Biochemistry I Lecture 22 Oct 24, 20051Atomic Resolution Structures: X-ray DiffractionProteins must be crystallized in a regular lattice. Crystal quality determines resolution.1. No real limitations as to the size of structures.2. X-rays are scattered by electrons. The amplitude of the wave scattered by an atom isproportional to the number of electrons.3. The scattered waves recombine. The scattered waves reinforce on another (constructiveinterference) if they are in phase and cancel each other out (destructive interference) if they areout of phase.4. The way in which scattered waves recombine depends only on the arrangement of atoms.Therefore, scattered X-rays can be used to determine the position of atoms.5. Fourier transform of the intensity and phases of the scattered X-rays produces an ‘electrondensity map’ or the number of electrons at each point in space in the crystal (r(x,y,z)). Thecrystallographer must figure out how to place, or “fit”, the known primary structure of the proteinonto this map (see diagram).Biochemistry I Lecture 22 Oct 24, 20052Lecture 22: LipidsCampbell: Chapter 8.1-8.2A. Fatty Acids:A1. Structure:A2. Nomenclature for fatty acids:#CarbonsNameMeltingTemp, C°12Laurate44.214Myristate53.916Palmitate63.118Stearate69.620Arachidate76.518:1 cD9(C9 = C10)Oleate16.0A3. Physical Properties - pure substance1. The melting temperature depends on thelength of the acyl chain due to van derWaals interactions between the chains.2. This van der Waals interaction is reduced infatty acids with cis double bonds, causing areduction in the meltingtemperature (morefluid).A4. Physical Properties -interaction with water1. Fatty acids formmicelles, aggregates offatty acids with a polar(charged) surface and ahydrophobic, waterless,interior. The assemblyof micelles is driven bythe hydrophobic effect.2. The non-polar interiorMelting Temp of Fatty Acids010203040506070809010 12 14 16 18 20Carbon LengthMelting Temperature (C)OOC18OOHHC18OOHHC18Fatty AcidsPhospholipidsTriglyceridesBiological MembranesCholesterolProteins(Energystorage)(Permeability Barrier)Biochemistry I Lecture 22 Oct 24, 20053can dissolve hydrophobic compounds (oily 'dirt'). The principal components of soaps are fatty acidsand modified fatty acids (e.g. SDS) .3. Free fatty acids are essentially toxic since they can dissolve normal cell membranes in the same waysoap would. Fortunately, fatty acids in cells are esterified to glycerol to form triglycerides orphospholipids.A5. Triacylglycerols (triglycerides) -stores of energy - not constituents of biological membranesTriacylglycerols are composed of three fatty acyl chains esterified to glycerol.Triacylglycerols are highly concentrated stores of metabolic energy because they are reduced andanhydrous. They are stored in the form of lipid droplets in adipose cells. Lipids are richer energy sourcesthan sugars because they are less oxidized than carbohydrates.A6. Phospholipids - major constituents of biological membranesPhospholipids are composed of two fatty acyl chains esterified to glycerol 3-phosphate. In mostphospholipids the phosphate group is also esterified to an alcohol. Resulting phospholipids are usuallyeither zwitterionic or have a net negative charge.OOOHOOHOHOHOHH6 carbon fatty acid6 carbon sugarCH2CHOCH2OOOOHOHOHOHOHOCH2N+CH2CH3CH3CH3CH2NH3+CH2OCH2CHNH3+COO-OOHCH2CHOCH2OOOOPO_XBiochemistry I Lecture 22 Oct 24, 20054Physical Properties1. Phospholipids self-assemble spontaneously in water to form bilayers(two opposing layers of phospholipids). Bilayers are formed insteadof micelles because the cross section of the head group is roughlyequal to the cross section of the 2 fatty acid chains.2. The bilayers form closed, water filled, compartments with a60-90A° thick wall.3. Permeability properties:• Charged compounds do not cross the bilayer.• Polar compounds cross infrequently.• Non-polar compounds cross readily.OPOOOOOOO10-1410-1210-1010-810-610-410-2Na+Cl-GlucoseH2OGlycerolK+PERMEABILITY COEFFICIENT