffusion High------Low (b) Active transport (ATP) Low High Uniport, symport antiport Primary, secondary

24 Transport May be Passive or Active Passive transport may or may not require protein transporters; active transport always requires transporter proteins.

25 The Electrochemical Gradient is the Determining Force for Ion Transport The electrochemical gradient is the combination of concentration and charge differences across the membrane.

26 Carrier Proteins Bind Solutes Tightly and Undergo Large Conformational Changes The tightness and specificity of carrier protein-solute binding is much like enzyme-substrate binding.

27 Protein phosphorylation changes conformation ATP + ADP

28 Transport Via Carrier Proteins is Saturable Simple Diffusion is Not

29 Active Transport Always Occurs Using Carrier Proteins (not channels)

30 Coupled Carriers Function as Symports or Antiports Coupled carriers always function in active transport; uniports function in facilitated diffusion.

31 A Glucose Antiport Driven by the Na + Electrochemical Gradient The Na + electrochemical gradient is harnessed to drive many transport processes.

32

33 The Na + -K + Pump is Abundant and Ubiquitous The Na + -K + pump maintains low ic [Na +] and high ic [K +] Ionic imbalance important for intracellular ph control, osmotic control, excitability, and transport

34 The Na + -K + Pump is a P-Type Transport ATPase This class of pumps autophosphorylates following ATP hydrolysis. The phosphorylation is reversible and changes the conformation of the pump, alternately exposing ion binding sites on the extracelluar and cytosolic faces of the membrane.

35 About 1/3 of the Cell s Metabolic Energy Goes to Powering the Na + -K + Pump

36 The Na + -K + Pump The Na + -K + Pump is abundant and ubiquitous The Na + -K + pump maintains low ic [Na + ] and high ic [K + ] It is actually an enzyme ATPase (P-type transport ATPase) The phosphorylation changes the conformation of the pump. Ionic imbalance important for intracellular ph control osmotic control excitability transport About 1/3 of the Cell s Metabolic Energy Goes to Powering the Na + -K + Pump

37 Vesicular Transport Transport of large particles and macromolecules across plasma membranes Exocytosis moves substance from the cell interior to the extracellular space Endocytosis enables large particles and macromolecules to enter the cell Phagocytosis pseudopods engulf solids and bring them into the cell s interior

38 Vesicular Transport

39

40 Endocytosis of cholesterol molecule (large)

41 Clathrin-Mediated Endocytosis Figure 3.13

42 Exocytosis Figure 3.12a

43 Ion Channels

44 Ion Channels are the Second Major Class of Transport Proteins Ion channels differ from carriers in always working in passive transport, their exquisite selectivity, and their higher rate of transport. A single ion channel may transport up to 100 million ions per second, a rate 100,000 times higher than the fastest carrier. Ion channels are on all cells, but reach their highest level of sophistication on electrically excitable cells like neurons.

45 Channel Opening and Closing is Regulated in Three Broad Ways

46

47 Receptors

48

49 SIGNALLING THROUGH 1. Ionotropic receptors 2. Metabotropic receptors/g-protein-linked 3. Tyrosine kinase-linked receptors 4. Cytokine receptors (Class I and II) 5. Tumour necrosis factor (TNF) family 6. Haematopoietic antigen receptors

50 Ionotrophic Receptors

51 G-protein-linked receptors

52 G-protein-linked receptors

53 Tyrosine kinase receptors Note steps involved: 1. Ligand Reception 2. Receptor Dimerization 3. Catalysis (Phosphorylization) 4. Subsequent Protein Activation 5. Further Transduction 6. Response

54