Metabolism - Stages 1 and 2 of Carb metabolism Flashcards
(36 cards)
What do monosaccharides contain?
Many OH groups
An aldehyde or keto group (Aldose/ketose sugar)
What is the empirical formula for carbohydrates?
(CH2O)n
What does saliva do to carbohydrates?
It contains glycosidase enzymes (e.g. amylase) which hydrolyses polysaccharides such as starch and glycogen into products such as glucose, maltose and dextrins.
What does the pancreas produce in relation to carbohydrate metabolism?
Pancreatic amylase for the small intestine.
Where does digestion of carbohydrates occur in the small intestine?
The jejeunum and duodenum.
How are polysaccharides hydrolyses into monosaccharides in the small intestine?
Enzymes such as lactase, sucrase and pancreatic amylase are in the brush border of the epithelial cells of the small intestine. These hydrolyse the glycosidic bonds to produce monosaccharides.
How are monosaccharides taken up by the small intestine?
Glucose and galactose by cotransporter with sodium (requires energy)
Fructose by GLUT 5 (no energy use)
Why is cellulose not hydrolysed?
It’s monomers are held by beta1-4 glycosidic bonds. We don’t contain the enzymes which digest these.
Why is the non digestion of cellulose helpful?
It provides a bulk in the gut = more s.a. for other food to stick on and be hydrolysed by enzymes.
Give the 6 basic steps of glycolysis and the link reaction.
1) . Glucose converted to G6P by hexokinase (or glucokinase in the liver) using ATP
2) . G6P undergoes numerous reactions, involving a step which requires ATP and the enzyme phosphofructokinase to convert F6P to F1,6bisphosphate.
3) . F16BP is converted to DHAP and 2 G3P
4) . G3P converted to pyruvate by a number of reactions producing 4ATP and 2NADH, the last step also requires pyruvate kinase.
5) . Pyruvate is decarboxylated by pyruvate dehydrogenase to give acetyl CoA.
What conformation do carbohydrates have?
Boat or chair and are always stereoisomers.
How do monosaccharides appear if they have 5 or more carbons?
Ring structures - double bond O reacts with OH group = hemiacetyl.
Can form an alpha or beta form of the carb as the carbon is chiral.
What are the features of glycolysis?
Can function anaerobically Exergonic Oxidative Irreversible Cytoplasmic
Which steps are irreversible and what does this mean?
They are committing steps (committing the glucose to glycolysis) and occur in steps 1, 3 and 10.
What is the pentose phosphate pathway?
G6P is converted to 5C sugar phosphates and CO2 by G6P dehydrogenase, which can be converted into G3P.
What are the features of the pentose phosphate pathway?
Irreversible due to loss of CO2
Requires no ATP
Cytoplasmic
What are the functions of the pentose phosphate pathway?
1). Produces NADPH
= biosynthesis reducing power
= prevents formation of disulphide bonds
2). Produce 5C sugars
= used to produce nucleotides
What occurs in G6P dehydrogenase deficiency?
Less NADPH can be produced so disulphide bonds begin to form.this causes proteins such as RBCs to aggregate, forming Heinz bodies. This leads to haemolytic as the spleen recognises the RBCs as damaged, and breaks them down.
How is hexokinase regulated?
Inhibited by high levels of G6P
How is pyruvate kinase regulated?
By covalent modification.
1). Dephosphorylated by a high insulin to glucagon ratio = activated.
2). Phosphorylation inhibits it.
How is PFK regulated?
Allosterically (2 sites that measure ATP and AMP).
1) . High ATP inhibits PFK in muscle.
2) . High AMP stimulates PFK in liver.
Also stimulated by high insulin to glucagon ratio and inhibited by citrate when the body is in starvation.
What is PDH and how is it regulated?
It is a multienzyme complex controlled by regulation-
Activated by dephosphorylation = pyruvate, ADP, NAD+, insulin.
Inhibited by phosphorylation = NADPH, ATP, acetylCoA
Where does the link reaction occur?
The mitochondrial matrix.
What occurs in PDH deficiency?
Lactic acidosis is a result as there is not enough energy produced by krebs reaction (link is almost non functional) so anaerobic respiration takes over. As this uses lots of NAD+, it needs regenerating by LDH, so lactate is produced.
