fructose/galactose Flashcards
what C are fructose and galactose phosphorylated on?
C1
major dietary sugar
glucose
2nd most common sugar
fructose
fructose comes from what disaccharide?
sucrose
sucrose = ___+____
glucose + fructose
lactose = ___+___
glucose + galactose
in liver, enters glycolytic pathway at
glucose –>?
fructose –>?
galactose–>?
glucose –> g6p
fructose –> DHAP and G3p (glyceraldehyde)
galactose –> glucose 1 phosphate –> g6p
where does glycerol enter glycolysis?
triglycerides break down into FA and glycerol
glycerol is phosphoryylated and enters at DHAP
2 sources of dietary fructose
1 disaccharide sucrose (fruits and sugar)
2 monosaccharide fructose corn syrup
fructose becomes what in fasting condition and well fed?
glucose in fasting times and pyruvate in fed state
special about fructose: enters glycolysis after what?
after control point PFK1
so it can rapidly produce lactate or FA
what sugar is useful in sports drinks and why?
fructose
because of its conversion to glucose and liver glycogen
max uptake rate is 60 g/hr = 240 cal/hr –> limited by glucose transporters in gut
but fructose enters by a different transporter so up to 336 cal/hr with fructose+glucose
two things aldolase B does?
cleaves F1P in fructose pathway and F16BP in glycolytic path
fructose metabolism
- phosphorylated by fructokinase to F1P (uses 1 ATP)
- F1P is cleaved by aldolase B –>DHAP and glyceraldehyde
- glyceraldehyde is phosphorylated by triose kinase (glyceraldehyde kinase) to G3P (uses 1 ATP)
in fructose metabolism, which step is fast and which is rate limiting enzyme?
fructose to F1P is rapid
and aldolase B is rate limiting
fructose metabolism in non-liver tissues
fructose is phosphorylated by hexokinase to F6P (but hexokinase has very low affinity for it)
what do sperm use as major energy source?
fructose
polyol pathway
glucose to fructose (makes it for sperm energy)
glucose is reduced to sorbitol by aldose reductase (aldehyde to alcohol)
sorbitol is then reoxidized at C2 by sorbitol dehydrogenase to make fructose
what causes cataracts?
elevated glucose or galactose –> sugar alcohols concentrate in lens –> increased osmotic pressure
non lethal fructokinase deficiency
essential fructosuria
essential fructosuria
deficient fructokinase
fructose is metabolized very slowly by hexokinase (low affinity); blood fructose levels rise and fructose appears in urine
no renal threshhold for fructose
HFI
hereditary fructose intolerance
serious fructose metabolism disorder?
HFI - hereditary fructose intolerance
HFI - hereditary fructose intolerance
defective hepatic aldolase B (liver isozyme), but aldolase B still functions in glycolysis to cleave F16BP
since aldolase B in liver is the only enzyme that can cleave F1P, F1P accumulates and inhibits glucose production –> severe hypoglycemia is fructose is ingested
symptoms: hepatomegaly, failure to thrive –> avoid dietary fructose!
major source of galactose?
disaccharide lactose (milk products)
infants have greater ability to metabolize what?
galactose
galactose to glucose metabolism
- galactose is phosphorylated by galactokinase to galactose 1 phosphate (uses 1 atp)
- galactose 1p reacts with UDP glucose to make glucose 1 phosphate and UDP galactose (via enzyme galactose 1 phosphate uriylyl transferase)
- UDP galactose is epimerized to UDP glucose by UDP glucose epimerase (reversible)
.
repeat steps to convert galactose to UDP glucose and g1p
net: galactose + atp –> glucose1P + adp
reversible step in galactose metabolism?
epimerase (step 3)
glucose 1 P from galactose breakdown can go on to…
form g6p and enter glycolysis
which step in galactose metabolism is irreversible?
phosphorylation of galactose by galactokinase to galactose1phosphate (uses an ATP)
net equation for galactose to glucose 1 P
galactose + atp –> glucose1P + adp
hit locations in pathwayy for nonclassical and classical galactosemia
nonclassical –> galactokinase
classical –> galactose 1 phosphate uridylyltransferase
galactose free diet - path from glucose to UDP galactose
glucose to g6p via glucokinase
g6p to g1p
g1p to UDP glucose (UTp used, PPi released; via UDP glucose phosphorylase)
UDP glucose to UDP galactose via epimerase
lactose intolerance
low level of lactase
unabsorbed lactose is turned into lactic acid by colonic bacteria and osmotic effects lead to diarrhea and dehydration
low lactase means…
lactase deficiency
galactosemia
high concentraton of galactose in blood
two types of galactosemia
galactokinase deficiency
uridylyl transferase defciency
what happens in galactosemia?
galactose accumulates and is reduced to galactitol;
cataracts from accumulation in lens
mental retardation from accumulation in brain
hepatomegaly
failure to thrive
if galactose is ingested –> hypoglycemia
avoid dietary galactose (milk products and artificial sweeteners)
uridylyl transferase deficiency
more serious type of galactosemia
elevates galactose 1 phosphate and inhibits phosphoglucomutase – interferes with glycogen synthesis and degradation
two ways to make UDP galactose
-from galactose OR glucose via UDP glucose and an epimerase rxn
what three products are made from UDP galactose?
glycoproteins
glycolipids
proteoglycans
what enzyme adds galactose to growing polysaccharide chains?
galactosyl transferase
whatenzyme reduces galactose to galactitol?
aldose reductase
how does lactating mammary gland use UDP galactose?
UDP galactose + glucose –> milk lactose
alpha-lactalbumin binds to galactosyl transferase and lowers its Km for glucose so that glucose adds galactose
entry of glycerol into glycolysis
glycerol –> glycerol phosphate (via glycerol kinase; uses ATP)
glycerol phosphate –> DHAP (via glycerol 3 phosphate dehydrogenase; NAD+–>NADH)
