Carbohydrate Metabolism Overview and Pathways

Carbohydrate Metabolism

Biochemical process converting carbohydrates into energy.

Seager et al.

Authors of the textbook on biochemistry.

Cengage

Publisher of the biochemistry textbook used.

General Biochemistry

Study of biochemical processes in living organisms.

Organic Biochemistry

Focus on carbon-containing compounds in biological systems.

Carbohydrate digestion

Hydrolysis of di- and polysaccharides to monosaccharides.

Blood glucose level

Amount of glucose in blood, mg per 100 mL.

Hypoglycemia

Lower-than-normal blood sugar level.

Hyperglycemia

Higher-than-normal blood sugar level.

Renal threshold

Blood sugar level where kidneys stop reabsorbing glucose.

Glucosuria

Excretion of glucose in urine due to high blood sugar.

Glycolysis

Pathway converting glucose to pyruvate, producing ATP.

Net reaction of glycolysis

glucose + 2Pi + 2ADP + 2NAD+ → 2Pyruvate + 2ATP.

Fates of pyruvate

Oxidized to acetyl CoA, reduced to lactate or ethanol.

Citric acid cycle

Oxidizes acetyl CoA to CO2, generates NADH and FADH2.

Regulation of glycolysis

Controlled by hexokinase, phosphofructokinase, pyruvate kinase.

Feedback inhibition

Glucose-6-phosphate inhibits hexokinase activity.

Oxidative phosphorylation

Process converting ADP to ATP during electron transport.

ATP yield from NADH

Each NADH generates 2.5 ATP during oxidation.

ATP yield from FADH2

Each FADH2 generates 1.5 ATP during oxidation.

Chemiosmotic hypothesis

Proton flow drives ATP synthesis in mitochondria.

Complete oxidation of glucose

Total ATP yield from glycolysis, citric acid cycle, and electron transport.

Glycogenesis

Process of converting glucose to glycogen for storage.

Glycogenolysis

Breakdown of glycogen to release glucose.

Gluconeogenesis

Synthesis of glucose from non-carbohydrate precursors.

Cori cycle

Recycles lactate to glucose in the liver.

Acetyl CoA

Key metabolite entering the citric acid cycle.

Electron transport chain

Series of reactions reducing oxygen to water.

Cytochromes

Iron-containing enzymes in the electron transport chain.

Lactate fermentation

Anaerobic conversion of glucose to lactate.

Alcoholic fermentation

Anaerobic conversion of glucose to ethanol.

Energy yield from glucose

Net yield of 32 ATP from complete oxidation.

ATP production from GTP

1 GTP is equivalent to 1 ATP.

Oxidation of pyruvate

Conversion of pyruvate to acetyl CoA in mitochondria.

Regulation of citric acid cycle

Stimulated by low ATP and high ADP levels.

Proton flow

Drives ATP synthesis via ATP synthase.

NAD+ regeneration

Necessary for glycolysis to continue after pyruvate processing.

Glucose Oxidation

Releases 686 kcal/mol of energy.

ATP Synthesis

Captures 234 kcal/mol from glucose oxidation.

Overall Reaction

C6H12O6 + 6O2 + 32ADP + 32Pi → 6CO2 + 32ATP + 38H2O.

Energy Efficiency

34% of free energy captured by cells.

Automobile Engine Efficiency

20-30% energy from gasoline is usable.

Glycogenesis

Synthesis of glycogen from glucose.

Glycogen Storage in Liver

Liver can store 110 g of glycogen.

Glycogen Storage in Muscles

Muscles can store 245 g of glycogen.

Glycogen Synthase

Enzyme forming α(1→4) linkages in glycogen.

Amylo (1,4-1,6)-trans-glycosylase

Inserts α(1→6) branch points in glycogen.

Glycogenolysis

Breakdown of glycogen to glucose.

Glycogen Phosphorylase

Enzyme cleaving α(1→4) linkages.

Debranching Enzyme

Cleaves α(1→6) linkages during glycogenolysis.

Glucose-6-Phosphate Hydrolysis

Converts glucose-6-phosphate to glucose.

Muscle Cell Limitations

Cannot form free glucose from glycogen.

Liver Function

Maintains blood glucose levels by releasing glucose.

Gluconeogenesis

Synthesis of glucose from noncarbohydrate sources.

Cori Cycle

Glucose-lactate cycle between muscle and liver.

Lactate Conversion

Lactate converted back to glucose in liver.

Insulin

Hormone enhancing glucose absorption into cells.

Glucagon

Hormone increasing blood glucose levels.