Glucose Maintenance: Hexokinase & Glucokinase

Enzymes Involved in Glucose Maintenance

Hexokinase

• Catalyzes the reaction of glucose to glucose 6-phosphate, consuming ATP.
• Glycolytic enzyme that operates anaerobically, requiring ATP and Mg2+.
• Keeps glucose inside the cell, but is not present in the liver and pancreas.
• Functions as a transferase enzyme.
• Upregulated by low ATP levels and insulin when blood glucose levels are high.
• Downregulated by end product inhibition from G6P (mixed inhibition).
• Converts fructose to F6P in muscle fructose metabolism, acting as a side reaction.

Glucokinase

• Catalyzes the reaction of glucose to glucose 6-phosphate (-ATP).
• Glycolytic enzyme that operates anaerobically, requiring ATP and Mg2+.
• Keeps glucose inside the cell, and is only active in the liver and pancreas.
• Functions as a Transferase reaction
• Upregulated by low ATP levels.
• Regulation bidirectionally controlled by insulin when blood glucose levels are high.
• Regulation does not involve end product inhibition.
• Fructose 1-phosphate binds to GKRP, causing conformational changes that promote glucose binding to glucokinase.
• Downregulated by Glucokinase Regulatory Protein (GKRP).
• GKRP binds to the active site of glucokinase and competes with glucose; it only binds in the presence of fructose 6-phosphate.

Phosphoglucose Isomerase

• Catalyzes the reaction of G6P to F6P.
• Glycolytic enzyme that operates anaerobically, with a simple isomerization reaction.
• Upregulated by low ATP levels and insulin.

Phosphofructokinase 1 (PFK-1)

• Catalyzes the reaction of F6P to F16BP, consuming ATP.
• Glycolytic enzyme that operates anaerobically, requiring ATP and Mg2+.
• Rate-determining step of glycolysis and functions as a transferase reaction.
• Upregulated by low ATP levels and low citrate levels which allow for the CATALYTIC site to binds to ATP at low levels with Mg2+.
• Upregulated by insulin, which prompts Domain 1 of PFK2 to secrete F26 Bisphosphate and by ALLOSTERIC activation by F 2,6 Bisphosphate which upregulates glycolysis.
• Downregulated by high ATP levels, acting as an allosteric inhibitor.
• Downregulated by glucagon which prompts Domain 2 of PFK2 to secrete F26 BisphosphotASE, and by F 2, 6 BisphosphotASE (secreted by Domain 2 of PFK2 which breaks down F26bisphosphate to downregulate glycolysis).
• Muscle PFK 1 deficiency is associated to Tarui disease.
• Homozygous cases in the fetus are not survivable however for heterozygous cases the small normal PFK amount is enough until exercise is initiated. - First manifestation: Muscle Fatigue and Weakness which results in exercise intolerance which causes breakdown of muscle protein that leads to MYOGLOBIN UREA.
  - Second manifestation: Hemolysis. - Third manifestation: Hyperuricemia. - Build up of G6P that goes into PPP, increases production of R5P and the abnormal increase of nucleotide prod causes breakdown of nucleotides that produces uric acid. - Fourth manifestation: Lactoacidosis enhancing Hyperuricemia. - Acidosis prevents excretion of uric acid into urine which build up uric acid in blood. - Fifth manifestation: Gout and Osteoarthritis. - Hyperuricemia causes uric acid to be get deposited in the joints as uric crystals.

Aldolase

• Catalyzes the reaction of F16BP to GAP and DHAP.
• Glycolytic enzyme that converts one 6C molecule into two 3C isomers.
• Only requires Zn in microbes and is targeted in drug development through chelation.
• Upregulated by low ATP levels and insulin. - Deficiency results to fetus not surviving. - First manifestation: Hemolytic Anaemia. - Second manifestation: Muscle DMG which results in Rhabdomyolysis and Exercise intolerance - Third manifestation: Jaundice like symptoms due to high RBC breakdown. - Fourth manifestation: Negative gain of ATP.

Triose Phosphate Isomerase (TIM)

• Catalyzes the reaction of DHAP to GAP.
• Glycolytic enzyme that isomerizes DHAP into GAP and if inhibited the net gain of ATP in glycolysis would be 0

GAP Dehydrogenase

• Catalyzes the reaction of GAP to 1,3-Bisphosphoglycerate.
• Glycolytic enzyme involved in dehydrogenation via oxidoreductase where NAD is reduced to NADH.
• Upregulated by low ATP levels and insulin.
• Downregulated by: - Irreversible inhibition by iodoacetate when thiol group (SH) of GAP dehydrogenase binds to the salt and forms disulfide bridges (covalent modification).
• Used in traditional medicine to help with gout and osteoarthritis, but resulted in chondrocyte DMG longterm.
• High concentration of salt consumed inhibits Glycolysis, which manifests as hemolytic anaemia.

