Outline the metabolic events for the conversion of glucose to pyruvate
The essence of glycolysis is that glucose is converted into two molecules of pyruvate in the cytoplasm of all cells, with no oxygen required. This is an oxidation reaction, that leads to the reduction of two NAD and the production of two molecules of ATP for every glucose molecule used.
Therefore, it is essential that there is enough NAD for glycolysis to continue. (This means that NADH+H+ needs to be recycled, and this happens by converting pyruvate (which is no longer needed in these anaerobic cells) into lactate, the reduction of which oxidises NADH+H+ back into NAD which can be used in further glycolysis reactions. Lactate leaves these cells (usually erythrocytes and muscle cells) and is converted back into pyruvate in the liver. This is the Cori Cycle)
HEXOKINASE or GLUCOKINASE converts D-glucose into glucose 6-phosphate -USES ATP IS IRREVERSIBLE, this phosphorylation is done to:
trap it within the cell
make start making it unstable
PHOSPHOGLUCOSE ISOMERASE converts the glucose 6-phosphate into fructose1,6-bisphosphate which further destabilizes it
*PHOSPHOFRUCTOKINASE 1 converts fructose 6-phosphate into fructose 1,6-bisphosphate – USES ATP, IS IRREVERSIBLE(this is also the rate-limiting step of glycolysis) (now there are two phosphate groups on a small molecule, this molecule is unstable and will have to break down)
ALDOLASE converts fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate which will form pyruvate after a lot more reactions
Basically, what to remember is that energy is used by kinases to phosphorylate and therefore destabilise molecules by adding negative charges of phosphate near each other.
It must be noted that you do need a OH group in order to add a phosphate (this becomes also important in recognizing where amino acids can be phosphorylated later – only in side chains with OH groups: serine, threonine and tyrosine).
Describe the substrate level phosphorylation reactions that result in the formation of ATP from ADP
Substrate-level phosphorylation (unlike oxidative phosphorylation) is the formation of ATP from ADP through the addition of a Pi group from another compound: in this case from
1,3-bisphosphoglycerate converted into 3-phosphoglycerate and ATP – phosphyglyxerate kinase
Phosphoenolypyruvate into pyruvate and ATP – pyruvate kinase
Note that these enzymes are called kinases, even though they are technically de-phosphorylating the organic molecules, they are still adding a phosphate group to ADP, so are therefore called kinases. Do not assume they will be phosporylating organic molecules when you see them!
Describe the processes for the regeneration of NAD+ from NADH under anaerobic conditions, and the role of lactate dehydrogenase in muscle As stated above: pyruvate + NADH + H+ → NAD+ + lactate (lactate dehydrogenase used)
In this way, NAD can be reduced again allowing the oxidation of further glucose – this reaction happens in erythrocytes and muscle. In the liver, lactate is converted back into pyruvate.
Indicate the major features of the control mechanisms associated with hexokinase, glucokinase, phosphofuctokinse and pyruvate kinase
Both convert glucose into glucose-6-phosphate, but:
Hexokinase – all tissues except liver and B pancreatic cells
It is not affected by insulin – so these tissues can carry out glycolysis even in times of low glucose and insulin in the blood
Inhibited by glucose-6-phosphate, so the more we glycolysis is done, the less hexokinase is activated
Glucokinase – only liver and B pancreatic cells
Effected by insulin – these tissues will not carry out glycolysis when levels of glucose are low in the blood
Not affected by glucose-6-phospate, so glycolysis can continue unaffected by the rate of glycolysis
Phosphofructokinase is the enzyme which converts fructose 6-phosphate into fructose 1,6-bisphosphate – it is the rate limiting step in glycolysis and is therefore very important. It is regulated by many things:
AMP (which shows that ATP levels are low)
Insulin (which shows that glucose levels are high)
Fructose 2,6-bisphosphate (allosteric promoter of PFK1)
ATP (there is enough ATP no more is needed)
Glucagon (there are low levels of glucose)
Low levels of glucose itself
Citrate (a product of the TCA cycle, which shows that no more of it is needed)
Note that all the things that promote PFK-1 inhibit gluconeogenesis, and all things that inhibit PFK-1 promote gluconeogenesis.
Summarise the roles of glycolysis in different tissues, eg red cells, liver, adipose tissue Glycolysis in red cells is the only source of ATP because these cells don’t have mitochondria.
Glycolysis in the liver is used to generate pyruvate, which is then used to synthesize fatty acids through lipogenesis.
Adipose tissue like the liver uses glucose to generate pyruvate, which is used to synthesise fatty acids in lipogenesis.
Indicate the entry points of fructose and galactose into the glycolysis pathway
Much of the ingested fructose is metabolised by the liver, using the fructose-1-phosphate pathway. So FRUCTOKINASE phosphorylates fructose into fructose-1-phosphate, which has to then be split into glyceraldehyde and dihydroxyacetone phosphate, glycolysis intermediate. Alternatively, fructose can be phosphorylated into fructose-6-phosphate by HEXOKINASE, but because the affinity of hexokinase for fructose is 20 times lower than that for glucose, little fructose-6-phosphate is formed in the liver this way.
There are no metabolic pathways to metabolise galactose, so it has to be converted into a metabolite of glucose. It is converted into glucose-6-phosphate in four steps. (galactokinase – galactose-1-phosphate, galactose-1-phosphate uridyl transferase – UDP-glucose, UDP-galactose-4-epimerase – glucose-1-phoshate, phosphoglucomutase – glucose-6-phosphate which can enter the normal glycolysis chain.