Carbohydrates metabolism Flashcards
(46 cards)
steps of digestion of carbohydrates
(1) Digestion of carbohydrate in the form of starch or glycogen starts in
the mouth by action of salivary amylase that partially digests them
into dextrins and few moles of maltose.
(2) In the stomach no digestion occurs for carbohydrate due to gastric
acidity.
(3) In the small intestine pancreatic amylase completely digests starch
and glycogen into maltose.
(4) The brush border of intestinal mucosa secretes enzymes maltase,
lactase and sucrase that split the disaccharides as follows:
Maltose maltase
2 glucose
Lactose lactase
glucose and galactose
Sucrose sucrase
glucose and fructose
enumerate non digestible carbohydrates
They include the dietery cellulose,
hemicellulose and pectins which are made of glucose unites linked by β1,4 glucosidic linkage. In humans, there is no β-1-4 glucosidase that can
digest such bond
define lactose intolerance
This is a deficiency of lactase enzyme, present in brush border of
enterocytes, which digests lactose into glucose and galactose.
b) It may be:
i. Congenital that occurs very soon after birth (rare).
ii. Acquired that occurs later on life (common).
effects of lactose intolerance
he presence of lactose in intestine causes:
a) Increased osmotic pressure: Water will be drawn from the tissue
(causing dehydration) into the large intestine (causing diarrhea).
b) Increased fermentation of lactose by bacteria with subsequent
production of CO2 gas that causes distention & abdominal cramps.
inherited sucrase deficiency
This is a deficiency of sucrase enzyme, present in brush
border of enterocytes, which digests sucrose into glucose and fructose. It
occurs later after few months of birth when sucrose is added to infant diet.
effects of inherited sucrase deficiency
The presence of sucrose in intestine cause abdominal
distension and colic
fate of glucose in tissues
- Oxidation:
a) Glycolysis followed by Kreb’s cycle are the major pathways.
b) Pentose phosphate pathway and uronic acid pathway are minor
pathways. - Storage in the form of glycogen and fat.
- Conversion to substances of biological importance for e.g.
a) Ribose, deoxyribose used for synthesis of RNA and DNA.
b) Lactose of milk.
c) Glucosamine and galactosamine of mucopolysaccharides.
d) Glucuronic acid glycosaminoglycans and mucopolysaccharides.
e) Fructose in semen.
Carriers of glucose
- Uptake by liver: Glucose uptake is affected by GLUT 2 insulin
independent transporters. Galactose and fructose are converted into
glucose - Glucose uptake in adipose tissue and skeletal muscles is affected by
GLUT 4 insulin dependent transporters. - RBCs use GLUT 1 transporters for glucose uptake. Glucose reaches
brain cells through GLUT 3 insulin independent transpporters.
define glycolysis
Glycolysis means oxidation of glucose to give pyruvate (in
the presence of oxygen) or lactate (in the absence of oxygen) with net gain of
8 or 2 ATPs moles respectively.
Location of glycolysis
- Organ location: it ocurrs in all tissue cells, but it is of physiological importance
in:
❖ Tissues with no mitochondria e.g., mature RBCs.
❖ Tissues with few mitochondria e.g., testes and leucocytes.
❖ Tissues that undergo frequent oxygen lack e.g., skeletal muscles especially
during exercise. - Cellular location: it ocurrs in the cytosol.
effects of pyruvate kinase deficiency
leads to hemolytic anemia
mention inhibtors of glyceraldehydye 3 - phosphate
Arsenate and iodoacetate
mention inhibitors of enolase enzyme
fluoride irreversibly inhibts enolase
Fate of pyruvate in aerobic conditions
In aerobic conditions: Under that condition pyruvate is transported from
cytoplasm into the mitochondria by symport mechanism involving one
proton which is co-transported. Inside the mitochondria, pyruvate acts as
substrate for two enzymes, pyruvate dehydrogenase complex and
pyruvate carboxylase producing acetyl CoA and oxaloacetate respectively
for Kreb’s cycle initiation.
fate of pyruvate in anaerobic conditions
In anaerobic conditions: Pyruvate is reduced to lactate-by-lactate
dehydrogenase enzyme (LDH) with reoxidation of NADH+H to NAD+
.
