页数：94页文件大小：4.64 MB来源： 生物化学ppt课件
本课件为英文教材第二章，以氨基酸为主题：先讲典型氨基酸的结构与聚合能力，再讲氨基酸的酸碱化学与滴定曲线、各类化学反应，随后讲氨基酸的旋光性与立体化学、光谱性质，最后讲氨基酸混合物的色谱分离与分析方法。
Chapter 2 Amino Acids
Proteins are the indispensableagents of biological function,andamino acidsare thebuilding blocks of proteins. Thestunning diversity of thethousands of proteins found innature arises from the intrinsicproperties of only 20 commonlyoccurring amino acids.
Because the resulting amino acid contains one positive andone negative charge, it is a neutral molecule called azwitterion. Amino acids are alsochiral
All objects have mirror images. Like many biomolecules, amino acids exist in mirror-image forms (stereoisomers). Only the L-isomers of amino acids commonly occur in nature.
The a-amino carboxylic acids (amino acids) can be of two stereoisomers
Amino Acids Can Join via Peptide BondsThe a-COOH and a-NH3+ groups of two amino acids can react with the resulting loss of a water molecule to form a covalent amide bond.
Amino acid residueThe peptidebond (M5.8)
Common Amino AcidsCommon Amino AcidsThe 20 amino acids that are the building blocks of most proteins can be classified as: (a)nonpolar (hydrophobic), (b) polar, neutral, (c) acidic, or (d) basic.
There are the one-letter and three-letter codes used to denote amino acids. For each amino acid, the ball-and-stick (left) and space-filling (right) models show only the side chain.
Structuralformula ofthestandardamino acids(L5-5)
All amino acids have the same general structurebut the side chain (R group) of each is different
Amino acid groups are classifiedaccording to their solubility in water:•Hydrophilic(charged or polar) aminoacids are often found on the surface ofproteins; by interacting with water they makeproteinssoluble inaqueous solutions. Theycan also act as binding sites for chargedmolecules.
Hydrophilic amino acids
Hydrophobic amino acids•Hydrophobic (non-polar) amino acids have aliphatic side chains, avoid water and form the water-insoluble core of proteins. They line the surface of membrane proteins that reside in the hydrophobic environment of the lipid bilayer.
“Special” amino acids•Cysteine: the side-chain contains a reactive sulphydryl group (-SH)-can oxidise to form a disulphide bond (-S-S-) with a secondcysteine. (Disulphide bonds are important in the cross-linking of
Proline a very“special”amino acid
Cysteine is also a “special” amino acid
Cysteine sidegroups can formdisulfide bondsbetween peptidechains of aminoacids(e.g. insulin)
The charge of an amino acid depends onpHAt pH 7 amino acids are zwitterionic
The simplestamino acids
The hydrophobic amino acids
The aromaticamino acids
Tyrosine and tryptophan absorbance of UV light is often used to quantitate proteins
Aliphatic hydroxyl amino acids
The basic amino acids(positively charged at neutral pH)
Thecarboxylateamino acidsand theiramidederivatives
Uncommon Amino AcidsCarboxyl-Hydroxyl-Methyl-Acetyl-
鸟氨酸瓜氨酸
Amino Acids Not Found in Proteins Certain amino acids and their derivatives, although not found in proteins, nonetheless are biochemically important.
4.2 •Acid-Base Chemistry of Amino AcidsAmino Acids Are Weak Polyprotic Acids From a chemical point of view, the common amino acids are all weak polyprotic( 多质子的)acids. The ionizable groups are not strongly dissociating ones, and the degree of dissociation thus depends on the pH of the medium. All the amino acids contain at least two dissociable hydrogens.
