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MCAT - BIOCHEMICAL PATHWAYS SUMMARY

Terms in this set (22)

Glucose enters the cell by either diffusion or active transport. Because the GLUT transporters are specific for glucose (not phosphorylated glucose) the
Occurs in cytosol

Products:

Requires initial input of 2 ATP per glucose molecule, regenerated in subsequent steps.

Generates 2 NADH and 2 ATP per glucose. Also generates 2 pyruvate, 2H2O, and 2H+ per glucose.

Important intermediates:

Dihydroxyacetone Phosphate (DHAP)
• Used in hepatic and adipose tissue for triacylglycerol synthesis. DHAP is formed from F16BP. It can be isomerized to glycerol 3-phosphate, which can then be converted to glycerol, the backbone of triacylglycerols

1,3-Bisphosphoglycerate (1,3-BPG) and phosphoenolpyruvate (PEP)
• High energy intermediates used to generate ATP by substrate-level phosphorylation. This is the only ATP generated in anaerobic respiraton.

Important enzymes:

Glucokinase/Hexokinase (step 1) Irreversible
• Glucokinase is found only in liver cells and pancreatic beta-islet cells (along with GLUT2, acts as the glucose sensor)
•Hexokinase is widely distributed in tissues. Require Mg2+ cofactor
• Hexokinase is FBI by its product, G6P
• Glucokinase is inducible by insulin
• Hexokinase has a low Km (reaches maximum velocity at low [glucose]) (Hexokinase has a higher affinity for glucose than glucokinase)
• Glucokinase has high Km (acts on glucose proportionally to its concentration) (Glucokinase has a higher vmax for glucose than hexokinase)


PFK-1 [Rate Limiting Step] (step 1) irreversible
• Inhibited by ATP, citrate (Allosterically), PEP (substrate of pyruvate kinase catalyzed reaction.
• Citrate is an intermediate of the TCA cycle; makes sense that if there's a buildup it means that the process needs to slow down a little.
• Stimulated by AMP and ADP (AMP more).
• Stimulated by F26BP

• F26 BP is synthesized by PFK2 [PFK2 is found mostly in the liver] from F6P; under conditions of high insulin to glucagon ratio, insulin dephosphorylates (via PPA) PFK2, production of F26BP continues. Glucagon causes PKA to phosphorylate PFK2, allowing for its FBPase2 domain to become active. This allows the cell to override the inhibition caused by ATP so that glycolysis can continue, even when the cell is energetically satisfied. This way the metabolites of glycolysis can continue to be fed into anabolic reactions. F26BP regulation becomes significant at very high levels of F6P. When there's too much F6P floating around, F26BP helps to kick PFK1 into high gear.

Glyceraldehyde-3-phosphate dehydrogenase (step 6)
reversible
• Generates 1 NADH
• Catalyzes an oxidation and addition of inorganic phosphate to its substrate, GAP. This results in the production of a high energy intermediate, 1,3-bisposphoglycerate and the reduction of NAD+ to NADH. If glycolysis is aerobic, the NADH can be oxidized (indirectly) by the mitochondrial electron transport chain, [GLYCEROL-3-phosphate-dehydrogenase) providing energy for ATP synthesis by oxidative phosphorylation. Gives the electrons to FAD. 1.5 ATP per NADH.

3-Phosphoglycerate kinase (step 7) reversible
• Substrate level phosphorylation
• Forms ATP and 3-phosphoglycerate
• Not O2 dependent

Pyruvate kinase (Step 10) irreversible
• Substrate level phosphorylation
• Generates 1 ATP and Pyruvate from OAA
• Hormonally and allosterically regulated
• Activated by F16BP from the PFK1 reaction [feed-forward activation]
• Inhibited by high energy charge; ATP acts as a negative modifier
• Inhibited by alanine
• Stimulated by PEP (its substrate)
• Stimulated by F16BP (you can only get effective metabolism if PFK1 and pyruvate kinase are working together well, F16BP is the connecting chain. The product of the control step is the positive modifier. If PFK1 slows down, then pyruvate kinase is going to slow down. PEP also inhibits PFK1.

• LIVER pyruvate kinase is hormonally regulated (glucagon and epinephrine and insulin?)
• Low I:G ratio = glucagon predominates. Glucagon initiates signalling cascade, PKA phosphorylates pyruvate kinase. Make it less active. Glycolysis in liver decreases during fasting.
• High I:G ratio = insulin predominates. Insulin stimulates protein phosphatase 1 to dephosphorylate pyruvate kinase. Helps to ensure that pyruvate kinase is not active at the same time as pyruvate carboxylase or PEP carboxykinase.
Glycogen synthesis and degradation occurs primarily in liver and skeletal muscle, but in smaller quantities in other tissues.

Liver glycogen is broken down to maintain a constant level of glucose in the blood.

