On food and cooking the science and lore of the kitchen ( PDFDrive ) 1517
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ProteinStructure
Proteinsareformedbylinkingtheamine
nitrogenofoneaminoacidwithacarbon
atomonanotheraminoacid,andthen
repeatingthis“peptidebond”tomakeachain
dozensorhundredsofaminoacidslong.The
carbon-nitrogenbackboneoftheprotein
moleculeformsasortofzigzagpattern,with
the“sidegroups”—theotheratomsoneach
aminoacid—stickingouttothesides.
TheProteinHelixOneeffectofthepeptide
bondisacertainkindofregularitythatcauses
themoleculeasawholetotwistandforma
spiral,orhelix.Veryfewproteinsexistasa
simpleregularhelix,butthosethatdotendto
jointogetherinstrongfibers.Theseinclude
connective-tissuecollageninmeat,an
importantfactorinitstenderness,andthe
sourceofgelatin(pp.130,597).
ProteinFoldsTheotherinfluenceonprotein
structureisthesidegroupsofitsaminoacids.
Becausetheproteinchainissolong,itcan
bendbackonitselfandbringtogetheramino
acidsthataresomedistancealongthechain
fromeachother.Aminoacidswithsimilar
sidegroupscanthenbondtoeachotherin
variousways,includingviahydrogenbonds,
vanderWaalsbonds,ionicbonds(p.813),
andstrongcovalentbonds(especiallybetween
sulfuratoms).Thisbondingiswhatgivesa
particularproteinmoleculethecharacteristic
shapethatallowsittocarryoutitsparticular
job.Theweak,temporarynatureofthe
hydrogenandhydrophobicbondsallowsitto
changeitsshapeasitworks.Theoverall
shapeofaproteincanrangefromalong,
extended,mostlyhelicalmoleculewithafew
kinksorloops,tocompact,elaboratelyfolded
moleculesthatarecalled“globular”proteins.
Collagenisanexampleofahelicalprotein,
andthevariousproteinsineggsaremainly
globular.
