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week 10 - gastrointestinal > carbohydrate metabolism > Flashcards

carbohydrate metabolism Flashcards

1
Q

ATP

A
  • adenosine triphosphate
  • energy fro anabolic precesses is provided by the hydrolysis of ATP

ATP+ energy liberation= ADP + Pi
ADP + Pi + atp resynthesis= ATP

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2
Q

replenishing ATP

A

– creatine phosphate (muscle – short term)
– anaerobic metabolism of CHO to lactate
-aerobic metabolism of CHO, fat and/or protein ( in mitochondria)

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3
Q

tissue dependent on glucose

A
  • erythrocytes: no mitochondria, therefore cannot oxidise fuels, only ATP from glycolysis
  • brain: fatty acids cannot cross blood- brain barrier
  • rate of tap production from fatty acids is too slow/requires too much oxygen
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4
Q

carbs in diet

A
  • polysacchrides: starch nd cellulose
  • dissacharides: maltose, sucrose, lactose
  • monosaccharides: glucose and fructose
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5
Q

digestion of carbs

A

Digestibility of starch varies with properties of food

Some starches slowly digested

*trapped in intact starch granules/plant cell wall structure
(e.g. raw cereals, vegetables)

*resistant to amylase as 3D structure too tightly packed
(some processed foods, raw/cold potato)

*associated with dietary fibre - slows absorption/digestion as increases gut content viscosity (e.g. beans/legumes)

  • CHO foods containing high levels of fat may have delayed gastric emptying
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6
Q

carbs homeostasis

A
  • Glucose is transport from of CHO in humans Glycogen is storage form of CHO in humans
  • Some tissues, including the brain and erythrocytes are dependent on the constant supply of glucose
  • Plasma glucose concentration is tightly regulated and maintained between
    concentrations of 4-5 mM (in fasted state). Can raise to 8-12 mM after a meal.
  • The principal regulator of glucose homeostasis are the hormones insulin and glucagon
  • glucose can be synthesised de novo by the liver or kidney to plasma glucose
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7
Q

glucose transport into tissues

A

-* Glucose transport into cells requires transporter proteins. Cannot simply diffuse into cells.

*Transported down concentration gradient by facilitated diffusion – GLUT1- GLUT14. (GLUT1-5 well-characterised)

*Transported against concentration gradient using energy provided by
cotransport of sodium (SGLT1 and 2). Required in intestine to absorb from gut lumen and kidney, to reabsorb from filtrate.

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8
Q

glucose transporters, SGLT-1/SGLT-2

A

site- intestinal mucosa, kidney tubules
characteristics- Co-transport one molecule of glucose or galactose along with sodium ions. Do not transport fructose.

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9
Q

glucose transporters, GLUT-1

A

site- ubiquitous
characteristics- Transports glucose (high affinity) and galactose, not
fructose.

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10
Q

GLUT-2

A

site- liver, small intestine, kidney
characteristics- Transports glucose, galactose and fructose. A low
affinity, high capacity glucose transporter

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11
Q

GLUT-3

A

site- brain, placenta and testes
characteristics- Transports glucose (high affinity) and galactose, not
fructose. The primary glucose transporter for
neurons.

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12
Q

GLUT-4

A

site- skeletal and cardiac muscle, adipocytes

characteristics- The insulin-responsive glucose transporter. High
affinity for glucose.

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13
Q

GLUT-5

A

site- small intestine, sperm
characteristics- transports fructose, but not glucose or galactose

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14
Q

insulin responsive GLUT-4

A

-Found in adipose & muscle, therefore, more glucose transported in (& converted to triglyceride or glycogen) when plasma [glucose] is raised after a meal because of increases insulin
- In muscle, GLUT4 translocates in response to physical activity/exercise (independent of insulin) therefore, more glucose used for ATP production

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15
Q

fate of glucose within a cell

A
  • production of ATP: * Glycolysis *TCA cycle & Oxidative Phosphorylation
  • storage as glycogen: glycogenesis
  • synthesis of sugars for RNA/DNA: pentose phosphate pathway
  • synthesis of other molecules: synthesis of triglycerides (lipogenesis), some amino
    acids, neurotransmitters etc.
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16
Q

phosphorylation of glucose

A
  • All pathways require phosphorylation of glucose to glucose-6-
    phosphate as first step
  • Phosphorylation traps glucose inside the cell, cannot be transported out.

