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🎓 FULL LECTURE: Biochemistry - Metabolism (Pages 70-78)

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PAGE 70: ETHANOL METABOLISM

What is ethanol metabolism?

• This happens in the cytosol (the liquid part of the cell, like the cell's "broth")
• The enzyme Alcohol dehydrogenase converts ethanol to acetaldehyde
• In this reaction, NAD+ gets converted to NADH
  • Think of NAD+ like an empty basket, and NADH like a full basket carrying hydrogen (electrons). When alcohol is broken down, the basket gets filled up.

• This happens in the mitochondria (the cell's powerhouse/energy factory)
• The enzyme Aldehyde dehydrogenase does this job
• Again, NAD+ → NADH
• Disulfiram (a drug used to treat alcoholism) blocks this enzyme. So acetaldehyde builds up → causes horrible nausea, flushing, vomiting → discourages drinking. That's why it's used as aversion therapy!

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Key Effects of Ethanol Metabolism - Explained One by One:

• When you drink a lot of alcohol, you make a HUGE amount of NADH
• The TCA cycle (energy production cycle) gets BLOCKED because it needs NAD+ (empty baskets) but they're all full (NADH)
• So acetyl-CoA (a key energy molecule) can't enter the TCA cycle
• Instead it piles up

• Because TCA cycle is blocked, acetyl-CoA gets diverted to make fat (triglycerides)
• This fat deposits in the liver → Fatty liver (hepatosteatosis)
• Think: "Drinkers get a fat liver because their TCA cycle is jammed"

• Excess acetyl-CoA also gets converted to ketones (acetoacetate, beta-hydroxybutyrate)
• This is why heavy drinkers can develop alcoholic ketoacidosis (blood becomes acidic from too many ketones)

• High NADH pushes the reaction: Pyruvate → Lactate
• So blood lactate increases → lactic acidosis (blood becomes acidic)
• Also, pyruvate gets used up, so it's not available for gluconeogenesis (making new glucose)

• Because:
  • Pyruvate is being converted to lactate
  • OAA (oxaloacetate - a key gluconeogenesis ingredient) is diverted
• The liver can't make new glucose
• So a person who's drunk and hasn't eaten can get dangerously low blood sugar!
• That's why drunk people sometimes pass out - partly low glucose.

• NAD+ is the "limiting factor" - when it runs out, metabolism goes haywire

• Confirms: the entire problem with alcohol is that NAD+ gets depleted

• Females are more susceptible to effects of alcohol than males because:
  • They have less gastric (stomach) alcohol dehydrogenase - so more alcohol reaches the blood
  • They have smaller body size
  • They have higher % body fat (alcohol distributes poorly into fat)
  • Result: same amount of alcohol = higher blood levels in women

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Fomepizole - Drug for Alcohol Poisoning

• Fomepizole = competitive inhibitor of alcohol dehydrogenase
• Used when someone drinks methanol (wood alcohol) or ethylene glycol (antifreeze)
• Why? Because alcohol dehydrogenase converts methanol → formaldehyde (toxic! causes blindness)
• And ethylene glycol → oxalic acid (causes kidney failure)
• Fomepizole blocks alcohol dehydrogenase → stops the toxic conversion
• Interestingly, ethanol itself can also be used as antidote because it has higher affinity for the enzyme and competes with methanol

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PAGE 71: ENZYME TERMINOLOGY - "What do these names MEAN?"

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Kinase

• What it does: Transfers a phosphate group from a high-energy molecule (usually ATP) to a substrate
• Think: A kinase "kindles energy" - it uses ATP's energy to add a phosphate to something
• Example: Glucokinase phosphorylates glucose → glucose-6-phosphate

Phosphorylase

• What it does: Removes a phosphate group from a substrate WITHOUT using ATP
• Different from kinase! Kinase ADDS phosphate using ATP. Phosphorylase REMOVES it.
• Example: Glycogen phosphorylase breaks down glycogen by removing phosphate groups

Dehydrogenase

• What it does: Catalyzes oxidation-reduction reactions (eg, pyruvate dehydrogenase)
• It "dehydrogenates" - removes hydrogen (electrons) from the substrate
• Example: Lactate dehydrogenase converts lactate → pyruvate by removing hydrogen

Hydroxylase

• What it does: Adds a hydroxyl group (-OH) to a substrate
• Example: Tyrosine hydroxylase converts tyrosine to L-DOPA

