Thema 1: koolhydraatmetabolisme

Kennis en inzicht verkrijgen in:

o (regulering van) het glucose-transport (GLUT1, 2, 3 en 4),

o de functie van de glycolyse in verschillende celtypen, o de hiërarchie van regelmechanismen in de glycolyse,

o de novo synthese van glucose, i.e. de gluconeogenese (substraten, specifieke enzymen, cellulaire en sub- cellulaire lokalisatie),

o (regulering van) het glycogeenmetabolisme,

o de werking van de pancreas-hormonen [insuline/glucagon] en de bijnier-hormonen [(nor)-adrenaline]

o de secretie van insuline door een β-cel in de pancreas

Na afloop van dit thema kun je:

o Precies de regel-punten en -mechanismen van de glycolyse, gluconeogenese, glycogeen-afbraak en glycogeen-opbouw aangeven,

o Het begrip glucostase uitleggen,

o Globaal uitleggen hoe het (koolhydraat-)metabolisme hormonaal wordt aangestuurd,

o De verschillende functies van glycogeen in spier- en lever-weefsel uitleggen.

HC1: introductie & metabologica

metabole kaart mag je bij tt gebruiken - wat staat er op wat je wel moet weten:
- namen vd metabole routes & enzymen
- regulering vd paden & enzymen

namen metabole routes

naamgeving enzymen

hoofdregels regulering

2 tegenovergestelde paden lopen niet tegelijk
snelheid glycolyse, krebs, oxfos wordt mede bepaald door ATP behoefte

HC2: regulering koolhydraatmetabolisme

1 overzicht regulering koolhydraat metabolisme

glucose heeft meerdere metabole mogelijkheden:
- glycolyse, KC, oxfos → E productie
- PPP → NADPH + ribose-5-P (voor antioxidanten, biosynthese lipides & DNA/RNA)
- opslag voor slechte tijden als
  - glycogeen in lever & spier
  - vet (via acetyl-CoA → FA → TAG)
hoge glucose conc → glucose kan worden opgenomen, gebruikt voor energie of opgeslagen - welke optie verschilt per orgaan!
al deze processen worden geregeld door de hormonen insuline & glucagon
- bij hoge glucose conc in bloed: insuline (uit beta cellen)
- bij lage glucose conc in bloed: glucagon (uit alpha cellen)

2 insuline afgifte

3 glucose tranporters (GLUT)

4 regulering glycogeenmetabolisme

5 regulering glycoluse & gluconeogenese (F-2,6-BP)

e-module: Glucostase

LOs

effects of insulin & glucagon on a cellular level (1)
relationships between organs regarding blood glucose levels and the roles of insulin & glucagon (2)
effects of rising & dropping glucose levels and reactions of different organs (2)
pathophysiology of glucostatic disorders diabetes mellitus & insulinoma (3)

0. introductie

0.1 properties insulin & glucagon

insulin & glucagon are peptide hormones with antagonistic functions - produced by cells in islets of Langerhans in endocrine pancreas
- after meal → blood glucose rises → insulin (produced in β-cells) released into bloodstream → lowers blood glucose by bringing it into tissues
- after fasting/exercise → blood glucose drops → glucagon (produced in α-cells) released → increases blood glucose by bringing it from liver into blood
insulin is an anabolic hormone = its effect is a net tissue buildup & energy supply formation
- high insulin in blood signals there’s high energy (glucose or TAG) →
  - surplus E must be stored as glycogen fat or protein
    - insulin has receptors in multiple organs so E can be stored
  - E store breakdown must be stopped
glucagon is a catabolic hormone = its effect is a net tissue breakdown & energy supply use
- high glucagon signals energy shortage (vnl glucose)
  - brings glucose into bloodstream by
    - breaking down E supplies
    - producing new glucose from dif precursors
peptide hormones so → receptors on outside of cell - binding causes signaling cascade
- insulin binds → activation protein/lipid kinase cascade → dephos of target p
- glucagon binds (G-p coupled) → production cAMP → activates PKA → phos target p
both secreted on basal level → both always present in blood
- → speak of insulin/glucagon ration ipv presence/absence
  - blood glu high →insulin high = INS/GCN-ratio high
  - glucagon high = INS/GCN-ratio low

