食品课件化学ch5proteins

1、10/7/20141Aliatic (no hydroilic groups)Glycine (GLY)Alanine (ALA)Valine (VAL)The only -chiral amino acidH-HHNH3 C COO+-NHC COO-CHNH3 C COO3HCH3CH3 CH3Isoleucine (ILE)Leucine (LEU)2 chiral carbonsHH-NH3 C COO-NH 3 C COOCHCH 2CH3 CH2CH3CHCH 3 CH 36Chemical Classification of AA*Based on the chemical na

2、ture of the functional group (R) *Ali atic amino acidsHydroxyl amino acidsAcidic amino acidsAmide amino acidsBasic amino acidsSulfur-containing amino acidsAromatic amino acidsSecondary amino acids5Classification of Amino Acids21 primino acidsDerived amino acidscross-linked amino acids or simple deri

3、vatives of single amino acidsChemicalBy functional groupNutritionalBy essential or-essential4Chirality of Amino Acids: ExampleL Alanine 丙氨酸D - AlanineOne amino acid is not chiral Can you name it?3OverviewAmino acidsStructureClassificationysical propertiesChemical propertiesThe peptide bondProtein st

4、ructure4 levelsDenaturation2Proteins IPu Jing, DFood Chemistry, Fall 2014110/7/20142Aromaticenylalanine ( E)Tyrosine (TYR)Trypto an (TRP)HHHNH C COO-NH C COO-NH3 C CO -O33CH2CH2CH2NOHHSummary:All very hydro obicAll contain aromatic groupAbsorb UV at 280 nm12SulfuricCysteine (CYS)Methionine (MET)Sele

5、nocysteine (SeCys)硒代半胱氨酸HHH+-NHC-NHC COO3COO+-3CH2NH3C COOCH2CH2CH2SHSSeHCH3Summary:Cysteine sulfur in form of sulfhydroyl, important in disulfide linkages, weak acid, can form hydrogen bondsMethionine very hydro obicSelenocysteine - an atom of selenium taking the place of the sulfur present in seve

6、ral enzymes (e g glu hione peroxidases)11BasicLysine (LYS)Arginine (ARG)Histidine (HIS)HHHNH3 C CO -NHC CO -NH C C -O3O3OOCH2CH2CH2CH2CH2CH2CH2 NHHN NH2CH2NH C NH2NH3Summary:Hydro illic nitrogenous basesitively charged at ysiological10AmideAsparagine (ASN)Glutamine (GLN)amide of aspartic acidamide o

7、f glutamic acidHH+-NH3+ C COO-NH 3C COOCH2CH 2C NHCH 22OC NH2OPolar uncharged amino acids9Acidic (carboxylic acid group)Aspartic Acid (ASP)Glutamic Acid (GLU)HH+-NH + C-NH3 C COO3COOCH2CH2CCOOOO- -CH2CCOOO-OSummaryContain carboxyl groups (weaker acids n a-carboxyl-group)Negatively charged at ysiolog

8、ical7 17 2 生理酸碱值, present as conjugate bases (therefore ate not ic acids)Carboxyl groups function as nucleo iles 亲核剂 in some enzymatic reactions8Hydroxyl Amino Acids -amino acids wilcoholSerine (SER)Threonine (THR)2 chiral carbonsHH-NHC-NH3 C COO3COOCH OOHHCH2OOHHCH3Polar uncharged side chain710/7/2

9、0143Example : Lysine1 carboxylic acid = 2.22 amino group = 9.03 R group = 10.5= ( 2+3)/2= (9+10.5)/2= 9.7518How to Determine the Isoelectric Potheat which amino acid carriese ectrical charge= ( a1 + a2)/2If no charge on R= ( a1 + a3)/2If acidic AA= ( a2 + a3)/2If basic AA17How to Determine the Isoel

10、ectric Po:Titrationa1= where COO-=COOH a2 = , where NH3 =NH2 a3 = , the a value for RAcid-Base Properties of AAAmino acids are zwitterions两性离子Theye boitive and negative charge on the same molecule.The isoelectric po( ) is theat which a molecule or surface carries e ectrical charge15Nutritional Class

11、ification of AA* Classified according to whether or not the human body can produce the AA.*9 of the 21 amino acids must be obtained from the dietThese are referred to as the essential amino acidsHistidine 组氨酸Isoleucine 异亮氨酸Leucine 亮氨酸Lysine 赖氨酸Methionine 蛋氨酸enylalanine 苯丙氨酸Threonine 苏氨酸Trypto an色氨酸V

12、aline 缬氨酸The remaining 12 amino acids are called-essential amino acids14Chem cal Classif cation of AA:Secondary Amino AcidProline (PRO)COO-H N+CH2H2CCH2 CH2cyclic “imino亚胺基 acid”1310/7/20144Protein Structure: Tertiary第三级3-Dimenanization of a polypeptide chainA linear protein chain with secondary str

13、ucture segments folds further o a compact 3-D formFolding of individual proteinMove hydro obic groups to the molecules eriorerior devoid of water molecules or charged groupsExcept for specific functionalityA protein has size and shas well as unique arrangement through hydrogen, ionic, hydro obic and

