Glycolysis and Glucose Metabolism

• Glucose is the main source of energy for most body tissues, especially brain cells, and a minimum amount is needed for normal function
• A normal fasting plasma glucose level is 70–100 mg/dL, rising after a heavy carbohydrate meal but staying below 150 mg/dL in a normal person
• Glycolysis, also known as the Embden-Meyerhof Pathway, involves carbohydrate metabolism, gluconeogenesis, glycogen metabolism, and the HMP shunt

Historical discoveries related to glycolysis

• Spallanzani showed in 1768 that living tissues consume oxygen and release carbon dioxide
• Pasteur demonstrated in 1860 the fermentation of glucose into alcohol by yeast
• Embden studied lactic acid formation from pyruvate in 1914
• Meyerhof described most steps of the glycolytic pathway in 1919, earning a Nobel Prize in 1922
• Euler-Chelpin identified hexokinase (HK) in 1915, earning a Nobel Prize in 1929
• Neuberg discovered pyruvate decarboxylase in 1911
• Harden identified phosphofructokinase (PFK) in 1920, earning a Nobel Prize in 1929
• Parnas discovered pyruvate kinase (PK) in 1934
• Leloir discovered phosphoglucomutase in 1938, earning a Nobel Prize in 1970
• Warburg crystallized and characterized all glycolytic pathway enzymes between 1935 and 1943, earning a Nobel Prize in 1931 for work on cellular respiration

Definition and Site of Glycolysis

• Glycolysis splits a 6-carbon glucose into two 3-carbon pyruvate molecules under aerobic conditions, or into lactate under anaerobic conditions, producing small amounts of energy
• Glycolysis comes from the Greek words "glykys" (sweet) and "lysis" (splitting)
• All reaction steps of glycolysis occur in the cytoplasm of cells in almost all tissues

Significance of Glycolysis

• Glycolysis happens in all body cells
• It's the only energy source for erythrocytes
• Under strenuous exercise, when muscles lack oxygen, anaerobic glycolysis is the major energy source
• The glycolytic pathway is a preliminary step for complete glucose oxidation, providing carbon skeletons for synthesizing nonessential amino acids and glycerol part of fat
• Most reactions in the glycolytic pathway are reversible and used for gluconeogenesis

Glucose Entry into Cells

• Glucose transporter-4 (GluT4) transports glucose from extracellular fluid to muscle cells and adipocytes, is influenced by insulin, hence, insulin promotes glucose uptake and utilization by peripheral tissues
• In diabetes mellitus, insulin deficiency hinders glucose entry into peripheral cells
• GluT2 is the transporter in liver cells and is not controlled by insulin and Insulin promotes glucose utilization by muscle cells, adipocytes, and other peripheral tissues

Steps of Glycolysis

• Step 1: Glucose is phosphorylated to glucose-6-phosphate by hexokinase or glucokinase, using ATP. Hexokinase is a key regulatory enzyme, and this step is irreversible, while glucokinase is under insulin influence.
• Step 2: Glucose-6-phosphate is isomerized to fructose-6-phosphate by phosphohexose isomerase in a reversible reaction to allow aldose to ketose isomerization.
• Step 3: Fructose-6-phosphate is further phosphorylated to fructose-1,6-bisphosphate by PFK-1, needing ATP, this is irreversible and uses the enzyme PFK
• Steps 1-3: Together step 1-3 are called the preparatory phase of glycolysis
• Step 4: Fructose-1,6-bisphosphate is cleaved into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (DHAP) by aldolase A.
• Step 4A: Dihydroxyacetone phosphate is isomerized to glyceraldehyde-3-phosphate by phosphotriose isomerase
• Steps 4 and 4A: Together these steps are called the splitting phase
• Step 5: Glyceraldehyde-3-phosphate is dehydrogenated and phosphorylated to 1,3-bisphosphoglycerate (1,3-BPG) by glyceraldehyde-3-phosphate dehydrogenase, using NAD+
• Step 6: One ATP molecule is synthesized due to the trapped energy of 1,3-BPG using bisphosphoglycerate kinase.
• Step 7: 3-Phosphoglycerate is isomerized to 2-Phosphoglycerate by shifting the phosphate group with phosphoglucomutase.
• Step 8: 2-Phosphoglycerate is converted to phosphoenol pyruvate (PEP) by the enzyme enolase, with a water molecule removed.
• Step 9: Phosphoenol pyruvate is dephosphorylated to pyruvate by pyruvate kinase, generating ATP.
• Step 10: Under anaerobic conditions, pyruvate is reduced to lactate by lactate dehydrogenase (LDH).

Comparison of hexokinase and glucokinase

• Hexokinase is present in all tissues, with a Km value of 10-2 mmol/L, a high affinity, it acts on glucose, fructose, and mannose, is not induced, and utilized glucose by body cells even when blood glucose is low.
• Glucokinase is only in the liver, with a Km value of 20 mmol/L, a low affinity, acts only on glucose, is induced by insulin and glucose, and acts only when blood glucose is more than 100 mg/dL

Impact of Arsenate and Fluoride on Glycolysis

• ATP formation can be inhibited by arsenate
• Fluoride will irreversibly inhibit glycolysis

Energy Yield from Glycolysis

• Glycolysis in anaerobic conditions yields a net of 2 ATP molecules, while in aerobic conditions it yields 7 ATP
• Complete glucose oxidation through glycolysis plus the citric acid cycle yields a net 32 ATPs

Regulatory Enzymes

• Glucokinase/HK, PFK, and PK are the key glycolysis enzymes

Factors Affecting Glycolysis

• Glucokinase/Hexokinase: Feedback inhibition (HK by glucose-6-phosphate) and activated by insulin (glucokinase is induced by insulin).
• Phosphofructokinase (step 3) has ATP and citrate as key allosteric inhibitors, while AMP acts as an allosteric activator.
• Fructose-2,6-bisphosphate (F-2,6-BP) increases the activity of PFK.
• Pyruvate kinase is inhibited when energy is available in the cell.
• Insulin favors glycolysis by activating three key glycolytic enzymes, and glucagon and glucocorticoids inhibit glycolysis and favor gluconeogenesis