Protein structure

Structure of protein with figures

1. Proteins
-Dr NidhiSharma

2. Protein Structure
Four levels of organization
•Primary structure
•Secondary structure
oAlpha helix
oBeta pleated sheets
•Tertiary structure
•Quaternary structure
Dr NidhiSharma

3. Protein Structure
Dr Nidhi Sharma

4. •Proteinsarethemajorcomponentsofliving
organismsandperformawiderangeof
essentialfunctionsincells.
•WhileDNAistheinformationmolecule,itis
proteinsthatdotheworkofallcells-
microbial,plant,animal.
•Proteinsregulatemetabolicactivity,catalyze
biochemicalreactionsandmaintainstructural
integrityofcellsandorganisms.
Dr NidhiSharma

5. Proteinscan be classified in a variety of ways, including
their biological function
Type: Example:
Enzymes-Catalyze biological
reactions
ß-galactosidase
Transportand Storage Hemoglobin
Movement
Actin
And Myosin in muscles
Immune Protection
Immunoglobulins
(antibodies)
Regulatory Function within
cells
Transcription Factors
Hormones
Insulin
Estrogen
Structural Collagen
Dr NidhiSharma

6. Proteinscanbeformedusing20differentbuildingblocks
calledaminoacids.
Eachoftheseaminoacidbuildingblockshasadifferent
chemicalstructureanddifferentproperties.
Eachproteinhasauniqueaminoacidsequencethatis
geneticallydeterminedbytheorderofnucleotidebases
intheDNA,thegeneticcode.
Sinceeachproteinhasdifferentnumbersandkindsof
thetwentyavailableaminoacids,eachproteinhasa
uniquechemicalcompositionandstructure
Dr Nidhi Sharma

7. Achangeinjustoneaminoacidcanchangethe
structureandfunctionofaprotein.
Forexample,sicklecellanemiaisadiseasethat
resultsfromanalteredstructureoftheprotein
hemoglobin,resultingfromachangeofthesixth
aminoacidfromglutamicacidtovaline.(Thisisthe
resultofasinglebasepairchangeattheDNAlevel.)
Thissingleaminoacidchangeisenoughtochange
theconformationofhemoglobinsothatthisprotein
clumpsatloweroxygenconcentrationsandcauses
thecharacteristicsickleshapedredbloodcellsofthe
disease.
Dr Nidhi Sharma

8. Aminoacidstructure:
Aminoacidsarecomposedofcarbon,hydrogen,
oxygen,andnitrogen.
Twoaminoacids,cysteineandmethionine,also
containsulfur.
Allaminoacidshaveanaminogroup(NH
2)anda
carboxylgroup(COOH)bondedtothesamecarbon
atom,knownasthealphacarbon.
Dr Nidhi Sharma

9. Dr Nidhi Sharma

10. AminoacidsdifferinthesidechainorRgroupthatis
bondedtothealphacarbon.Glycine,thesimplest
aminoacidhasasinglehydrogenatomasitsRgroup
-Alaninehasamethyl(-CH
3)group.
ThechemicalcompositionoftheuniqueRgroupsis
responsiblefortheimportantcharacteristicsof
aminoacidssuchaschemicalreactivity,ioniccharge
andrelativehydrophobicity.
Dr Nidhi Sharma

11. Theaminoacidsaregroupedaccordingtotheir
polarityandcharge.
Theyaredividedintofourcategories,thosewith
polarunchargedRgroups,thosewithapolar
(nonpolar)Rgroups,acidic(charged)andbasic
(charged)groups.
Dr Nidhi Sharma

12. Dr Nidhi Sharma

13. Thepolaraminoacidsaresolubleinwaterbecause
theirRgroupscanformhydrogenbondswithwater.
Forexample,serine,threonineandtyrosineallhave
hydroxylgroups(OH).
Dr Nidhi Sharma

14. The acidic amino acids carry a net negative charge at
neutral pH contain a second carboxyl group.
These are aspartic acid and glutamicacid, also called
aspartateand glutamate, respectively.
Dr Nidhi Sharma

15. The basic amino acids have R groups with a net positive
charge at pH 7.0.
These include lysine, arginineandhistidine.
Dr Nidhi Sharma

