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1、教育部化學生物與資訊人才培育銜接計畫蛋白質結構與功能之化學基礎Part I錢嘉琳5/14/2008Amino Acids, Peptides, and Proteins Amino Acids Peptides and Proteins Protein Structure Protein SequencesGeneral structure of amino acid -carbon -carboxyl group -amino groupSide chain*For all the standard amino acids except glycine,the -carbon is cova
2、lently bonded to four different groups (-COO-, -NH3+, -H and -R).The -carbon is a chiral center.*All molecules with a chiral center are also optically active.The two stereoisomers of alinine, L- and D-alanine, are nonsuperimposable mirror images of each other (enantiomers).Steric relationship of the
3、 stereoisomers of alanine to the absolute configuration of L- and D-glyceraldehyde. The amino acid residues in proteins are L stereoisomersAmino acids can be classified by R groupTryptophan and tyrosine absorb ultraviolet light. The absorbance maxima for both occurs near a wavelength of 280 nm. Phen
4、ylalanine generally contributes little to the absorbance properties of proteins,Reversible formation of a disulfide bond by the oxidation of two molecules of cysteine. Disulfide bonds between Cys residues stabilize the structures of many proteinsThe additional carbons in an R group are commonly desi
5、gnated , , , , and so forth, proceeding out from the carbon, Peptides and ProteinsThe polymers of amino acids that are linked by peptide bonds. The protein is a polypeptide.Formation of a peptide bond by condensationThe pentapeptide Ser-Gly-Tyr-Ala-LeuBiologically active peptides and polypeptides oc
6、cur in a vast range of sizesOxytocin: nine amino acid residues, secreted by the posterior pituitary and stimulates uterine contractions.Glucagon: 29 residues, a pancreatic hormone, opposing the action of insulin.Lengths of proteins vary considerablyDifferent proteins contain different amino acid com
7、positionsSome proteins contain chemical groups other than amino acidsLevels of structure in proteins Column Chromatography3. Ion-exchange Chromatography4. Size-exclusion chromatography5. Affinity chromatography1. Electrophoresis: a technique for the separation of proteins is based on the migration o
8、f charged proteins in an electric field.2. Electrophoresis of proteins is generally carried out in gels made up of the cross-linked polymer polyacrylamide.3. The polyacrylamide gel acts as a molecular sieve. Separation of proteins is based on their charge-to mass ratio.4. Electrophoresis in the pres
9、ence of SDS separates proteins on the basis of molecular weight. SDS binds to most proteins in amount roughly proportional to the molecular weight of the protein. The bound SDS contributes a large net negative charge, rendering the intrinsic charge of the protein insignificant. The native conformati
10、on of a protein is altered when SDS is bound.Different samples are loaded in well at the top of the polyacrylamide gel.After electrophoresis, the proteins are visualized by adding a dye such as Coomassie blue, which binds to proteinsSDS-PAGE can estimate the molecular weight of a protein Isoelectric
11、 focusing a technique used to determine the isoelectric point. A pH gradient is established by allowing a mixture of ampholytes to distribute themselves in an electric field generated across the gel. When a protein mixture is applied, each protein migrates until it reaches the pH that matches its pI
12、. Isoelectric focusingTwo-dimensional electrophoresisMore than 1,000 different proteins from E coli can be resolved using this techniqueThe Function of a protein depends on its amino acid sequence The bacterium E. coli produces more than 3,000 different proteins; a human produces 25,000 to 35,000. E
13、ach type of protein has a unique three-dimensional structure and this structure confers a unique function. Each type of protein has a unique amino acid sequence. Thousands of human genetics diseases have been traced to the production of defective proteins. Perhaps one-third of these proteins are def
14、ective because of a single change in their amino acid sequence; hence, if the primary structure is altered, the function of the protein may also be changed. On comparing functionally similar proteins often have similar amino acid sequences, as an extreme case is ubiquitin.Is the amino acid sequence
15、absolutely fixed, or invariant, for a perticular protein? NO; some flexibility is possible. An estimated 20% to 30% of the proteins in human are polymorphic, having amino acid sequence variants in the human population. Many of these variations in sequence have little or no effect on the function of
16、the protein. Although the amino acid sequence in some regions of the primary structure might vary considerably without affecting biological function, most proteins contain crucial regions that are essential to their function and whose sequence is therefore conserved.Methods of protein sequencing1. Automated Edman degradation2. DNA sequencing3. Mass spectrometryAmino acid sequence of protein can be deduced by its DNA sequenceJ. B. Fenn (1917)Kurt WthrichThe Nobel Prize Winners in Chemistry 2002 Protein
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