Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2001
Lecture Notes:: 5 October	© R. Paselk 2001
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Biological Membranes

(Chap 10)

Fluid Mosaic Model: (Figure 10-10, p 248) [overhead 11-21 V&V Check out the Guided Exploration on your CD] This model has as its core element lipid bilayer (predominantly glycero-phospholipid). This bilayer makes a very effective barrier for the flow of charged and polar species between aqueous compartments. Within the bilayer itself, however, flow occurs readily - it is a two- dimensional liquid with a viscosity similar to olive oil. Thus we see rapid exchange between adjacent phospholipid molecules on a face of the bilayer, but very rare exchange between faces (the polar "head" groups would have to cross the non-polar bilayer interior). A lipid bilayer membrane thus separates the interior of the cell from the outside.

Of course a cell also needs to communicate with the outside world - doors and windows are needed. Such communication occurs largely through proteins acting as pores, gates, and shuttles. Note that these proteins "float" in the bilayer. They have unconstrained movement in the two-dimensions of the sheet. Changes in protein conformation can also cause them to "sink" into the hydrophobic interior of the bilayer etc. Protein movement can be constrained by linkage to protein networks (cytoskeleton) within the cell as is exemplified by red blood cells (RBC's). (Figure 10-15, p 252) [overhead 12.17 P]

Membrane Assembly/Protein Targeting

Assembly occurs on scaffolding of previous membrane

Membrane lipids are synthesized in/on membrane (Eukaryotes synthesis on cytosolic face of ER, transport by budding and Phospholipid exchange protein. Signal hypothesis for targeting of many membrane and exported proteins:

• Membrane proteins and export proteins synthesized on rER (Figure 10.20, p 256) [overhead V&V 11-40]. SRP= signal recognition protein; signal sequence= 13-36 residues with 7-13 residue hydrophobic core flanked by hydrophilic and basic residues near N-terminii; Transmembrane proteins contain an approx. 20 hydrophobic residue "stop transfer" or "membrane anchor" sequence which stays in membrane, other proteins continue through. Note that this model indicates that the amino terminus would always be extra-cytosolic in membrane proteins (this is not always the case, thus other mechanisms must be operating).
• Mitochondria illustrates more complex situations: Cytosolically synthesized precursor proteins have signal sequence. Hsp 70 helps to unfold precursor protein (ATP required), which can then pass through both membranes of mito. Inside Hsp 70 maintains in partially folded state (ATP required). It is then aided in folding by Hsp 60 ("barrel" chaperon) (ATP required). Finally it is released (ATP required), and the N-terminus is removed to give the mature protein in the matrix.

Proteins are transported in coated vesicles: membranous sacs encased in polyhedral frameworks of clathrin.[overhead 11-42, 21-17, V&V]

INTRODUCTION TO ENZYMES

(Chap 11)

Enzymes are the heart of Biochemistry

• protein based catalysts (for us RNA based catalysts are Ribozymes)
• enormously effective catalysts: typically enhance rates by 10⁶ to 10¹² fold
• operate under mild conditions: 0 - 100 °C (or even 300+ for some bacteria), 20 -40 °C for most organisms; and low pressures (atmospheric)
• very specific: generally catalyze reaction for a very restricted group of molecules, sometimes for a single naturally occurring molecule of a single chirality.

Enzymes generally have a cleft for active site, generally <5%of surface: look like pac man. Need large structure to maintain shape etc. with many weak bonds.

Look at major aspects of enzyme study:

• Specificity
• Molecular mechanisms of catalysis
• Kinetics, including review
  

Pathway Diagrams

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Last modified 5 October 2001