Glycolysis Flashcards
How is glycogen broken down?
1 → Glycogen phosphorylase uses free phosphate in solution to break the glycosidic bond and phosphorylate each product, forming 2 x glucose-1-phosphate.
2 → Phosphoglucomutase then moves the phosphate group, forming glucose-6-phosphate.
What are the steps in the energy investment stage of glycolysis?
1 → glucose is phosphorylated by hexokinase into glucose-6-phosphate. ATP hydrolysis drives this reaction forward, and this step is regulated.
2 → G-6-P is isomerised into fructose-6-phosphate by glucose phosphate isomerase, as this makes C1 easier to phosphorylate in the next step.
3 → F-6-P is phosphorylated by phosphofructokinase, forming fructose-1,6-bisphosphate. ATP hydrolysis also drives this step forward, and this the most important regulatory step. The enzyme is regulated allosterically by ATP inhibition.
How is 3-phosphoglycerate formed from fructose-1,6-bisphosphate in glycolysis?
1 → Aldolase splits F-1,6-BP into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (DHAP).
2 → DHAP is isomerised into G-3-P by triosephosphate isomerase, allowing both halves of the glucose molecules to be used.
3 → G-3-P is oxidised by G-3-P dehydrogenase, reducing NAD+ to NADH in the process. There is enough energy left over to attach one inorganic phosphate ion to form 1,3-bisphosphoglycerate.
4 → Phosphoglyceratekinase transfers one phosphate group from 1,3-BPG to ADP, forming ATP (via substrate level phosphorylation) and 3-phosphoglycerate.
How is pyruvate formed from 3-phosphoglycerate in glycolysis?
1 → Phosphoglycerate kinase moves the phosphate group from C1 to C2 on 3-phosphoglycerate, forming 2-phosphoglycerate. This moves the phosphate into a less stable position.
2 → Enolase dehydrates 2-PG, forming phosphoenolpyruvate. The phosphate is now in an even less stable position.
3 → Pyruvate kinase removes the phosphate group, forming enol-pyruvate; because the phosphate was in a very unstable position, this reaction also drives forward the substrate level phosphorylation of ADP. This enzyme is regulated because the reaction is highly exothermic.
4 → Enol-pyruvate spontaneously breaks down into pyruvate, meaning that this last step is not reversible.
How is NADH converted back into NAD+?
Glycolysis occurs in the cytosol, and the mitochondria can use O2 to convert NADH back into NAD+. Therefore, this process in aerobic.
Lactate fermentation → Lactate dehydrogenase drives the reduction of pyruvate into lactate, thereby oxidising NADH into NAD+.
All tissues in humans produce lactate, especially RBCs (no mitochondria) and exercising white muscle. The liver takes up lactate and uses it to produce glucose via gluconeogenesis, which requires energy.
Alcoholic fermentation → Pyruvate is decarboxylated by pyruvate decarboxylase, forming ethanal. This is then reduced by alcohol dehydrogenase, oxidising NADH to NAD+. This process occurs in yeast.
What is the yield of glycolysis?
Glycogen → 2x NADH, 2x pyruvate and 3x ATP (due to being converted into G-1-P without using up ATP)
All other sugars → 2x NADH, 2x pyruvate, 2x ATP
What happens to glycolysis in the fed state?
Brain → only can use glucose for fuel, so the rate of glycolysis stay the same.
Liver → First organ to receive glucose (due to blood vessel connecting it to the small intestine). Glycolysis increases, glycogenesis increases, and fat synthesis increases.
Adipose tissue → Glucose is taken up and broken down for fat synthesis.
Skeletal muscle → Glycolysis increases while fatty acid oxidation decreases.
What happens to glycolysis during exercise?
Brain → only uses glucose for fuel, so rate of glycolysis doesn’t change
Liver → Glycogen breakdown is stimulated, producing glucose which is released into the blood.
Skeletal muscle → Glucose is taken up from the blood and glycogen stores are broken down. Glycolysis increases, and lactate may be produced with intense exercise.
What happens to glycolysis during fasting?
Brain → only uses glucose for fuel, so rate of glycolysis doesn’t change
Liver → Glycogen is broken down and gluconeogenesis occurs in order to try and keep the blood glucose level constant.
Adipose tissue → Fats are broken down and the products are released into the blood so that tissues have an alternative fuel source to glucose.
Skeletal muscle → Glycolysis is inhibited and fats are used. Amino acids are also released into the blood
What happens to glycolysis during starvation?
Brain → Tries to use less glucose by using ketone bodies
Liver → Glycogen stores have run out. Some gluconeogenesis and ketone body formation occurs.
Kidneys → Gluconeogenesis helps to maintain glucose level
Adipose tissue → Fats are broken down and released into the blood to provide an alternative fuel source to glucose
Skeletal muscle → Glycolysis is inhibited and fats are broken down. Amino acids are released into the blood.
