TBL 8: Glycolysis & PPP

Glycolysis Irreversible Steps

1, 3, 10

Glycolysis Irreversible Step 1

Glucose → Glucose-6-phosphate

via Hexokinase

Glycolysis Irreversible Step 3

Fructose-6-phosphate → Fructose-1,6-bisphophate

via Phosphofructokinase-1 [PFK1]

Glycolysis Irreversible Step 10

Phosphoenolpyruvate [PEP] → Pyruvate

via Pyruvate Kinase [PK]

*happens 2x for payoff

Activators of glycolysis

• ADP

  • low E, stim PFK 1

• AMP

  • stress signal [product of cAMP, which indicates glucagon fasting state]

• Calcium

  • signaling & insulin connection

• F-1,6-BP & F-2,6-BP

  • allosteric

inhibitors of glycolysis

• ATP

  • high E, inhibits PFK1

• Citrate

  • downstream product [negative feedback]

• Glu-6-phosphate

  • roadblock

glycolysis in fed state…

insulin increases → activates PFK-2 → increases fructose-2,6-BP → allosteric activation of PFK1 → high glycolytic flux

glycolysis in fasting state….

glucagon increases → activate PKA → phosphorylates/inactivates PFK-2 → inhibits PFK-1 & PK → glycolysis slows → conserves glucose for brain & RBCs

Which process replenishes NAD+ supply for glycolysis to continue?

lactate production

Glycolysis pyruvate kinase deficiency

• 2nd most genetic cause of hemolytic anemia

  • lack of ATP → RBC death

• autosomal recessive

• Na+/K+-ATPase → loss of ion balance → osmotic fragility → swelling/lysis

• characteristic histopathology: echinocytes [RBCs w/ thorny projections]

• elevated 2,3-bisphosphoglycerate [2,3-BPG]

  • created from diverted 1,3-BPG [from PEP]

  • used by RBCs to alter Hb oxygen binding in order to cope with lack of ATP

PPP

alternative fate of glucose-6-phosphate

produces NADPH & ribose-5-phosphate

PPP in fasting state

AMPK [AMP-activated protein kinase] driven energy conservation + Pyruvate dehydrogenase [PDH] inhibition [prevents pyruvate→acetyl-CoA for TCA]

• spares carbon skeletons for gluconeogenesis

• increased mitochondrial activity/respiration → ROS → Nrf2 signaling to activate antioxidant gene

Nrf2 signaling

increases expression of G6PD [glucose-6-phosphate dehydrogenase] AKA PPP Enzyme

PPP in refeeding state

G6P driver PPP surge → NADPH to power fat synthesis & clear fasting-induced oxidative stress via glutathione reduction

• oxidized glutathione [GSSG] uses NADPH to get to reduced glutathione [GSH]

  • GSH neutralizes ROS & protects cells

Part of PPP to run if you need NADPH only?

run oxidative PPP; recycle sugars back to glycolysis

Part of PPP to run if you need Ribose-5-P only?

run reverse non-oxidative PP using glycolytic intmds

Part of PPP to run if you need NADPH & Ribose-5-P?

run complete forward PPP pathwayPart of PPP to run if you need NAPH only?

Part of PPP to run if you don’t need NADPH or Ribose-5-P?

push G6P into glycolysis for ATP or glycogenesis for storage

G6PD Deficiency

Glucose-6-phosphate dehydrogenase deficiency

• most common disease-producing enzyme deficiency in humans

• female carriers have increased malaria resistance

• X-linked recessive

• hemolytic anemia

  • w/o reduced glutathione produced, RBCs die from metabolic poisons

• asymptomatic until exposed to triggers → oxidative stress

  • fava beans, oxidants, infections, medications [primaquine - anti-malarial, sulfa drugs - antibiotics]

• effects

  • bite cells [splenic macrophages removing heinz bodies]

  • heinz bodies [inclusions of denatured, oxidized Hb]

Thiamine

Vitamin B1

• required cofactor for transketolase activity in non-oxidative phase [reversible] of PPP

• water soluble so readily absorbed

• in many foods, but short-lived w/ little storage

Vitamin B1/Thiamine deficiency clinically associated with…

1. Wernicke encephalopathy

2. Beriberi

*Alcoholism also has severe Vitamin B1 deficiency from inability to absorb or store it

Wernicke encephalopathy

• acute: ocular abnormalities, ataxia, confusion

• chronic: severe memory loss, korsakoff psychosis

• clinical test: RBC TKT assay

  • RBC transketolase activity

Beriberi

• dry: severe peripheral neuropathy

• wet: high-output cardiac failure, edema

• common: muscle weakness, fatigue