Hexokinase vs Glucokinase

Hexokinase

• Catalyzes the conversion of glucose to glucose 6-phosphate, consuming one ATP
• Considered a glycolytic enzyme, functions anaerobically
• Requires ATP and Mg2+
• Facilitates glucose retention within cells
• Not present in the liver and pancreas
• Functions via a transferase reaction
• Low ATP levels and insulin upregulate its activity, the latter indicating high blood glucose levels
• G6P (glucose-6-phosphate) downregulates it by end product inhibition
• Can convert fructose to fructose-6-phosphate in muscle tissue as a side reaction

Glucokinase

• Catalyzes the conversion of glucose to glucose 6-phosphate, consuming one ATP
• Considered a glycolytic enzyme, functions anaerobically
• Requires ATP and Mg2+
• Facilitates glucose retention within cells
• Exclusively found in the liver and pancreas
• Functions via a transferase reaction
• Low ATP levels and bidirectional regulation with insulin, indicative of high blood glucose levels, upregulates it
• Exhibits no end-product inhibition
• Fructose 1-phosphate enhances glucose binding by causing conformational changes in GKRP (Glucokinase Regulatory Protein)
• GKRP competes with glucose by binding to the active site of glucokinase
• GKRP exclusively binds in the presence of fructose 6-phosphate, downregulating glucokinase

Phosphoglucose Isomerase

• Catalyzes the conversion of G6P to F6P
• Considered a glycolytic enzyme, functions anaerobically
• Performs a simple isomerization reaction
• Upregulated by low ATP levels and insulin

Phosphofructokinase 1

• Catalyzes the conversion of F6P to F16BP, consuming one ATP
• Considered a glycolytic enzyme, functions anaerobically
• Requires ATP and Mg2+
• Considered the rate-determining step of glycolysis
• Functions via a transferase reaction
• Low ATP levels and low citrate upregulate its activity
• PFK-1(Phosphofructokinase-1) contains two ATP binding sites, a high-affinity catalytic site that binds ATP at low levels along with Mg2+
• Insulin stimulates Domain 1 of PFK2 to secrete F26BP (fructose-2,6-bisphosphate)
• F-2,6-BP (fructose-2,6-bisphosphate) allosterically activates it, secreted by Domain 1 of PFK2, thus upregulating glycolysis
• High ATP levels inhibit it allosterically, PFK-1 contains two ATP binding sites, a low-affinity allosteric site that binds ATP at high levels
• Glucagon stimulates Domain 2 of PFK2 to secrete F26BPase (fructose-2,6-bisphosphatase)
• F-2,6-BPase, secreted by Domain 2 of PFK2, degrades F26BP to downregulate glycolysis

Muscle PFK1 Deficiency (Tarui Disease)

• Condition associated with Muscle PFK1 Deficiency
• Homozygous cases are fatal for the fetus
• Heterozygous individuals can survive with limited normal PFK amount until exercise commences
• Exercise intolerance and muscle fatigue, leading to muscle protein breakdown and myoglobin urea, signify its first manifestation
• Hemolysis marks its second manifestation
• Hyperuricemia its third manifestation, caused by G6P buildup shunted into PPP, increasing R5P production and nucleotide breakdown into uric acid
• Lactoacidosis intensifies hyperuricemia, which is its fourth manifestation
• Acidosis inhibits uric acid excretion into urine, causing its accumulation in the blood
• Gout and osteoarthritis represent its fifth manifestation
• Hyperuricemia results in uric acid deposition in joints, causing both gout and osteoarthritis

Aldolase

• Catalyzes the conversion of F16BP into GAP and DHAP
• Considered a glycolytic enzyme that converts a 6-carbon molecule (6C) into two 3-carbon isomers (3C)
• Requires zinc (Zn) exclusively in microbes
• Drug development targets it through chelation
• Upregulated by low ATP levels and insulin
• Fetal deficiency is generally lethal

Triose Phosphate Isomerase (TIM)

• Catalyzes the conversion of DHAP to GAP, isomerizing DHAP into GAP
• Considered a glycolytic enzyme
• Net ATP yield in glycolysis would be zero if inhibited

GAP Dehydrogenase

• Catalyzes the conversion of GAP to 1,3-Bisphosphoglycerate
• A glycolytic enzyme, operating via oxidoreductase mechanism, reducing NAD to NADH
• Low ATP levels and insulin upregulate its activity
• Iodoacetate irreversibly inhibits it by binding to the thiol group (SH) of GAP dehydrogenase, forming disulfide bridges (covalent modification)
• Traditionally used in medicine for gout and osteoarthritis, but has resulted in chondrocyte damage in the long term
• Excess salt consumption leads to the inhibition of glycolysis, manifesting as hemolytic anemia