- Lactate formation is an obligatory end product of the glycolytic pathway in
red blood cells, parts of the retina, and in skeletal muscle cells during
strenuous exercise.
importance of lactate formation by lactate dehydrogenase from pyruvate
1-Under anaerobic conditions NADH is re-oxidized. This allows glycolysis
to proceed in the absence of oxygen. The process generates enough NAD
needed for glyceraldehyde 3- phosphate dehydrogenase for another cycle of
glycolysis.
2- Wash out excess pyruvate which cannot pass membranes
energy produced under anaerboic condition
- 2ATPs are gained by substrate level phosphorylation via the reaction
catalysed by 1,3 diphosphoglycerate kinase. - 2ATPs are gained at substrate level via the reaction catalyzed by
pyurvate kinase - 2ATPs are lost for the phosphorylation of glucose by hexokinase
enzymes and for phosphorylation of fructose 6-phosphate by
phosphofructokinase -1. - 2ATPs are the net gain under anaerobic condition.
energy production under aerboic conditions
- 2ATPs are gained at substrate level via reaction catalyzed by 1,3
diphosphoglycerate kinase. - 2ATPs are gained at substrate level via reaction catalyzed by pyurvate
kinase. - Shuttling of the two cysolic NADH+ +H+
to the mitochondria and their
subsequent oxidation via ETC will produce 6 or 4 ATP molecules if
malate or glycerol phosphate shuttles are used respectively. - 2ATPs are lost for the phosphorylation of glucose and fructose-6-P
- 8 or 6 molecules of ATPs are the net gain under aerobic condition
when does substrate level phosphrylation occur in glycolysis
Substrate Level Phosphorylation in glycolysis ocurrs at the level of:
** Phosphoglycerate kinase [1,3 bisphosphoglycerate to 3 phosphoglycerates]
** Pyruvate kinase [phosphoenol pyruvate to pyruvate].
importance of glycolysis
- Important source of energy (8 ATPs or 2 ATPs under aerobic or
anaerobic). - It is the main route for glucose metabolism.
- It is an important metabolic pathway for fructose and galactose.
- It connects between carbohydrate, lipid and protein metabolism.
- Its reversibility is important for gluconeogenesis.
- Production of important metabolic intermediates e.g:
* Dihydroxy acetone phosphate (DAHP) which gives glycerol 3-
phosphate that is used for synthesis of triacylglycerol and phospholipids.
* 3 phosphoglycerates can give serine.
give an account of short term regulation of glycolytic pathway
1- Hexokinase is allosterically inhibited by glucose-6-P (feedback
inhibition).
2- Phosphofructokinase-1 is the rate limiting enzyme of glycolysis and is:
* Allosterically activated by AMP (low energy signal) & fructose 2,6
bisphosphate
* Allosterically inhibited by ATP and citrate (high energy signals).
{Pasteur effect}
3- Pyruvate kinase is:
* Allosterically activated by fructose 1,6 bisphosphate.
* Allosterically inhibited by ATP.
* Regulated by phosphorylation and dephosphorylation (covalent
modification). The active form is dephosphorylated.
effect of fructose 2,6 biphosphate
Fructose 2,6 bisphosphate stimulates
glycolysis by allosteric stimulation of phosphofructokinase-1. It also inhibits
gluconeogenesis by inhibiting fructose 1,6 bisphosphatase enzyme. This
substrate is produced from fructose-6-phosphate by phosphofructokinase-2
(PFK-2). This enzyme is a bifunctional enzyme that has two catalytic sites;
one of them acts as a kinase that converts fructose-6-phosphate into
fructose 2,6 bisphosphate. The other acts as phosphatase converting
fructose 2,6 bisphosphate into fructose-6-phosphate.
* Insulin activates the kinase moity of the enzyme while the phosphatase
is activated by glucagon.
define pasteur effect
It means that aerobic oxidation of glucose via Kreb`s cycle inhibits the
anaerobic degradation of glucose via glycolysis and consequently
inhibiting phosphofructokinase-1 causing accumulation of G6P with
consequent allosetric inhibition of hexokinase. Low AMP level & high
citrte concenteration are the causes.
give an account of long term regulation of glycolytic pathway ( effect of hormones )
a) Insulin:
It is secreted after meal in response to high blood glucose level. It
stimulates synthesis of all key enzymes of glycolysis except hexokinase
which is a constitutive enzyme.
Also, it stimulates pyruvate kinase by dephosphorylation.
b) Glucagon: It is secreted in response to low blood glucose level. It inhibits
the synthesis of all key enzymes of glycolysis except hexokinase. Also, it
inhibits pyruvate kinase by phosphorylation of the enzyme protein
