other sugars Flashcards
composition of sucrose
glucose + fructose with a1,2 bond
composition of lactose
glucose + galactose with b1,4 bond
what type of glycosidase is a-amylase?
endoglycosidase = hydrolyzes internal a1,4 bonds to yield smaller sugar units (dextrins)
following action of pancreatic a-amylase, what remains?
maltose (a1,4)
isomaltose (a1,6)
limit dextrins (3-8 sugars with branching)
maltotriose- trioses+other oligosaccharides
what breaks down the products of pancreatic a-amylase?
glycosidases in the intestinal brush boarder
what are the 2 a-glucosidases?
- sucrose-isomaltase complex
- glucoamylase
where us the sucrose-isomaltase complex found? activity?
found most in the jejunum, sucrose subunit breaks sucrose into fructose and glucose, isomaltase subunit hydrolyzes a1,6 bonds; both enzymes are active on a1,4 bonds
what is glucamylase?
2 subunits, both exonucleases, activity increases along the length of the small intestine
what does b-galactosidase breakdown?
b1,4 bonds of lactose
where is b-galactosidase located?
mainly jejunum
what is the basis of the hydrogen breath test?
if lactose is not digested by b-galactosidase, then in will be converted to CH4 and H2 by intestinal flora, excess presence of H2 in the breath after lactose load suggests lactose intolerance
what are a-glucosidase inhibitors?
DM med that reversibly inhibits enzyme to slow digestion and absorption of sugars and reduce post-prandial blood sugar spikes
examples of a-glucosidase inhibitors
acrabose, migitol
what contains trehalose?
insect blood, algae, fungi
consequences of trehalase deficiency
illness after consumption of larger amount of edible mushrooms
how does gut flora contribute to our caloric extraction from foods?
bacterial flora degrade soluble fiber, leads to products of gas/water and some short chain fatty acids, short chain FA then absorbed and carried to liver
type of bond in cellulose
b1,4
2 general ways monosaccharides are taken up by cells
1- Na/glucose cotransporter
2-faciltated diffusion
GLUT1
on most cells, low Km, transports glucose and galactose
GLUT2
on liver/pancreatic b cells, high Km, transports glucose/galactose/fructose
GLUT3
on neurons, placenta and testes, transport glucose and galactose
GLUT4
on skeletal mm and fat cells, activated by insulin, specific for glucose only
GLUT5
mucosa of small intestine and sperm, specific for fructose
enzyme that converts fructose to fructose-1-phosphate
fructokinase
enzyme that converts fructose-1-phosphate to DHAP
aldolase B
why is aldolase B rate limiting in fructose digestion?
has lower affinity for fructose-1-P than fructose-1,6-bisphophate (in glycolysis), thus fructose will accumulate in the liver after excess ingestion
hereditary fructose intolerance
aldolase B deficiency caused by missense mutation in exon 5 that converts G to C and causes Ala to Pro. can cause hepatic/renal failure, hypoglycemia due to accumulation of fructose-1-P and inhibition of glycogenolysis/gluconeogenesis, also causes phosphate depletion
what is activated when phosphate levels are low?
adenosine deaminase = AMP to IMP = IMP to uric acid = gout
essential fructosuria
def of fructokinase, more benign, fructose excreted in urine
what are “polyols”?
sugar alcohols
enzyme that forms polyols
aldose reductase
polyols formed from glucose, galactose
glucose- sortibol
galactose- galactiol
what is a consequence of accumulated polyols?
cataracts, esp in hyperglycemic pts
what enzyme forms fructose from sorbitol?
sorbitol dehydrogenase
how is UDP-glucose formed?
reaction of glucose-1-P + UTP –> UDP-glucose + PPi by UDP-pyrophsophorylase
what are the uses of UDP-glucuronate (4)
- drug excretion
- use in GAGs
- conjugation of bilirubin
- re-entry into glucose metabolism
what enzyme is responsible for the conjugation of bilirubin?
UDP-gluconyltransferase
what is lactose synthesized from?
UDP-galactose and glucose
how is UDP-galactose formed?
from UDP-glucose with UDP-4-glucose-epimerase
structure of lactose synthase
2 subunits- galactotransferase and a-lactalbumin (responsive to prolactin to increase activity of galactotransferase)
2 enzymes in galactose metabolism
1- galactokinase
2- galactose-1-P uritidyltransferase
classical galactosemia
deficiency of galactose-1-P uritidyltransferase leads to excess of galactose-1-P= mental retardation, vomiting, jaundice, cataracts
why does classical galactosemia cause jaundice?
high levels of galactose-1-P inhibit formation of UDP-glucose
dec UDP glucose = dec UDP-glucaronate = dec bili excretion = jaundice
what is non-classical galactosemia?
def of galactokinase, galatose accumulates and is excreted
what can result from both forms of galactosemia?
cataracts from excessive galactitol (more likely to form cataracts in this dz than from hyperglycemia)