16. ThereareeightaminoacidswithnonpolarR
groups.Asagroup,theseaminoacidsareless
solubleinwaterthanthepolaraminoacids.
Thesearealanine,valine,leucine,isoleucine,
tryptophan,phenylalanine,methionine,and
proline
IfaproteinhasagreaterpercentageofnonpolarR
groups,theproteinwillbemorehydrophobic
(waterhating)incharacter.
Dr Nidhi Sharma

17. Aproteinisformedbyaminoacidsubunitslinked
togetherinachain.
Thebondbetweentwoaminoacidsisformedbythe
removalofaH
20moleculefromtwodifferentamino
acids,formingadipeptide.
Thebondbetweentwoaminoacidsiscalleda
peptidebondandthechainofaminoacidsiscalleda
peptide(20aminoacidsorsmaller)orapolypeptide.
Dr Nidhi Sharma

18. I
H
Dr Nidhi Sharma

19. PrimaryStructurereferstothelinearsequenceof
aminoacidsthatmakeupthepolypeptidechain.
Thissequenceisdeterminedbythegeneticcode,the
sequenceofnucleotidebasesintheDNA.
Thesequenceofaminoacidsdeterminesthe
positioningofthedifferentRgroupsrelativetoeach
other.
Thispositioningthereforedeterminesthewaythatthe
proteinfoldsandthefinalstructureofthemolecule.
Dr Nidhi Sharma

20. Fig. Apentapeptide. The chain starts at the
amino acid end
Dr Nidhi Sharma

21. Thesecondarystructureofproteinmoleculesrefersto
theformationofaregularpatternoftwistsorkinksof
thepolypeptidechain.
Theregularityisduetohydrogenbondsforming
betweentheatomsoftheaminoacidbackboneofthe
polypeptidechain.
Thetwomostcommontypesofsecondarystructure
arecalledthealphahelixandßpleatedsheet.
Dr Nidhi Sharma

22. Fig. Right handed αhelix
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23. Fig. Antiparallelβpleated sheet
Dr Nidhi Sharma

24. Tertiarystructurereferstothethreedimensional
globularstructureformedbybendingandtwistingof
thepolypeptidechain.
Thisprocessoftenmeansthatthelinearsequenceof
aminoacidsisfoldedintoacompactglobularstructure.
Dr Nidhi Sharma

25. Thefoldingofthepolypeptidechainisstabilizedby
multipleweak,noncovalentinteractions.These
interactionsinclude:
HydrogenbondsthatformwhenaHydrogenatom
issharedbytwootheratoms.
Electrostaticinteractionsthatoccurbetween
chargedaminoacidsidechains.Electrostatic
interactionsareattractionsbetweenpositiveand
negativesitesonmacromolecules.
Dr Nidhi Sharma

26. Hydrophobicinteractions:Duringfoldingofthe
polypeptidechain,aminoacidswithapolar(water
soluble)sidechainareoftenfoundonthesurfaceof
themoleculewhileaminoacidswithnonpolar
(waterinsoluble)sidechainareburiedinthe
interior.Thismeansthatthefoldedproteinis
solubleinwateroraqueoussolutions.
Covalentbondsmayalsocontributetotertiary
structure.
Theaminoacid,cysteine,hasanSHgroupaspartof
itsRgroupandtherefore,thedisulfidebond(S-S)can
formwithanadjacentcysteine.Forexample,insulin
hastwopolypeptidechainsthatarejoinedbytwo
disulfidebonds.
Dr Nidhi Sharma

27. Quaternarystructurereferstothefactthatsomeproteins
containmorethanonepolypeptidechain,addingan
additionallevelofstructuralorganization:theassociationof
thepolypeptidechains.
Eachpolypeptidechainintheproteiniscalledasubunit.
Thesubunitscanbethesamepolypeptidechainordifferent
ones.Forexample,theenzymeß-galactosidaseisatetramer,
meaningthatitiscomposedoffoursubunits,and,inthis
case,thesubunitsareidentical-eachpolypeptidechainhas
thesamesequenceofaminoacids.
Dr Nidhi Sharma

28. Hemoglobin,theoxygencarryingproteininthe
blood,isalsoatetramerbutitiscomposedof
twopolypeptidechainsofonetype(141amino
acids)andtwoofadifferenttype(146amino
acids).
Inchemicalshorthand,hemoglobinisreferredto
asa
2ß
2.
Forsomeproteins,quaternarystructureis
requiredforfullactivity(function)oftheprotein.
Dr Nidhi Sharma

29. Fig. Four levels of organization of protein structure
Dr Nidhi Sharma

30. Dr Nidhi Sharma