1,3 Bisphosphoglycerate kinase

• Catalyzes the reaction of 1,3 Bisphosphoglycerate to 3-phosphoglycerate, producing ATP
• Glycolytic enzyme involved in substrate level phosphorylation which is the first step in glycolysis that produces ATP.
• 3-phosphoglycerate gets isomerized to 2-phosphoglycerate.

Enolase

• Catalyzes the reaction of 2-phosphoglycerate to PEP.
• Glycolytic enzyme
• Downregulated by inhibition by fluoride during groundwater poisoning (excess fluoride binds to enolase in the presence of Mg2+).
• Inhibition of glycolysis manifests as hemolytic anaemia

Pyruvate Kinase

• Catalyzes the reaction of PEP to Pyruvate, producing ATP.
• Glycolytic enzyme involved in substrate level phosphorylation which is the second step in glycolysis to prod ATP. Net gain of ATP is now +2.
• Upregulated by: - ChREBP: transcription factor that upregulates Pyr Kinase due to high levels of glucose in hepatocytes. - High levels of F16BP, allosterically activate PYR Kinase.
• Deficiency is associated to Malaria (Malarial Shiver)

Fructokinase

• Catalyzes the reaction of fructose to fructose 6 phosphate
• Fructose catabolism in the liver results to Fructose 1 phosphate

Pyruvate Dehydrogenase Complex

• Catalyzes the reaction of Pyruvate to Acetyl CoA.
• Enzyme involved in link reaction (preceding TCA), oxidative decarboxylation where CO2 and NADH gets produced.
• Composed of 3 core catalytic subunits: E1, E2, E3 and 2 regulatory subunits: PDK and PDP. - E1: Pyruvate Dehydrogenase responsible for Decarboxylation that req TPP As Cofactor. - Deficiency of TPP results in: - Impairment of the enzymes PDH, Alpha ketoglutarate Dehydrogenase, Transketolase, Branched Chain Alpha KetoAcid Dehydrogenase. - Associated diseases that occurs in Beri Beri disease (malnutrition with two types- wet and dry where Diabetics have a high chance of getting Beri Beri because of the high polyuria), Warnick Korsakoff disease (Chronic Alcoholism) and Lead poisoning (malnutrition that causes brain problem bc of lack of ATP as well as Nephrotoxicity bc of rapid excretion of thiamine) - E2: Dihydrolipoamide Acetyl Transferase responsible for transferring Acetyl to CoA that req Lipoic Acid and CoA As Cofactor. - Associated diseases that occur in Arsenic poisoning when Lipoic acid gets inhibited impairing E2, - E3: Dihydrolipoamide Dehydrogenase responsible for regenerating oxidised lipoamide that req FAD As Cofactor. - Associated diseases that occurs in Mercury poisoning (eating a lot of ink which causes mutations in E3
• Regulation of PDH: - Upregulated by low ATP, NADH, FADH2 levels and insulin which activates PDH by dephosphorylating it. - Downregulated by glucagon which inactivates PDH by phosphorylating it.

Citrate Synthase

• Catalyzes the reaction of Oxaloacetate + Acetyl CoA to Citrate.
• TCA enzyme.
• Downregulated by high levels of Citrate which results in allosteric inhibition aka end product inhibition.

Isocitrate Dehydrogenase

• Catalyzes the reaction of Isocitrate to Alpha Ketoglutarate.
• TCA enzyme and rate determining step of TCA, Dehydrogenase meaning NADH produced. Contains many isoforms: Isoform 1 found in RBCs and peroxisomes and uses NADP instead and.
• Isoform 2 contains Isoform 3 because mitochondria wants to produce both NADPH and NADH so isoform 2 which uses NADP also contains Isoform 3 which uses NAD
• Downregulated by high levels of ATP and NADH.

Alpha Ketoglutarate Dehydrogenase

• Catalyzes the reaction of Alpha ketoglutarate to Succinyl CoA.
• TCA enzyme, Oxidative Decarboxylation meaning CO2 and NADH being produced.
• Req TPP As Cofactor.
• Effects of Arsenic and Alchohol on PDH apply here too.
• Downregulated by high levels of ATP, NADH, and Succinyl CoA via end product inhibition. - Succinyl CoA precursor for heme synthesis and is req for acetoacetate metabolism (ketone body).