14、 disulfide bonds24Protein Structure: SecondaryFormed by hydrogen bonding betn different parts of the amino acid chainSmall negative charge on the oxygen atom (-) is attracted to the small itive charge on the hydrogen atom ( )C OH NWhen a lot of these hydrogen bonds occur along one polypeptide chain,

15、 the chain will form two maypes of secondary structures:Helix structures: e g - Helix- sheet structures: (anti)parallel23Protein Structure: PrimaryGly Ile Val Cys Glu Gln22Protein StructurePrimary structureAA sequence & chain lengthSecondary structureHeliand pleated sheetsTertiary structureSpatial a

16、rrangementQuaternary structure3-D arrangement (2 or more polypeptide chains)21Peptide Bond CharacteristicsType of bond: amideC-terminalCO -OR3N-terminalOCH HCHR1CNCNCNHHHR2OH1 unit of amino acid3 units of amino acid = Tripeptide20Protein Structure1910/7/20145Protein denaturationUndesirableLoss of so

17、lubility and some function propertiesFlocculation and preciion during storage of protein beveragesDesirablePartial denaturationimprove their foaming and emulsifying properties at air- water anl-water erfaMore digestible n native oneHeat-induced gelation30Protein StructureCoded in DNASelf assembly to

18、 a single (native) structure. Depends on primary structure and solution conditionsCommon in foods. Many -Denaturednative forms depending on protein structure, solution conditions (& history) and29ingredient eractions21 Amino AcidsPrimarySecondaryTertiaryQuaternaryProtein Denaturation28Researches fou

19、nd protein contains 30% hydro obic AA residues exhibit a great tendency to form a quaternary structureHydro obic surfaDimer27TetramerProtein Structure: Quaternary第四级-covalent 非共价键 asso tion of protein unitsIn other words, two or more polypeptide chains eract to form one proteinMany examples in food:

20、Soy globulinsCasein (milk)Specifice P- nteractionHydrogen bondhydro obicelectros ic eraction26Protein Structure: TertiaryHydro obic eraction2510/7/20146Hydro obic eraction favor the folded s eGND= G+ GconfConformational entropy:Favor the unfolded s eRelative changes in free energy contributions by h

21、ydrogen bonding , hydro obic eractions, and conformational entropy to the stability of proteins as a function of36temperatureThermal DenaturationGD= HD-T SDRate of denaturation depends on the temperature 40 80 CAs Temperature is increasedHydro obic eractions conformational entropy both increaseTo a

22、mild degreeAffect eractions of tertiary structureIncreased flexibility reversibleTo a severe degreeLoss solubility and viscosityIrreversible denaturationStructures different from native proteinUpon coolingAggregationLoss of solubility35and DenaturationProteins are more stable against denaturation at

23、At extremea high net chargestrong ramolecular分子间 repul 排斥swelling and unfoldingThe degree of unfolding is greater at extreme alkalinen it is at acidMostly reversible However, deamidation of Asn and Gln, destruction of sulfhydryl巯基groups, and aggregation can result in irreversible denaturation of pro

24、teins34Factorst can cause DenaturationHeat (thermal)Pre reShear forExtremeanic solventsanic compoundsHigh concentrations of saltCold temperature (sometimes)33Protein DenaturationOne native s eIncreasedchain entropy熵Many denatured s es32Protein DenaturationProtein denaturation is acovalent changehe s

25、tructure of a protein Any modification in conformation notpanied by rupture ofpeptide bondsCompare with “conformational adaptability”This change may alter the secondary, tertiary or quaternary structure of the moleculesUltima y might correspond to a totally unfolded polypeptide chain.Can be reversib

26、le or irreversible3110/7/20147anic Compounds and DenaturationUrea and Guanidine saltsDisrupts H-bondsDecrease hydro obic eractionsSurface-active agents (SDS，十二烷基硫酸钠)Disrupt hydro obic eractionsIncrease ernal repulsive forUnfoldReducing agentsDisulfide crosslinks42anic Solvents and DenaturationLess p

27、olar solvent will lower the dielectric constant of the system.At high concentrations, allanic solvents cause proteins denaturation.polar side chains are buried inside proteins they are more soluble inanic solvents, and so undergo unfolding inanic solventsHas application in protein purification.41She

28、ar and DenaturationHigh mechanical shear can cause denaturation of proteins.shaking, kneading 揉，捏, whip搅打Causes:incorporation of air bubbles and adsorption of proteins to air-liquiderface.Proteins undergo conformational change.polar residues of denatured protein orient to gas ase and the polar resid

29、ues toward the aqueousase.40Hydrosic Prere 静水压力 and DenaturationMost proteins undergo prere-induced denaturationhe range of 1-12 kbar.A reduction of volume usually occurs for denatured globular proteins.Prere-induced protein denaturation is highly reversible.Many enzymes regaheir activity once pre re returns to normal atmos eric pre re.39Factorst Affect Thermal DenaturationAmino acid comitionProteins containing more hydro obic AA residues (Val, Ile, Leu and e) tend to be more stableCrosslinke