The degree of protonation of amino acid side chains can be calculated from pH and pKa of the side chain
The Henderson-HasselbachequationpH = pK + log([A-]/[HA])For acetic acid, pK = 4.8At pH = 4.8, pH = pK and [A-] = [HA]At pH = 3.8, pH-pK= -1 so [A-]/[HA] = .1 or ~90% of A is protonated
Ionization of Side Chains
Green Fluorescent Protein—The “Light Fantastic” from Jellyfish to Gene Expression
Chemical rearrangement of amino acid residues occurs in the Green Fluorescent Protein
Aquorea victoria, a species of jellyfish found in the northwest Pacific Ocean, contains a green fluorescent protein (GFP)that works together with another protein, aequorin, to provide a defense mechanism for the jellyfish. When the jellyfish is attacked or shaken, aequorin produces a blue light. This light energy is captured by GFP, which then emits a bright green flash that presumably blinds or startles the attacker.
Remarkably, the fluorescence of GFP occurs without the assistance of a prosthetic group—a “helper molecule” that would mediate GFP’s fluorescence. Instead, the light-transducing capability of GFP is the result of a reaction between three amino acids in the protein itself. As shown below, adjacent serine, tyrosine,and glycinein the sequence of the protein react to form the pigment complex—termed a chromophore. No enzymes are required; the reaction is autocatalytic.
Because the light-transducing talents of GFP depend only on the protein itself (upper photo, chromophore highlighted), GFP has quickly become a darling of genetic engineering laboratories. The promoter of any gene whose cellular expression is of interest can be fused to the DNA sequence coding for GFP. Telltale green fluorescence tells the researcher when this fused gene has been expressedAutocatalytic oxidation of GFP amino acids leads to the chromophore shown on the left. The green fluorescence requires further interactions of the chromophore with other parts of the protein.
4.3 • Reactions of Amino Acids
Carboxyl and Amino Group Reactions
The Ninhydrin ReactionRuhemann’s Purple can be quantified spectrophotometrically at 570 nm
Specific Reactions of Amino Acid Side Chains
4.4 •Optical Activity and Stereochemistry of Amino AcidsAmino Acids Are ChiralMolecules Except for glycine, all of the amino acids isolated from proteins have four different groups attached to the a-carbon atom. In such a case, the a-carbon is said to be asymmetricor chiral(from the Greek cheir, meaning “hand”), and the two possible configurations for the a-carbon constitute nonsuperimposable mirror image isomers, or enantiomers.
Enantiomeric molecules display a special property calledoptical activity—the ability to rotate the plane of polarization of plane-polarized light. Clockwise rotation of incident light is referred to as dextrorotatory(+)behavior, and counterclockwise rotation is called levorotatory(-) behavior.
L-Alanine+1.8L-Arginine+12.5L-Aspartic acid+5.0L-Glutamic acid+12.0L-Histidine-38.5L-Isoleucine+12.4L-Leucine-11.0L-Lysine+13.5L-Methionine-10.0L-Phenylalanine-34.5L-Proline-86.2L-Serine-7.5L-Threonine-28.5L-Tryptophan-33.7L-Valine+5.6
The configuration of the common -amino acids can be related to the configuration of L(-)-glyceraldehyde as shown. These drawings are known as Fischer projections. The horizontal lines of the Fischer projections are meant to indicate bonds coming out of the page from the central carbon, and vertical lines represent bonds extending behind the page from the central carbon atom.
In spite of its widespread acceptance, problems exist with the D,L system of nomenclature. For example, this system can be ambiguous for molecules with two or more chiral centers. To address such problems, the (R,S) systemof nomenclature for chiral molecules was proposed in 1956 by Robert Cahn, Sir Christopher Ingold, and Vladimir Prelog. In this more versatile system, priorities are assigned to each of the groups attached to a chiral center on the basis of atomic number, atoms with higher atomic numbers having higher priorities
A Deeper LookThe Murchison Meteorite —Discovery of Extraterrestrial HandednessThe predominance of L-amino acids in biological systems is one of life’s most intriguing features. Prebiotic syntheses of amino acids would be expected to produce equal amounts of L-and D-enantiomers. Some kind of enantiomeric selection process must have intervened to select L-amino acids over their D-counterparts as the constituents of proteins. Was it random chance that chose L-over D-isomers? Analysis of carbon compounds—even amino acids—from extraterrestrial sources might provide deeper insights into this mystery.