1,3 Bisphosphoglycerate Kinase

• Catalyzes the conversion of 1,3-Bisphosphoglycerate to 3-phosphoglycerate, generating one ATP
• A glycolytic enzyme that performs substrate-level phosphorylation, producing ATP
• Performed twice, it marks the first step in glycolysis producing ATP
• 3-phosphoglycerate is isomerized to 2-phosphoglycerate

Enolase

• Catalyzes the conversion of 2-phosphoglycerate to PEP
• Considered a glycolytic enzyme
• Fluoride inhibits it, excess fluoride binds to enolase in the presence of Mg2+ when there is fluoride poisoning from groundwater

Pyruvate Kinase

• Catalyzes the conversion of PEP to Pyruvate, generating one ATP
• Glycolytic enzyme that performs substrate level phosphorylation, the second step in glycolysis to produce ATP
• Net gain of ATP is now +2
• ChREBP upregulates it due to high glucose levels in hepatocytes
• High levels of F16BP allosterically activate it
• Deficiency is linked to malaria

Manifestations of Pyruvate Kinase Deficiency

• Hemolytic anemia is the first indication
• Spleenomegaly marks the second manifestation, due to increased RBC breakdown enlarging the reticuloendothelial system
• Compensatory reticulocytosis enhances the first and second manifestations as the third indication

Fructokinase

• Catalyzes the conversion of fructose to either fructose-6-phosphate or fructose-1-phosphate in fructose metabolism in the liver

Pyruvate Dehydrogenase Complex

• Catalyzes the conversion of Pyruvate to Acetyl CoA
• Enzyme involved in the link reaction (preceding TCA)
• Oxidative decarboxylation produces CO2 and NADH
• Consists of 3 core catalytic subunits and 2 regulatory subunits
• E1, i.e. Pyruvate Dehydrogenase, is responsible for decarboxylation and requires TPP as a cofactor

Deficiency of Thiamine Pyrophosphate (TPP)

• Leads to impairment of several enzymes
• Pyruvate Dehydrogenase
• Alpha-ketoglutarate Dehydrogenase
• Transketolase
• Branched-chain Alpha KetoAcid Dehydrogenase
• Occurs in Beri Beri disease and Wernicke-Korsakoff syndrome
• Beri Beri is associated with malnutrition
• Diabetics at a high risk due to polyuria excreting thiamine
• Wernicke-Korsakoff is linked to chronic alcoholism
• Also occurs in lead poisoning and is linked to malnutrition, causes brain problems due to ATP deficiency and nephrotoxicity

E2: Dihydrolipoamide Acetyl Transferase

• Transfers Acetyl to CoA
• Requires lipoic acid and CoA as cofactors

Associated Diseases of E2:

• Arsenic poisoning inhibits lipoic acid leading to lipoic acid deficiency -> impairment of E2
• E3, i.e. Dihydrolipoamide Dehydrogenase, regenerates oxidized lipoamide and Requires FAD as a cofactor
• Mercury poisoning that can cause mutations in E3

Regulation of PDH

• Low ATP, NADH, and FADH2 levels upregulate its activity because insulin activates PDH by dephosphorylating it
• Glucagon downregulates it, inactivating PDH by phosphorylating it

Citrate Synthase

• Catalyzes the conversion of oxaloacetate and Acetyl CoA to Citrate
• A TCA enzyme
• High citrate levels downregulate it via allosteric or end-product inhibition

Isocitrate Dehydrogenase

• Catalyzes the conversion of isocitrate to alpha-ketoglutarate
• A TCA enzyme that constitutes the rate-determining step
• Produces NADH through dehydrogenase activity
• Contains various isoforms
• Isoform 1 is found in RBCs and peroxisomes and uses NADP
• 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
• High ATP and NADH levels downregulate it

Alpha Ketoglutarate Dehydrogenase

• Catalyzes the conversion of alpha-ketoglutarate to succinyl CoA
• A TCA enzyme that produces CO2 and NADH via oxidative decarboxylation
• Requires TPP as a cofactor
• High ATP and NADH levels and succinyl CoA end-product inhibition downregulate it
• Succinyl CoA is a precursor for heme synthesis and is required for acetoacetate metabolism

