Koolhidraat

Koolhidrate is organiese verbindings wat 'n belangrike rol in lewende organismes speel. Hulle het die algemene formule (CH2O)n, waarvan ook hul benaming - "hidrate van koolstof" - afgelei is. Die eenvoudigste koolhidrate is suikers wat uit vyf (pentoses) of ses (heksoses) koolstofatome kan bestaan.

Hierdie suikermolekule kom gewoonlik as ringe voor en staan bekend as monosakkariede, soos byvoorbeeld glukose en fruktose. Verbindings van twee molekule word disakkariede genoem; tafelsuiker of sukrose (met een glukose- en een fruktose-eenheid per molekuul) is die bekendste voorbeeld.

Kettings van tussen drie en ses monosakkariede word oligosakkariede genoem, terwyl langketting-polimere van suikermolekule as polisakkariede bekend staan. Polisakkariede speel 'n belangrike rol as verbindings wat energie kan stoor, soos byvoorbeeld stysel in plante en glikogeen in diere. Ander polisakkariede is strukturele molekule soos byvoorbeeld sellulose (wat in die wande van plantselle voorkom) en chitien (wat in die selwande van fungi en in die eksoskelette van baie ongewerwelde diere aangetref word).

Koolhidrate in die menslike dieet

Koolhidrate kom - naas vette en proteïene - ook in menslike voedsel voor en maak 60% van die gemiddelde totale dieet uit. Voorbeelde waar koolhidrate in plantaardige stowwe voorkom, sluit in: brood, mielies, rys, aartappels, rietsuiker, en konfyt. Dit kom ook in melk van soogdiere voor.

Soorte koolhidrate

Koolhidrate kan ongeveer met die formule C(H2O)n weergegee word, maar dit bestaan uit molekule waarvan die meeste koolstofatome 'n -OH alkoholgroep en een 'n C=O-karbonielgroep dra. [1]

Monosakkariede

Glukose in sy lineêre en sikliese piranose-vorm.

Die α- en β-anomere van glukose. Die posisie van die hidroksielgroep (rooi of groen) kan verskillend wees teenoor koolstofatoom 5 se CH2OH groep: hulle het ofwel dieselfde absolute konfigurasies (R,R of S,S) (α), ofwel die teenoorgestelde absolute configurasies (R,S or S,R) (β).[2]

Monomere sakkariedmolekule kan verdeel word:

  • volgens ketting lengte
    • n=3: triose
    • n=4: tetrose
    • n=5: pentose
    • n=6: heksose
    • n=7: heptose
  • volgens funksionele groep
    • aldose met 'n aldehied-groep, byvoorbeeld glukose, 'n aldoheksose
    • ketose met 'n ketoon-groep, byvoorbeeld fruktose, 'n ketoheksose

Die pentoses en heksose kan lineêre kettings of ringe vorm. Die ringvorming tree op deurdat die karbonielgroep van 'n molekuul reageer met 'n hidroksielgroep op dieselfde molekuul en daarby 'n sikliese hemiasetaal of hemiketaal vorm.

Die ringsluiting kan tot verskeie stereo-isomere (anomere) lei. Hierdie verskil is baie belangrik vir die vorming van polimere sakkariede. Die produk van hierdie ringsluitingsreaksie word 'n piranose genoem indien n=6. Byvoorbeeld D-Glukose vorm α- of β-D-glukopiranose. Indien n=5 word dit 'n furanose genoem. Hierdie reaksie is 'n ewewig en die β-vorm is ~63% teenwoordig teenoor ~36% vir di α-vorm.[3]

Disakkariede

Dimere molekule van twee monosakkariede, wat met 'n α of β-1,4-glikosiedskakel verbind is.

  • homodimere: twee dieselfde monosakkariede
  • heterodimere: twee verskillende dimere, soos sukrose = fruktose+glukose

Oligosakkariede

In die natuur kom ook kort kettings van 3 or 4 sakkaried molekule voor.

Polisakkariede

  • Stysel
    • Onvertakte ketting met alfa-skakel: amilose
    • Vertakte ketting met alfa-skakel: amilopektien (plantstysel), glikogeen (dierlike stysel)
  • Sellulose - onvertakte ketting met beta-skakels

Verwysings

  1. ↑ brooklyn.liu.
  2. ↑ Carolyn R Bertozzi. "Structural Basis of Glycan Diversity - Essentials of Glycobiology - NCBI Bookshelf". nih.gov (in Engels). Geargiveer vanaf die oorspronklike op 19 Mei 2020.
  3. ↑ "Purdue" (in Engels). Geargiveer vanaf die oorspronklike op 30 Oktober 2019. Besoek op 5 Maart 2017.

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