Metabolism - Stages 1 and 2 of Carb metabolism Flashcards
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.
What is the LDH reaction?
NADH + H+ + pyruvate —–> lactate + NAD+
Uses enzyme lactate dehydrogenase
Where is lactate produced and what increases levels?
Produced by RBCs and skeletal muscle. The levels increase during exercise and in certain conditions such as shock.
How does the heart and liver utilise lactate?
They both contain LDH to convert lactate back to pyruvate.
Heart oxidises pyruvate to CO2, the liver converts it into glucose.
What are the features of hyperlactaemia?
2-5mM
Below renal threshold.
What are the features of lactic acidosis?
> 5mM
Above renal threshold
How is fructose metabolised?
Fructose is converted to F1P by fructokinase, then into G3P by aldolase.
What happens if fructose are is missing?
No clinical signs
Fructose present in urine
What happens if aldolase is missing?
Build up of F1P in the liver causes damage by scarring. The liver then struggles to break down bilirubin = jaundice.
How is galactose metabolised?
Galactose is converted to galactose-1-phosphate by galactokinase, then into glucose-1-phosphate by galactose transferase. This is then converted to G6P.
The galactose-1-phosphate can also be converted into UDP galactose, then UDP glucose by epimerase.
What happens if galactokinase is missing?
Only galactose builds up.
It is converted to galacticol by aldose reductase using NADPH. This depletes NADPH stores, so disulphide bridges can form, and in the eyes, this causes cataracts.
What happens if galactose transferase is missing?
G1P and galactose build up.
Galactose is converted to galacticol by aldose reductase using NADPH. This depletes NADPH stores, so disulphide bridges can form, and in the eyes, this causes cataracts.
Also, the liver becomes scarred by the build up of G1P = can’t break down bilirubin and jaundice occurs as a result.
What are some basics about monosaccharides?
Hydrophilic
Partially oxidised
3-9 C atoms long (usually 3, 5 or 6)
Undergo condensation reactions to form disaccharides.