0.2 metabolism

insulin & glucagon also have opposite effects on metabolism
- insulin → glycolyis & formation glycogen and fat
- glucagon → gluconeogenesis & breakdown glycogen and fat

catabolism

cell can make E (ATP) via dif routes - from glucose, FA or AA
breakdown of glucose (glycolysis)
- glucose in cytoplasm → 2x pyruvate + 2x ATP
- pyruvate transported to mitochondria and irreversibly converted by pyruvate dehydrogenase (PDH)→ acetyl-CoA
breakdown of FA (β-oxidation)
- FA transported from cytoplasm to mitochondria and via β-oxidation broken down into multiple acetyl-CoA
in Kreb’s cycle 1x acetyl-CoA broken down into 2x CO2 + e- acceptors: 3x NAD+ & 1x FAD (can both accept 2 e-)
- they give their e- away in e- transport chain to build up H+ gradient - used for ATP formation

anabolism

formation of glucose (gluconeogenesis)
- (in mitochondrium) pyruvate → oxaloacetate
- oxaloacetate transported to cytosol (via malate) and converted into phosphoenolpyruvate (PEP)
- PEP then follows glycolysis backwards til → glucose (in liver) or glucose 6-phosphate (in other cells)
formation of FA (lipogenesis) & TAG synthesis
- acetyl-CoA transported from mitochondrium (via citrate) to cytosol and converted → malonyl-CoA
- both acetyl-CoA & malonyl-CoA put onto fatty acid synthase (FAS)
  - multiple cycles til chain is 16C long, then released from FAS
- the FA can then be connected to a glycerol phosphate and w 2 more FA form triacylglycerol (TAG)

buildup and break down of glycogen

once activated via phosphorylation, glucose can go into different pathways
1) glycolysis
2) pentose phosphate pathway: glucose → ribose-5-P
- used to make nucleotides (for DNA & RNA) & NADPH (reducing power for biosynthesis)
3) glucose-6-P can be built into glycogen for storage
- glucose-6-P → glucose-1-P
- glycogen synthase adds glucose-1-P to the growing chain
  - branching enzymes cause the branch formation in the glycogen molecule → fast break down & buildup glycogen
glycogen is broken down into glucose-1-P by glycogen phosphorylase
- branches broken by debranching enzymes (bc gly phosphorylase can’t)
  - → some free glucose released - needs to be activated into glucose-6-P before it can be broken down via glycolysis
- glucose-1-P can then be converted → glucose-6-P

0.3 metabolic map

0.4 summary

rising blood glucose → insulin released → lowers blood glucose
- by stimulating glycolysis & synthesis of glycogen, FA, p
- has inhibitory effect on lipolysis in adipocytes
dropping blood glucose → glucagon released → raises blood glucose
- by stimulating breakdown TAG and glycogen & stimulates gluconeogenesis
- has inhibitory effects on glycolysis in liver

1. glucostasis on cellular level

1.1 introduction: cell types & GLUT

every organ has a different role in maintaining stable blood glucose levels
- endocrine pancreas: β-cells secrete insulin, α-cells secrete glucagon
- liver: store glycogen (oiv insulin) or release glu into blood (oiv glucagon and adrenaline)
- muscle: store and use glycogen (oiv insulin) - no glucagon rec
- adipocytes: take in glucose (oiv insulin) & convert to glycerol-3-P to store FA by making TAG. TAG breakdown (oiv glucagon) → FA release as alt E source so brain can use glucose
- neurons: brain only uses glucose as E source
  - exceptional circ (bv prolonged fasting) → partial switch to ketone bodies as E source - uptake depends on conc in blood NOT hormonally regulated
  - use of glucose (or ketone bodies) NOT hormonally regulated
glucose taken into cell by GLUT - there’s dif isotypes
- GLUT1: standard GLUT, normal affinity, in every cell.
  - ensures basal & continuous uptake → cell always enough E
- GLUT2: low affinity → uptake only if (relatively) high blood glucose
  - in pancreatic β-cells (measure blood glucose here to adjust their insulin secretion) & liver cells
- GLUT3: high affinity → uptake even is (relatively) low blood glucose - daarom in brain cells (bc glucose only E source)
- GLUT4: normal affinity, only muscle & adipocytes. insulin-regulated!
  - high insulin → incr GLUT4 in memb → cell takes in more glu
  - insulin conc dcr → GLUT4 taken up by cell (wait in vesicle til insulin rec triggered again)

