carbohydrate metabolism 1/2 WF Flashcards
carbohydrates in a typical western diet
starch (polysaccharide) - 160g/day
sucrose (disaccharide) 120g/day
lactose (disaccharide) - 30g/day
glucose (monosaccharide) - 10g/day
carbohydrates in the diet - general points?
carbs meet up to 50% of energy requirement
free glucose/glycogen usually unimportant
all dietary carbs are convertible to glucose
there are no essential dietary sugars
glucose structure?
6 carbon sugar
ring structure when combined with other sugars or in solution, formed by a link between C5 and C1
starch structure?
composed of glucose units joined together
two components in starch - amylose, amylopectin
amylose structure?
formed by linking glucose units
unbranched chains
between C1 and C4 - alpha(1-4) link
amylopectin structure?
branched chains
alpha(1-4) and alpha(1-6) links (this forms a branch between otherwise straight chains)
enzymes involved in starch digestion? what is the general name for them?
alpha-amylase
glucoamylase
isomaltase
general name = alpha-glucosidase
alpha-amylase features
present in saliva - levels variable
also secreted by the pancreas into the duodenum
endoglycosidase - hydrolyses alpha(1-4) links
products are oligosaccharides
glucoamylase features?
present on luminal side of intestinal wall
hydrolyses alpha(1-6) link in isomaltose
alpha-glucosidase inhibitors action? what is this useful?
lower rate of starch digestion -> lower the rate of uptake of glucose by the intestine -> lowering blood glucose
useful in diabetics
dietary disaccharides?
maltose (from starch)
isomaltose (from starch)
lactose
sucrose
what is maltose hydrolysed to? by which enzyme? where?
maltose alpha(1-4) link hydrolysed by glucoamylase (intestinal). produces 2 glucose
what is isomaltose hydrolysed to? by which enzyme? where?
isomaltose alpha(1-6) links hydrolysed by isomaltose (intestinal). produces 2 glucose
what is lactose hydrolysed to? by which enzyme? where?
lactase (aka beta-galactosidase) hydrolyses lactose to galactose + glucose in the intestine
what is sucrose hydrolysed to? by which enzyme? where?
sucrase hydrolyses sucrose into glucose and fructose in the intestine
what is sucralose? structural differences to sucrose? affect on hydrolysis?
an artificial sweetener
hydroxyl groups replaced by chloride groups
cannot be hydrolysed by sucrase therefore excreted.
sucrose in the blood?
bad sign! not absorbed as sucrose, possibly entered blood via stomach ulcer.
how is free glucose taken up in the intestine?
secondary active transport:
sodium potassium pump on basolateral side uses ATP to pump Na+ out of cell creating a concentration gradient of Na+ into cell from apical side.
SGLT1 uptakes 2Na+, glucose.
how does glucose pass from the intestinal cells into the bloodstream
down a concentration gradient via the uniporter GLUT-2
diarrhoea treatment that uses glucose uptake?
oral rehydration therapy - glucose and salt water.
glucose promotes sodium uptake expanding the plasma
plasma fatty acid levels after feeding
decrease then gradually increase
Glucose transporters in humans?
GLUT1-5
SGLT1-2
GLUT1 tissue?
erythrocytes, placenta, brain
GLUT2 tissue? features?
liver, kidney, intestine, pancreas
high Km - uptake rate increases as blood glucose rises
GLUT3 tissue? features?
brain, testis
low Km - constant uptake rate, independent of blood glucose
GLUT4 tissue? features?
muscle, adipose, heart
insulin-responsive
GLUT5 tissue? features?
jejunum
specific for fructose
SGLT1 tissue? features?
duodenum, jejunum, kidney
symporter, 2Na+/glucose
high affinity, low capacity
SGLT2 tissue? features?
kidney
symporter, 1Na+/glucose
low affinity, high capacity
glucose phosphorylation in the liver?
designed to cope with high concentrations of glucose.
glucose phosphorylated to form glycose-6-phosphate by hexokinase (in most tissues) and glucokinase.
glucokinase has high Km, rate of phosphorylation can continue to increase in high glucose concentrations.
metabolism fates of glucose?
converted to glycogen in liver and muscle (G-6-P -> G-1-P -> UDP-glucose -> glycogen)
glycolytic pathway (G-6-P -> pyruvate/lactate)
pentose phosphate pathway (G-6-P -> 5-carbon sugars)
structure of glycogen
contains only glucose. glucose chains linked alpha(1-4) with occasional alpha(1-6) branches.
end with free C-1 = reducing end
end with C-4 = non-reducing ends (where glucose is added/removed)
UDP-glucose to glycogen?
glycogen synthase adds glucose units from UDP-glucose onto non-reducing ends of glycogen.
glycogen to G-1-P? what does this lead to?
glycogen phosphorylase allows Pi to split off glucose units from glycogen non-reducing ends forming G-1-P. this can isomerised to G-6-P and metabolised.
hormonal control of glycogen metabolism?
glycogen synthase activated by insulin, inhibited by adrenaline and glucagon.
glycogen phosphorylase activated by adrenaline and glucagon, inhibited by insulin