Succinyl CoA Synthase

• Catalyzes the reaction of Succinyl CoA to Succinate, producing GTP.
• TCA enzyme involved in substrate level phosphorylation.

Succinate Dehydrogenase

• Catalyzes the reaction of Succinate to Fumarate.
• TCA enzyme, Dehydrogenase meaning FADH2 produced.
• This enzyme is present in both TCA and ETC, so FADH stays bound and regen in the same place so, FADH is the prosthetic group of TCA cycle.
• Deficiency of complex 2 of succinate Dehudrogenase results in: - Weakness of muscle - Impaired brain function - Lactoacidosis due to increased reliance on anaerobic resp

Malate Dehydrogenase

• Reversible reaction of Malate to Oxaloacetate or Oxaloacetate to Malate
• TCA and Gluconeogenesis enzyme, cytosolic and mitochondrial isoforms.

Acetyl CoA carboxylase

• Increased activity by insulin for increased availability of Acetyl CoA for TCA.
• Enzyme involved in Fatty acid synthesis that produced Malonyl CoA, and Malonyl CoA metabolism produced more Acetyl CoA .
• Anatplerotic reactions of TCA; deficiency leads to lactoacidosis. - 1. Pyruvate Carboxylase: Pyruvate -> Oxaloacetate. - 2. Aspartate aminotransferase: Aspartate -> Oxaloacetate. - 3. Glutamate Dehydrogenase/ Transaminase: Glutamate-> Alpha Ketoglutarate. - 4. Propionyl CoA Carboxylase: Propionyl CoA-> Succinyl CoA which uses biotin as cofactor.

G6P Dehydrogenase

• G6P catalyzed to 6 phospho-glucono-beta-lactone, PPP enzyme, rate determining step of PPP.

• Upregulated by: -1. High NADPH demand and NADP levels. - 2. Insulin (Stimulates expression of this enzyme). - 3. High levels of excess G6P. - 4. Oxidative stress. - 5. High levels of Thiamine.

6-Phosphogluconolactase

• Catalyzed to : 6 phospho-glucono-beta-lactone -> 6 phosphogluconate and is a PPP enzyme.

6-phosphogluconate dehydrogenase

• Catalyzed to 6 phosphogluconate to to Ribulose 5 phosphate
• PPP enzyme, oxidative decraboxylation is the prod of CO2 and NADPH

Transketolase and Transaldolase

• Enzymes involved in nonoxidative phase of PPP that req TPP As Cofactor

Glucose 6 Phosphatase

• Catalyzed to G6P -> Glucose
• Gluconeogenesis enzyme that is only present in liver and kidney.
• Upregulated by glucagon (low blood glucose levels).

Fructose 1 6 Bisphosphatase

• Catalyzed to Fructose 16 Bisphosphate -> Fructose 6 Phosphate
• Gluconeogenesis enzyme
• Upregulated by: -Low AMP levels - Allosteric Activation by ATP and Citrate. - Regulation of Fructose 1 6 Bisphosphatase and PFK 1 is reciprocal.

Pyruvate Carboxylase

• Catalyzed to Pyruvate -> Oxaloacetate (-ATP)
• Gluconeogenesis enzyme (and anaplerotic for TCA)
• Req HCO3- and ATP and BIOTIN as coenzyme
• Upregulated by allosteric activation of Acetyl CoA; if oxaloacetate to go into gluconeogenesis unavailable then Acetyl CoA build up.
• Downregulated by low ATP and Acetyl CoA levels, and insulin which suppresses glucocorticoids and glucagon stimulated cAMP
• Inhibition due to a high concentration of avidin (high affinity for biotin) by eating raw eggs therefore no gluconeogenesis occurs

Malate Dehydrogenase:

• Reversible reaction of Malate-> Oxaloacetate or Oxaloacetate-> Malate
• TCA and Gluconeogenesis enzyme, cytosolic and mitochondrial isoforms.

PEP Carboxylase

• Catalyzed Oxaloacetate CTP and PEP-> Oxaloacetate.
• Gluconeogenesis enzyme, Req GTP and Prod CO2 and need x2 GTP to go from 3C to 6C

Methylmalonyl CoA Mutase

• Involved in formation of Propionyl CoA, and req VITB12 as for its cofactor. - Deficiency of VIT B12: impairs Methylmalonyl CoA Mutase, resulting accumulation of Lmethylmalonyl CoA, that gets converted to methylmalonic acid, gets excreted in urine resulting methylmalonic aciduria.