Download Chapter 27 biochem notes [10-2

Chapter 27 – Digestion, absorption, and transport of Carbohydrates
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IV.
Introduction
A. Starches amylose and amylopectin are broken apart by α-amylase to αdextrins, then pancreatic α-amylase breaks the dextrins to maltose,
maltotriose, and limit dextrins (which have a branch call an
isomaltose branch (two glucosyl residues).
B. They are further digested by disaccharidases on the membranes surfaces of
the brush border (microvilli) of the intestine.
C. Amylose – glucosyl units bonded by α-1,4 bonds straight
D. Amylopectin – glucosyl units bonded by α-1,4 bonds with branches via α1,6 bonds.
E. Sucrose – glucose and fructose disaccharide
F. Fiber – made of plant polysaccharides and lignan
Digestion of Dietary Carbohydrates
A. α-amylase is an endoglucosidase, it hydrolyzes internal α-1,4 bonds at
random points and can not do α-1,6 bonds or 1,4 at the nonreducing end
B. Glucoamylase – an exoglucosidase that starts at the nonreducing end and
breaks off glucose residues at α-1,4 bonds until it hits a α-1,6 branch
C. Sucrase-Isomaltase Complex – very similar to glucoamylase in structure,
it gets inserted with roughly the same structure but is then cleaved by an
intestinal protease. This is 100% responsible for the hydrolysis of sucrose.
It has a strong maltse activity as well breaking apart α-1,6 bonds.
D. Trehalase – it breaks apart trehalose, a disaccharide of two glucosyl units
linked by an α bond at their anomeric carbons, this is in insects, algae,
mushrooms, and not a major dietary component in the USA. People can be
deficient of this enzyme and get nauseous, vomit, and GI distress
E. ß-glycosidase complex (lactase-glucosylceramidase) – has 2 catalytic
sites, it breaks ß- bonds in lactose (glucose and galactose) at 1 site, the
other is the ß- bonds between glucose and galactose and ceramide in
glycolipids.
F. Sucrase-isomaltase is high in the jejunum as well as ß-glycosidase
activity.
G. Glucoamylase is high in the ileum.
Metabolism of Sugar by Colonic Bacteria
A. Beans are high in amylose, they make it to the colon and are digested by
bacteria making propionic acid, acetic acid, and butyric acid, as well as
gases (H2, Methane) (flatulence)
B. This is also the breath test to measure H2 that is exhaled. If high, it
means sugars are reaching the colon to be digested by the bacteria
and not by normal intestinal cells.
Lactose Intolerance
A. Infants have a high lactase activity, as an adult we only have 10% of
theirs and is called hypolactasia. In places where they drink milk all the
time Western Northern Europeans and Saharan Africa, their lactase
activity is just as high as infants
V.
VI.
VII.
B. Kwashiorkor (protein malnutrition), colitis, gastroenteritis, tropical
and nontropical sprue, and EtOH, damage the intestine and cause
secondary lactose intolerance. Other disaccharidases can go but this is the
first to go and the last to come back.
Dietary Fiber
A. Soluble fiber – pectins, mucilages, and gums
B. Insoluble fiber – cellulose, hemicellulose, and lignins
C. Should take 25 – 38g of fiber, we get 10% of our calories from bacteria in
our digestive tract
Absorption of Sugars
A. Some foods have a high glycemic index (absorbed rapidly) best eaten
after exercise (wheat, crackers) and those with low are best taken before
exercise (yogurt, skim milk, pasta)
B. Beans, peas, soybeans, have 1,6 linked galactose that can’t be digested
until it hits the colon and bacteria use them
C. Glucose absorbed in cells by facilitated diffusion and Na-dependant
facilitated transport
D. Na-dependant uses the low intracellular sodium level to pull glucose in,
the low intracellular sodium is kept that way by the Na, K, ATP pump
E. Facilitated is done using the GLUT 1-5 proteins (all have 12 membrane
spaning domains)
F. GLUT 1 – blood-brain barrier, RBCs, and placenta (high affinity)
G. GLUT 2 – Liver, kidney, pancreatic ß-cells, Intestinal mucosa
H. GLUT 3 – neurons
I. GLUT 4 – adipose, skeletal, and heart
J. GLUT 5 – intestinal epithelium and spermatozoa (actually a fructose
transporter!!!!!)
K. Know the above for sure, galactose and fructose go through the glucose
transporters as well
L. RBCs GLUT 1 – has such a strong affinity for glucose that even low
levels in blood don’t effect how much it can take up. It also is 5% of the
proteins in its membrane (there’s a lot of it basically)
M. In the liver, GLUT 2 has a high Km (low affinity) which makes sense
because it can make its own glucose (gluconeogenesis)
N. Insulin activates the GLUT 4 (very very important) in adipose tissue,
promoting the uptake of glucose for storage and making fats (want to stop
gaining weight, block this step and you will make billions of dollars)
O. GLUT 1 in the blood-brain barrier is not strong enough when the blood
levels drop below 80 mg/dL and this causes coma, dizziness, and lightheadedness.
The fun stuff
A. Commercial Milk has been subjected to lactase activity with a 70%
reduction in lactose before we drink it.
B. People who are lactose intolerant, can eat some lactose because our
bacteria flora increases and helps to handle the load.
C. Also, a lot of drugs are packaged with lactose and this can cause
problems for these people
D. Cholera – diarrhea caused by Vibrio Cholerae, fecal-oral route, it binds to
the brush border and secrets an exotoxin that binds irreversibly to GM1
ganglioside receptor on the cell curface. It causes cAMP levels to rise in
the cell, it causes the cells to decrease its absorption of NA, anions, and
water from the gut, while stimulating the crypt cells to secrete chloride
and cations and water from the bloodstream into the lumen of the gut.
Causing extreme diarrhea exceeding 1L/hour. Treatment involves
increased glucose and Na+ by mouth as well as antibiotics.
Just so you know, incase you didn’t or forgot, an Alpha bond is a bond between the
hydroxyl group of two sugars that are either both in the up or down configuration.
A Beta Bond is a bond when one OH is up and the other is down. This is lactose