Phosphorylation of glucose catalysed by hexokinases I-IV

17
Q

hexokinases I-IV

A
  • glucokinase (hexokinase IV) expressed by cells of pancreas and liver –has high KM
    –Enzyme synthesis regulated
    hexokinase I-III
    –expressed in all other tissues
    –has low KM
    –is inhibited by G6P (feedback inhibition)
18
Q

glycolysis

A
  • breakdown of glucose to yield energy
  • occurs in cytoplasm of all cells

Glucose + 2ADP + 2P+ 2NAD+
into
2 Pyruvate + 2ATP + 2NADH + 4H+

19
Q

glucose to glucose-6-phosphate

A

ATP into ADP
hexokinase/glucokinase

20
Q

glucose-6-phospahte to fructose-6-phosphate

A
  • phosphoglucose isomerase
21
Q

fructose-6-phosphate to fructose-1,6- bisphosphate

A

ATP- ADP
phosphofructokinase-1

22
Q

phosphofructokinase (PFK)

A
  • inhibited by ATP, citrate, downstream products
  • activity determines wether G6P from hexokinase/glucokinase used for glycolysis or other purposes
23
Q

glycolysis net gains and uses

A

Uses 2ATP and generates 4ATP and 2NADH

  • Net gain of 2ATP and 2NADH
  • Phosphofructokinase is the committed step for glycolysis
  • Under anaerobic conditions get lactate formation (no
    further ATP produced
  • Under aerobic conditions, NADH can be used to make more ATP in mitochondria
24
Q

fructose and galactose

A

fructose: converted to Fructose-1-phosphate (F1P) by fructokinase *F1P is converted into DHAP and Glyceraldehyde-3- phosphate
*DHAP and Glyceraldehyde 3-phosphate are intermediates of glycolysis

galactose: converted to Fructose-1-phosphate (F1P) by fructokinase *F1P is converted into DHAP and Glyceraldehyde-3- phosphate
*DHAP and Glyceraldehyde 3-phosphate are intermediates of glycolysis

25
Q

fates of pyruvate

A
  • lactate (anaerobic conditions)
  • pyruvate —> acetyl CoA
  • occurs in mitochondria by pyruvate dehydrogenase
  • Produce CO2 and NADH + H+
    Aerobic
    Acetyl-CoA can be used in the TCA cycle
26
Q

lactate production

A

pyruvate into lactate
NADH-NAD
by lactate dehydrogenase
- 2 ATP from every glucose

27
Q

citric acid cycle

A

TCA/KREBS

3 NAD+ + FAD + GDP + Pi +acetyl-CoA
into
3 NADH + FADH2 + GTP + CoA + 2 CO2

28
Q

pentose phosphate pathway

A

importnat for synthesis:
- NADPH
- required for generation of lipids by reductive biosynthesis
- required for reduction of glutathione (antioxidant)
- ribose 5-phosphate, formation of nucleotides (DNA/RNA)
- dehydrogenation of glucose 6-phosphate is the committed step

29
Q

glycogen

A

-synthesized from glucose
- liver: storage fro blood glucose maintenance
- muscle: storage for local energy production (only used by muscle itself)

30
Q

hormonal regulation of glycogen synthesis

A
  • glucagon (liver) and (nor) adrenaline (liver and muscles)
    –Increase glycogen phosphorylase activity rapidly
    –Inhibit glycogen synthase activity rapidly

-insulin (liver and muscle)
–Increase glycogen synthase activity rapidly
–Inhibit glycogen phosphorylase activity rapidly

31
Q

gluconeogenesis

A
  • the synthesis of glucose from a noncarbohydrate (nonhexose) source.
    –Lactate
    –Pyruvate
    –Glycerol
    –Certain amino acids
32
Q

gluconeogenesis in liver

A
  • occurs mainly in the liver
  • kidneys can contribute with prolonged starvation
  • essentially a reversal of glycolysis ( hexokinase/glucokinase, PFK, pyruvate kinase not reversible)
33
Q

glucose 6-phosphatase

A
  • only expressed at high levels in liver
  • therefore, only liver can convert glucose-6-phosphate back into glucose
  • glucose can then be transported by GLUT2 out of hepatocyte into the blood
  • allows maintenance of blood glucose levels in the fasted state
34
Q

substrates for glycogenesis

A

lactate: converted to pyruvate first
glycerol: converted to DHAP (dihydroxyacetone phosphate), intermediate in glycolysis
amino acids- converted into pyruvate or various in TCA cycle intermediates

week 10 - gastrointestinal (5 decks)

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