BIOC2600 1.2

• Dietary carbohydrates are absorbed by our body through the digestive system, starting in the mouth cavity where starch is broken down to amylose, and continuing in the gastrointestinal tract until the carbs are broken down into monosaccharides.
• Monosaccharides are absorbed and travel to the liver via portal blood circulation, while cellulose and other fibres remain undigested.
• Glucose plays a central role in metabolism in all organisms, serving as a rich source of potential energy and a precursor of metabolic intermediates for biosynthetic reactions to make amino acids, nucleotides, coenzymes and fatty acids.
• Glucose can be stored as polysaccharide or as sucrose, oxidised to a 3-carbon compound (Pyruvate) via glycolysis to provide ATP and metabolic intermediates, oxidised via the pentose-phosphate pathway to give ribose 5-phosphate for nucleic acid synthesis and NADPH, or used for synthesis of structural polymers.
• Glycolysis is the first step in glucose breakdown, converting glucose into 2 pyruvate, with various fates including 4 ATP used for energy-requiring processes within the cell, 2 NADH that must be reoxidized to NAD+ in order for glycolysis to continue, and 2 ATP used for the synthesis of structural polymers.
• Carbohydrates other than glucose are transformed into one of the glycolytic intermediates to enter glycolysis.
• Dephosphorylation of fructose 1,6 - phosphate is catalysed by another enzyme fructose - 1,6 - bisphosphatase, no ATP is needed.
• Reversal of the reaction would require the generation of ATP, which is energetically unfavourable.
• By using another enzyme, glucose - 6 - phosphatase, for the reverse reaction, no synthesis of ATP is required.
• Bypass 2: Conversion of Fructose 1,6 - phosphate to Fructose 6 - phosphate involves bypassing the phosphorylation of fructose - 6 - Phosphate to fructose 1,6 - Bisphosphate by phosphofructokinase - 1.
• Bypass 3: Conversion of glucose 6 - phosphate to glucose involves bypassing the reaction of glucose to glucose - 6 - phosphate by the enzyme hexokinase.
• In most animal tissues, the major catabolic fate of glucose-6-phosphate (G6P) is glycolysis to pyruvate, with G6P having other fates including the pentose phosphate pathway (PPP).
• The pentose phosphate pathway (PPP) is important because ribose-5-phosphate are made for the synthesis of nucleotides and NADPH is produced for providing reducing power for biosynthetic reactions.
• Whether G6P enters glycolysis or PPP depends on the current need of the cell or the concentration of NADPH.
• The reverse reaction of glycolysis is energetically unfeasible.
• Glucose is the universal fuel and building block, used by some tissues, such as the brain, for energy.
• Lactate is readily converted to pyruvate by lactate dehydrogenase.
• Glycolysis and gluconeogenesis are not identical pathways running in opposite directions.
• The supply of glucose from stores may not be sufficient.
• In mammals, gluconeogenesis takes place mainly in the liver.
• Major non-carbohydrate precursors are lactate, amino acids and glycerol.
• Bypassing the reaction of phosphoenolpyruvate to pyruvate is achieved by a sequence of reactions using enzymes in both the cytosol and mitochondria.
• Glucose has to be synthesised.
• The brain, nervous system, and red blood cells depend solely on glucose from the blood as fuel.
• The gluconeogenic pathway converts pyruvate to glucose.
• The pathway for bypassing the reaction of phosphoenolpyruvate to pyruvate mainly in anaerobic muscles with lactate produced after vigorous exercise.
• If all the enzymes are the same, the two pathways will be impossible to regulate.
• Three bypass reactions from pyruvate up are: conversion of pyruvate to phosphoenolpyruvate, conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, and conversion of glucose 6-phosphate to glucose.
• The glucose produced is supplied to other tissues via blood circulation.
• The predominant pathway for bypassing the reaction of phosphoenolpyruvate to pyruvate is in the liver.
• There are three irreversible steps in glycolysis.
• The equilibrium of glycolysis favours pyruvate formation.
• Bypass the irreversible glycolytic reactions.
• Non-carbohydrate precursors are converted to pyruvate or enter the pathway at later stages.