1.2 pancreas

pancreas is blood glucose meter of the body → hormone release adjustment
pancreatic β-cells have GLUT2 → can incr insulin (if blood glu high)
insulin secretion is directly regulated by blood glucose level
- glucose enters cell → metabolized via glycolysis → ATP
- incr ATP/ADP ratio → K+ channel closes
- dcr K+ efflux → change memb potential → opens Ca2+ channel
- Ca2+ influx → granules w insulin fuse to plasma memb → insulin into blood

1.3 muscle

insulin (minorly) & adrenaline affect glucose metabolism in muscle cells

1.3.1 glycolysis & gluconeogenesis

barely any enzymes for gluconeogenesis in muscle → almost no gluconeogenesis
glycolysis regulated to meet ATP needs - 3 control sites:
- main 1) phosphofructokinase (PFK): turns fructose-6-P → fructose 1,6-bisP
  - ATP inhibits PFK, AMP reverses inhibition → adjusts speed to E consumption
- 2) hexokinase: glucose → glucose-6-P (1st step glycolysis)
  - PFK inhibition → glucose-6-P accumulates & inhibits hexokinase → dcr glucose into glycolysis
- 3) pyruvate kinase (PK): dephos of PEP → pyruvate (last step)
  - ATP inhibits PK

1.3.2 pyruvate dehydrogenase (PDH)

PDH converts pyruvate → acetyl-CoA, links glycolysis to Krebs cycle
- important bc acetyl-CoA can’t be converted back into glucose!
PDH regulated in 2 main ways
- 1) allosteric interaction of acetyl-CoA & NADH
  - PDH is inhibited by its products: acetyl-CoA, NADH & ATP
    - when high conc, they form a signal that E needs are met and no more glucose has to be broken down
- 2) phosphorylation: PDH kinase phosphorylates PDH which inactivates it
muscle cell at rest: ATP/ADP ratio high → directly inhibits PDH + stims PDH kinase →phosphorylates PDH so inactive PDH
muscle cell contraction: ATP/ADP ratio drops (ATP used) & glycogen/glucose converted to pyruvate → ADP + pyruvate inhibit PDH kinase → stimulate PDH
- also phosphatase (stimulated by Ca2+ which also initiates contraction) is activated to dephosphorylate PDH

1.3.3 glycogen metabolism

when muscle cell needs ATP → glycogen phosphorylase activation, so glycogen can be broken down → glucose-6-P → ATP
- contraction → incr ATP → incr AMP → stim glycogen phosphorylase
- before AMP can stimulate it, it has to be activated by phosphorylase kinase which must be activated in 2 ways:
  - Ca2+ binds to camodulin subunits of phosphorylase kinase
  - phosphorylation of it by PKA
    - PKA is stimulated by adrenalin

when insulin binds to muscle cell it has 2 main effects
- 1) more GLUT4 put into membrane → more glucose in
- 2) activation of protein phosphatase (PP1)
  - inactivates phosphorylase kinase & glycogen phosphorylase → inhibition glycogen breakdown
  - activates glycogen synthase → stims glycogen synthesis
adrenaline has a similar double-sides effect by activating PKA which:
- 1) activates glycogen phosphorylase → stims glycogen breakdown
- 2) inactivates glycogen synthase → inhibits glycogen breakdown

1.4 liver

ATP & Ca2+ fluctuates less than in muscles cells so less of an effect
- instead dif molecules with reg effects on the various enzymes

1.4.1 glycolysis

glycolysis speed hormonally regulated (as response to blood glucose) by insulin & glucagon at 2? regulation points:
- main 1) phosphofructokinase (PFK): turns fructose-6-P → fructose 1,6-bisP
  - ATP & citrate inhibit PFK
- 2) hexokinase: glucose → glucose-6-P (1st step glycolysis)
  - PFK inhibition → glucose-6-P accumulates & inhibits hexokinase → dcr glucose into glycolysis
- 3) pyruvate kinase (PK): dephos of PEP → pyruvate (last step)
  - ATP & alanine inhibit PK

1.4.2 gluconeogenesis

1.4.3 pyruvate dehydrogenase

1.4.4 glycogen metabolism

1.4.5 ketogenesis

1.5 adipose tissue

1.6 questions