Succinyl CoA Synthase

• Catalyzes the conversion of succinyl CoA to Succinate, generating one GTP
• TCA enzyme and functions via substrate-level phosphorylation

Succinate Dehydrogenase

• Catalyzes the conversion of Succinate to Fumarate
• A TCA enzyme and produces FADH2 through dehydrogenase activity
• Present in both TCA and ETC
• FADH stays bound and regenerated in the same place
• FADH is the prosthetic group
• Deficiency causes muscle weakness, impaired brain function, and lactoacidosis

Malate Dehydrogenase

• Catalyzes the reversible reaction of Malate to Oxaloacetate
• A TCA and gluconeogenesis enzyme with cytosolic and mitochondrial isoforms

Acetyl CoA Carboxylase

• Its activity is increased by insulin for increased availability of Acetyl CoA for TCA
• Enzyme involved in fatty acid synthesis that produces Malonyl CoA
• Increased Malonyl CoA metabolism leads to increased Acetyl CoA production, boosting TCA cycle activity

Anaplerotic Reactions of TCA

• Deficiency in any of these reactions can lead to Lactoacidosis
• Pyruvate Carboxylase: Pyruvate -> Oxaloacetate
• Aspartate aminotransferase: Aspartate -> Oxaloacetate
• Glutamate Dehydrogenase/ Transaminase: Glutamate -> Alpha Ketoglutarate
• Propionyl CoA Carboxylase: Propionyl CoA-> Succinyl CoA, uses biotin as a cofactor

G6P Dehydrogenase

• Catalyzes the conversion of G6P to 6-phospho-glucono-beta-lactone
• PPP enzyme that is also the rate-determining step
• High NADPH demand, insulin, high levels of excess G6P, oxidative stress, and high thiamine levels upregulate it

6-Phosphogluconolactase

• Catalyzes the conversion of 6-phospho-glucono-delta-lactone to 6-phosphogluconate
• PPP enzyme

6-phosphogluconate dehydrogenase

• Catalyzes the conversion of 6-phosphogluconate to Ribulose 5-phosphate
• A PPP enzyme that produces CO2 and NADPH during oxidative decarboxylation

Transketolase and Transaldolase

• Enzymes participate in the nonoxidative phase of PPP
• Transketolase requires TPP as a cofactor

Glucose 6 Phosphatase

• Catalyzes the conversion of G6P to Glucose
• Gluconeogenesis enzyme exclusively located in the liver and kidney
• Glucagon upregulates it

Fructose 1,6 Bisphosphatase

• Catalyzes the conversion of Fructose 1,6 Bisphosphate to Fructose 6 Phosphate
• Gluconeogenesis enzyme
• Low AMP levels and allosteric activation by ATP and citrate upregulate it
• When PFK1 is downregulated by glucagon and domain 2 of PFK2, reciprocal regulation upregulates Fructose 1,6 Bisphosphatase

Pyruvate Carboxylase

• Catalyzes the conversion of pyruvate to oxaloacetate, consuming one ATP
• Gluconeogenesis enzyme and anaplerotic for TCA
• Requires HCO3-, ATP, and biotin as coenzyme

Regulation of Pyruvate Carboxylase

• Allosteric activation via Acetyl CoA occurs so that when there isn't oxaloacetate to go through gluconeogenesis, Acetyl CoA levels rise
• Low ATP and Acetyl CoA levels downregulate it
• Insulin inhibits glucocorticoids- and glucagon-stimulated cAMP
• Avidin inhibits it
• Avidin prevents gluconeogenesis

Malate Dehydrogenase - Gluconeogenesis

• Catalyzes the reversible reaction between malate and oxaloacetate
• A cytosolic and mitochondrial enzyme involved in TCA and gluconeogenesis

PEP Carboxylase

• Catalyzes the conversion of Oxaloacetate to PEP, by using GTP and producing CO2
• Gluconeogenesis enzyme
• Requires two GTP molecules to convert a 3-carbon molecule to a 6-carbon molecule

Methylmalonyl CoA Mutase

• Involved in the creation of propionyl CoA and requires VITB12 as a cofactor

Deficiency of Vitamin B12

• Causes impairment of methylmalonyl CoA mutase, leading to accumulation of L-methylmalonyl CoA
• L-methylmalonyl CoA gets converted to methylmalonic acid and is excreted in urine, resulting in methylmalonic aciduria