John Cronin and Sandra Pizzarello have examined the enantiomeric distribution of unusual amino acids obtained from the Murchison meteorite, which struck the earth on September 28, 1969, near Murchison, Australia. (By selecting unusual amino acids for their studies, Cronin and Pizzarello ensured that they were examining materials that were native to the meteorite and not earth-derived contaminants.) Four of amino acid were found to have an L-enantiomeric excess of 2 to 9%.
This may be the first demonstration that a natural L-enantiomer enrichment occurs in certain cosmological environments. Could these observations be relevant to the emergence of L-enantiomers as the dominant amino acids on the earth? And, if so, could there be life elsewhere in the universe that is based upon the same amino acid handedness?
4.5 •Spectroscopic Properties of Amino AcidsOne of the most important and exciting advances in modern biochemistry has been the application of spectroscopic methods,which measure the absorption and emission of energy of different frequencies by molecules and atoms. Spectroscopic studies of proteins, nucleic acids, and other biomolecules are providing many new insights into the structure and dynamic processes in these molecules.
Ultraviolet Spectra
The aromatic amino acids also exhibitrelatively weakfluorescence, and it hasrecently been shown thattryptophan canexhibitphosphorescence—a relativelylong-lived emission of light. These
Nuclear Magnetic Resonance SpectraThe development in the 1950s of nuclear magnetic resonance(NMR), a spectroscopic technique that involves the absorption of radio frequency energy by certain nuclei in the presence of a magnetic field, played an important part in the chemical characterization of amino acids and proteins. Several important principles rapidly emerged from these studies.
Proton NMR spectra of two amino acids
4.6 •Separation and Analysis of Amino Acid Mixtures Chromatographic Methods
Tswett (1950), a Russian botanist, poured solutions of the pigments through columns of finely divided alumina氧化铝and other solid media, allowing the pigments to partition between the liquid solvent and the solid support. Owing to the colorful nature of the pigments separated, Tswett called his technique chromatography.This term is now applied to a wide variety of separation methods, regardless of whether the products are colored or not.
The methods important for amino acids include separations based on partitionproperties (the tendency to associate with one solvent or phase over another) and separations based on electrical charge.
Methods important for aminoacid separations includeionexchange chromatography,gaschromatography (GC), andhigh-performance liquidchromatography (HPLC).
ion exchange chromatography
HPLC has rapidly become the chromatographic technique of choice for most modern biochemists. The very high resolution, excellent sensitivity, and high speed of this technique usually outweigh the disadvantage of relatively low capacity.
The End of Chapter2
polymerizeNovelChiralityFeatureFunctionalityTypicalTetrahedralCovalentlyCarboxylVariableIdentityNEW WORDSPositiveNegativeNeutralChiralEnantiomersConfigurationNonidenticalIsomerStereoisomerPeptideCovalentresidueCovalentAromaticAliphaticHydrophobicDerivativePolyproticProtonationFluorescentFantasticRearrangementStartlePresumably
NEW WORDSvarietyvarietyVarietyPigmentBotanistBotanyBotanicbotanicalTendencyResolutionExcellentSensitivityOutweighDisadvantagepro and conMagneticResonanceSpectroscopicAbsorptionFrequencyNuclearNucleiEmergemolarExtinctionCoefficientAbsorbanceUltravioletDynamicVersatilePrioritiesNomenclatureHorizontalVerticalRepresentTerrestrialStandardquantitate
DenoteKinkAdjacentprosthetic groupAssistanceTelltaleClockwiseCounterclockwiseAmbiguousAcceptancePrioritiesHandedness
Topics of Discussion1 What Are Properties of 20 Amino Acids?2 How To Judge Hydrophobic and Hydrophilic of An Amino Acid? 3 Why the Charge of R Side Chains Are SoImportant for the Structures and Features of Proteins?4 Why We Can Use Ultraviolet SpectraMethodTo Assay Proteins.
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