Carbohydrate - digestion and storage Flashcards
What are the carbohydrates in our diet?
Starch
Glycogen
Meat (however when the animal dies enzyme activity in the tissue degrades much of the glycogen stores)
Cellulose and hemicellulose
Plant cell walls – we don’t digest this
Oligosaccharides containing (α1→6) linked galactose
Peas, beans, lentils – not digested
Lactose, sucrose, maltose
Milk, table sugar, beer
Glucose, fructose
Fruit, honey
How are carbohydrates digested?
-mouth
salivary amylase hydrolyses
(alpha1 to 4) bonds of starch
stomach- no carb digestion
duodenum- pancreatic amylase works like mouth
small intestine-Final digestion by mucosal cell-surface enzymes
What do the different mucosal cell surface enzymes do?
Isomaltase – hydrolyses (α1→6) bonds
Glucoamylase – removes Glc sequentially from non-reducing ends
Sucrase – hydrolyses sucrose
Lactase – hydrolyses lactose
What are the main products that gut will actually absorb?
Glucose , galactose and fructose
Absorption of glucose?
Glucose is absorbed through an indirect ATP-powered process
ATP-driven Na+ pump maintains low cellular [Na+], so glucose can continually be moved in to the epithelial cells
This system continues to work even if glucose has to be moved into the epithelial cells against it’s concentration gradient (i.e. When blood glucose is high)
Absorption of galactose ?
Galactose has a similar mode of absorption as glucose, utilising gradients to facilitate it’s transport
Absorption of fructose?
Fructose is slightly different,
Binds to the channel protein GLUT5
Simply moves down it’s concentration gradient (high in gut lumen, low in blood)
What are the uses of cellulose and hemicellulose?
increase faecal bulk and decreases transit time
better gut health
cancer of gut if food left for too long in gut
What breaks down polymers?
Gut bacteria and yields CH4 and H2
Dissacharidase deficiencies causes?
Deficiencies may be genetic#
Or, they can result from,
Severe intestinal infection
Other inflammation of the gut lining
Drugs injuring the gut wall
Surgical removal of the intestine
symptoms of dissacharidase deficiences?
abdominal distension and cramps
what are the symptoms of someone with disaccharidase deficiency for e.g. lacking lactase activity?
Undigested lactose is broken down by gut bacteria causing gas build up and irritant acids
Lactose is osmotically active, thus drawing water from the gut into the lumen causing diarrhoea
Fate of absorbed glucose?
Glc diffuses through the intestinal epithelium cells into the portal blood and on to the liver
Glc is immediately phosphorylated into glucose 6-phosphate by the hepatocytes (or any other cell glucose enters)
Glucose 6-phosphate cannot diffuse out of the cell because GLUT transporters won’t recognise it
This effectively traps the glucose in the cell
Characteristics of glucokinase?
in liver
High K m for glucose (low affinity for glucose)
High V max (efficient enzyme)
Characteristics of hexokinase?
in other tissues
Low K m (high affinity for glucose)
Low V max (not as efficient enzyme)
Why does glucokinase in liver have low affinity for glucose?
Blood [Glc] normal – the liver doesn’t “grab” all of the glucose, so other tissues have it
What happens when blood glucose is high after a meal?
Blood [Glc] high (after meal) - liver “grabs” the Glc as it has a high V max
What does a high glucokinase V max mean for glucose?
High glucokinase Vmax means it can phosphorylate all that Glc quickly, thus most absorbed Glc is trapped in the liver
Why does hexokinase have a high affinity for glucose?
Hexokinase low Km means even at low [Glc] tissues can “grab” Glc effectively
Why does hexokinase have a low enzyme efficiency compared to glucokinase?
Hexokinase low Vmax means tissues are “easily satisfied”, so don’t keep “grabbing” Glc
Where is most glycogen stored?
90% in liver and skeletal muscle
What happens in liver - if blood glucose drops?
-Glycogen broken down into glucose 6 phosphate
- enzyme glucose 6 phosphatase removes phosphate
-now glucose and can enter blood stream
What happens in skeletal muscle?
there is no glucose 6-phosphatase,
glycogen to G-6-P (Glycolosis) to lactate
what is von Gierke’s disease?
Liver (and kidney, intestine) glucose 6-phosphatase deficiency
High concentrations of glycogen in the liver
Low blood glucose levels
high blood lactate levels
