11.4 Carbohydrate Metabolism

Carbohydrate metabolism is a complex network of pathways that break down and build up sugars. These processes, including glycolysis, gluconeogenesis, and the pentose phosphate pathway, are crucial for energy production and cellular building blocks.

Hormones like insulin and glucagon tightly regulate these pathways, ensuring proper blood sugar levels. Understanding carbohydrate metabolism is key to grasping energy production, biosynthesis, and metabolic disorders like diabetes and glycogen storage diseases.

Carbohydrate Metabolism Pathways

Pathways of carbohydrate metabolism

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• Glycolysis
  • 10-step process occurring in cytoplasm breaks down glucose into pyruvate
  • Generates 2 ATP and 2 NADH per glucose molecule fueling cellular energy needs
  • Key enzymes regulate flux (hexokinase, phosphofructokinase, pyruvate kinase)
  • Provides building blocks for biosynthesis (glyceraldehyde-3-phosphate, pyruvate)
• Gluconeogenesis
  • Reverses glycolysis synthesizing glucose from non-carbohydrate precursors
  • Occurs primarily in liver and kidney cortex maintaining blood glucose levels
  • Key substrates include lactate from anaerobic glycolysis, amino acids from protein breakdown, and glycerol from triglyceride hydrolysis
  • Unique enzymes bypass irreversible steps of glycolysis (glucose-6-phosphatase, fructose-1,6-bisphosphatase, phosphoenolpyruvate carboxykinase)
• Pentose Phosphate Pathway
  • Parallel pathway to glycolysis oxidizes glucose-6-phosphate
  • Generates NADPH for reductive biosynthesis and antioxidant defense
  • Produces ribose-5-phosphate for nucleotide and nucleic acid synthesis
  • Two phases: oxidative (NADPH production) and non-oxidative (interconversion of sugars)

Regulation of carbohydrate metabolism

• Insulin
  • Promotes glucose uptake in muscle and adipose tissue via GLUT4 translocation
  • Stimulates glycogen synthesis by activating glycogen synthase
  • Inhibits gluconeogenesis by suppressing key enzymes (PEPCK, G6Pase)
• Glucagon
  • Stimulates glycogenolysis by activating glycogen phosphorylase
  • Enhances gluconeogenesis by inducing enzyme expression
  • Increases blood glucose levels during fasting or low blood sugar
• Epinephrine
  • Promotes rapid glycogenolysis and lipolysis during fight-or-flight response
  • Increases blood glucose levels to meet energy demands during stress
  • Acts through $β$-adrenergic receptors and cAMP signaling
• Allosteric regulation
  • Phosphofructokinase inhibited by high ATP and citrate levels indicating energy sufficiency
  • Pyruvate dehydrogenase complex regulated by acetyl-CoA (product inhibition) and NADH (redox state)
• Substrate availability
  • High glucose levels increase glycolysis rate through mass action
  • Abundance of gluconeogenic precursors (lactate, amino acids) enhances glucose production

Importance in energy and biosynthesis

• Energy production
  • Glycolysis provides rapid ATP through substrate-level phosphorylation
  • Pyruvate oxidation feeds acetyl-CoA into TCA cycle driving electron transport chain
  • Theoretical maximum ATP yield: 38 ATP per glucose (2 from glycolysis, 36 from oxidative phosphorylation)
• Biosynthesis
  • Glucose-6-phosphate serves as precursor for glycogen storage
  • Ribose-5-phosphate forms backbone of nucleotides (ATP, DNA, RNA)
  • NADPH from pentose phosphate pathway powers reductive steps in lipid synthesis
  • Oxaloacetate from TCA cycle acts as starting point for amino acid synthesis (aspartate family)
• Metabolic flexibility
  • Glucose serves as primary energy source for brain (20% of body's glucose consumption)
  • Glycogen storage in liver (10% of its mass) and muscles (1-2% of mass) provides quick energy
  • Gluconeogenesis maintains blood glucose during prolonged fasting (after 8-12 hours)

Disorders of carbohydrate metabolism

• Diabetes mellitus
  • Type 1: autoimmune destruction of pancreatic β-cells leads to insulin deficiency
  • Type 2: insulin resistance and impaired insulin secretion cause hyperglycemia
  • Consequences include ketoacidosis, microvascular complications (retinopathy, nephropathy)
• Glycogen storage diseases
  • Von Gierke's disease: glucose-6-phosphatase deficiency causes severe hypoglycemia
  • McArdle's disease: muscle glycogen phosphorylase deficiency leads to exercise intolerance
  • Symptoms often include hepatomegaly, growth retardation, and muscle weakness
• Galactosemia
  • Inability to metabolize galactose due to enzyme deficiency (galactose-1-phosphate uridylyltransferase)
  • Symptoms include failure to thrive, liver damage, cataracts, and intellectual disability
  • Managed through strict dietary galactose restriction
• Fructose intolerance
  • Deficiency in aldolase B enzyme impairs fructose metabolism in liver and kidney
  • Accumulation of fructose-1-phosphate depletes ATP and causes cellular dysfunction
  • Symptoms include hypoglycemia, liver and kidney damage, and growth retardation