Muscle glycogen is broken down to provide glucose to the muscle during vigorous exercise.

Starts with a core protein called glycogenin. Glucose addition to a granule begins with G6P, which is converted to G1P. This G1P is activated by coupling to a molecule of uridine diphosphate (UDP), which permits its integration into the glycogen chain by glycogen synthase. This activation occurs when G1P interacts with UTP, forming UDP-glucose and a pyrophosphate (PPi).

Glycogen synthase is the rate-limiting enzyme of glycogen synthesis and it forms the alpha-1,4 glycosidic bond found in the linear glucose chains of the granule. it is stimulated by G6P and insulin. It is inhibited by epinephrine and glucagon through a protein kinase cascade that phosphorylates and inactivates the enzyme

Branching enzyme (Glycosyl alpha-1,4:alpha-1,6 transferase): responsible for introducing alpha-1,6-linked branches into the granule as it grows. Branching enzyme hydrolyzes one of the alpha-1,4 bonds to release a bond of oligoglucose (a few glucose molecules bound together in a chain), which is then moved and added in a slightly different location. It then forms an alpha-1,6 bond to create a branch.

Glycogen synthase is stimulated by G6P and insulin. Glycogen synthase is inhibited by epinephrine and glucagon through a protein kinase cascade that phosphorylates and inactivates the enzyme.


Glycogen synthase and branching enzyme are stimulated by insulin in liver and muscle.
The liver maintains glucose levels in blood during fasting through either glycogenolysis or gluconeogenesis. The kidney can also carry out gluconeogenesis, but its effect is much smaller. These pathways are promoted by glucagon and epinephrine and inhibited by insulin.

During fasting, glycogen reserves drop dramatically in the first 12 hours, during which time gluconeogenesis increases. After 24 hours it represents the sole source of glucose.

Source of acetyl coa for gluconeogenesis MUST come from fatty acid oxidation; if it came from glycolysis, it would just burn the glucose that is being generated in gluconeogenesis

Important substrates for gluconeogenesis are:
• Glycerol-3-phosphate (from stored fats, or triacylglycerols, in adipose tissue)
• Lactate (from anaerobic glycolysis)
• Glucogenic amino acids (from muscle proteins)

Amino acids can be subclassified as glucogenic, ketogenic, or both. Glucogenic amino acids (all except leucine and lysine) can be convtered into intermediates that feed into gluconeogenesis, while ketogenic amino acids can be converted into ketone bodies, which can be used as an alternative fuel, particularly during periods of prolonged starvation.

Pyruvate carboxylase
activated by acetyl coa from beta-oxidation
produces OAA (TCA intermediate)
OAA cant leave the mitochondrion, is reduced to malate, which can leave the mitochondria via the malate-aspartate shuttle. Once in the cytoplasm, malate is oxidized to OAA. Acetyl coa inhibits pyruvate dehydrogenase and activates pyruvate carboxylase. Acetyl coa here will come from oxidation of beta-fatty acids.

phosphoenolpyruvate carboxykinase
• in the cytoplasm, induced by glucagon and cortisol.
• converts OAA to PEP in a rxn that requires GTP.


fructose-1,6-bisphosphatase
• in the cytoplasm, a key control point of gluconeogenesis and represents the rate limiting step of the process. It reverses the action of PFK1, the rate-limiting step of glycolysis, by hydrolyzing phosphate from F16BP to produce F6P.
• Activated by ATP and inhibited by AMP and F26BP.

glucose-6-phosphatase
• Found only in the lumen of the endoplasmic reticulum in liver cells.
• G6P is transported into the ER, and free glucose is transported back into the cytoplasm, from where it can diffuse out of the cell using GLUT transporters.
• The absence of glucose-6-phosphatase in skeletal muscle means that muscle glycogen cannot be serve as a source of blood glucose, and rather is for use only within the muscle.
• Although alanine is the major glucogenic amino acid, almost all amino acids are also glucogenic. Most of these are converted by individual pathways to citric acid cycle intermediates, then to malate, following the same path from there to glucose.

Glucose produced by hepatic gluconeogensis does not represent an energy source for the liver. Gluconeogenesis requires expenditure of ATP that is provided by beta-oxidation of fatty acids. Therefore, as mentioned above, hepatic gluconeogenesis is always dependent on beta-oxidation of fatty acids in the liver. During periods of low blood sugar, adipose tissue releases these fatty acids by breaking down triacylglycerols to glycerol (which can also be converted to the gluconeogenic intermediate DHAP) and free fatty acids.

Although the acetyl-CoA from fatty acids cannot be converted into glucose, it can be converted into ketone bodies as an alternative fuel for cells, including the brain. Extended periods of low blood sugar are thus usally accompanied by high levels of ketones in the blood. Ketone bodies can be thought of as transportable form of acetyl coa that is primarily utilized in periods of extended starvation.