carbohydrate metabolism 1/2 WF

Question 1

carbohydrates in a typical western diet

Answer 1

starch (polysaccharide) - 160g/day
sucrose (disaccharide) 120g/day
lactose (disaccharide) - 30g/day
glucose (monosaccharide) - 10g/day

Question 2

carbohydrates in the diet - general points?

Answer 2

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

Question 3

glucose structure?

Answer 3

6 carbon sugar
ring structure when combined with other sugars or in solution, formed by a link between C5 and C1

Question 4

starch structure?

Answer 4

composed of glucose units joined together
two components in starch - amylose, amylopectin

Question 5

amylose structure?

Answer 5

formed by linking glucose units
unbranched chains
between C1 and C4 - alpha(1-4) link

Question 6

amylopectin structure?

Answer 6

branched chains
alpha(1-4) and alpha(1-6) links (this forms a branch between otherwise straight chains)

Question 7

enzymes involved in starch digestion? what is the general name for them?

Answer 7

alpha-amylase
glucoamylase
isomaltase

general name = alpha-glucosidase

Question 8

alpha-amylase features

Answer 8

present in saliva - levels variable
also secreted by the pancreas into the duodenum
endoglycosidase - hydrolyses alpha(1-4) links
products are oligosaccharides

Question 9

glucoamylase features?

Answer 9

present on luminal side of intestinal wall
hydrolyses alpha(1-6) link in isomaltose

Question 10

alpha-glucosidase inhibitors action? what is this useful?

Answer 10

lower rate of starch digestion -> lower the rate of uptake of glucose by the intestine -> lowering blood glucose

useful in diabetics

Question 11

dietary disaccharides?

Answer 11

maltose (from starch)
isomaltose (from starch)
lactose
sucrose

Question 12

what is maltose hydrolysed to? by which enzyme? where?

Answer 12

maltose alpha(1-4) link hydrolysed by glucoamylase (intestinal). produces 2 glucose

Question 13

what is isomaltose hydrolysed to? by which enzyme? where?

Answer 13

isomaltose alpha(1-6) links hydrolysed by isomaltose (intestinal). produces 2 glucose

Question 14

what is lactose hydrolysed to? by which enzyme? where?

Answer 14

lactase (aka beta-galactosidase) hydrolyses lactose to galactose + glucose in the intestine

Question 15

what is sucrose hydrolysed to? by which enzyme? where?

Answer 15

sucrase hydrolyses sucrose into glucose and fructose in the intestine

Question 16

what is sucralose? structural differences to sucrose? affect on hydrolysis?

Answer 16

an artificial sweetener
hydroxyl groups replaced by chloride groups
cannot be hydrolysed by sucrase therefore excreted.

Question 17

sucrose in the blood?

Answer 17

bad sign! not absorbed as sucrose, possibly entered blood via stomach ulcer.

Question 18

how is free glucose taken up in the intestine?

Answer 18

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.

Question 19

how does glucose pass from the intestinal cells into the bloodstream

Answer 19

down a concentration gradient via the uniporter GLUT-2

Question 20

diarrhoea treatment that uses glucose uptake?

Answer 20

oral rehydration therapy - glucose and salt water.
glucose promotes sodium uptake expanding the plasma

Question 21

plasma fatty acid levels after feeding

Answer 21

decrease then gradually increase

Question 22

Glucose transporters in humans?

Answer 22

GLUT1-5
SGLT1-2

Question 23

GLUT1 tissue?

Answer 23

erythrocytes, placenta, brain

Question 24

GLUT2 tissue? features?

Answer 24

liver, kidney, intestine, pancreas

high Km - uptake rate increases as blood glucose rises

Question 25

GLUT3 tissue? features?

Answer 25

brain, testis

low Km - constant uptake rate, independent of blood glucose

Question 26

GLUT4 tissue? features?

Answer 26

muscle, adipose, heart

insulin-responsive

Question 27

GLUT5 tissue? features?

Answer 27

jejunum

specific for fructose

Question 28

SGLT1 tissue? features?

Answer 28

duodenum, jejunum, kidney

symporter, 2Na+/glucose
high affinity, low capacity

Question 29

SGLT2 tissue? features?

Answer 29

kidney

symporter, 1Na+/glucose
low affinity, high capacity

Question 30

glucose phosphorylation in the liver?

Answer 30

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.

Question 31

metabolism fates of glucose?

Answer 31

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)

Question 32

structure of glycogen

Answer 32

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)

Question 33

UDP-glucose to glycogen?

Answer 33

glycogen synthase adds glucose units from UDP-glucose onto non-reducing ends of glycogen.

Question 34

glycogen to G-1-P? what does this lead to?

Answer 34

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.

Question 35

hormonal control of glycogen metabolism?

Answer 35

glycogen synthase activated by insulin, inhibited by adrenaline and glucagon.

glycogen phosphorylase activated by adrenaline and glucagon, inhibited by insulin