Carboxylase

• What it does: Transfers a carboxyl group (-COOH) using biotin as a cofactor
• Remember: Carboxylase needs BIOTIN (Vitamin B7)
• Example: Pyruvate carboxylase adds CO2 to pyruvate → makes oxaloacetate

Mutase

• What it does: Relocates a functional group WITHIN a molecule (same molecule, different position)
• Example: Phosphoglycerate mutase moves the phosphate group from position 3 to position 2

Synthase

• What it does: Catalyzes synthesis reactions WITHOUT using ATP as energy source
• Example: Citrate synthase makes citrate from acetyl-CoA + oxaloacetate

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Rate-Determining Enzymes of Metabolic Processes

Regulators:

• Glycolysis (PFK-1): ACTIVATED by AMP (energy is low, need more!), fructose-2,6-bisphosphate; INHIBITED by ATP (already have enough energy), citrate
• Gluconeogenesis (FBPase-1): ACTIVATED by citrate, AMP inhibits it; fructose-2,6-bisphosphate inhibits
• TCA cycle (isocitrate dehydrogenase): ACTIVATED by ADP; INHIBITED by ATP, NADH
• Glycogen synthase: ACTIVATED by glucose-6-phosphate, insulin; INHIBITED by epinephrine, glucagon
• Glycogen phosphorylase: ACTIVATED by epinephrine, glucagon, AMP; INHIBITED by glucose-6-phosphate, insulin, ATP

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PAGE 72: METABOLIC COMPARTMENTATION

Where does each process happen?

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Mitochondria (the powerhouse - deep inside the cell)

• Fatty acid oxidation (β-oxidation) - burning fat for fuel
• Acetyl-CoA production (from pyruvate)
• TCA cycle - the main energy-producing cycle
• Oxidative phosphorylation - making ATP using electron transport
• Fatty acid synthesis (partially)
• Ketogenesis (making ketone bodies)

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Cytoplasm/Cytosol (the main liquid inside the cell)

• Glycolysis - breaking down glucose
• HMP shunt (Pentose phosphate pathway)
• Synthesis of cholesterol (in the SER - smooth endoplasmic reticulum)
• Synthesis of proteins (in ribosomes/RER)
• Fatty acid synthesis
• Synthesis of nucleotides

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Both mitochondria AND cytoplasm:

• Heme synthesis (starts in mitochondria, finishes there too, but some steps in cytoplasm)
• Urea cycle (starts in mitochondria, finishes in cytoplasm)
• Gluconeogenesis (steps in both)

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Summary of Metabolic Pathways (Big Map - Page 72 Diagram)

1. Glucose enters glycolysis (steps 1-13 numbered on diagram)
2. Glucose → Glucose-6-phosphate (enzyme: Hexokinase/Glucokinase)
3. → Fructose-6-phosphate → Fructose-1,6-bisphosphate (enzyme: PFK-1 - the rate-limiting step!)
4. → Glyceraldehyde-3-P (G3P) and DHAP (Dihydroxyacetone phosphate)
5. → Eventually → Pyruvate
6. Pyruvate → Acetyl-CoA (via PDH - pyruvate dehydrogenase)
7. Acetyl-CoA → enters TCA cycle → makes NADH, FADH2, GTP
8. NADH/FADH2 → Electron transport chain → ATP

• It can enter the HMP shunt (makes NADPH and ribose-5-phosphate)
• It can enter Glycogenesis (stored as glycogen)

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PAGE 73: ACTIVATED CARRIERS + HEXOKINASE vs GLUCOKINASE

Activated Carriers - "Delivery trucks of metabolism"

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Universal Electron Acceptors - NAD+ vs NADPH

• NAD+ = empty battery (oxidized form) - used in CATABOLIC (breaking down) reactions
  • When a molecule is broken down, NAD+ accepts the electrons → becomes NADH (full battery)
  • NADH is then "drained" in the electron transport chain to make ATP
• NADPH = full battery ready to GIVE electrons - used in ANABOLIC (building up) reactions
  • Used in: Fatty acid synthesis, Cholesterol synthesis, RBC protection (glutathione reduction), Respiratory burst (immune cells killing bacteria), Cytochrome P-450 system
  • NADPH is a product of the HMP shunt

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Hexokinase vs Glucokinase - "Two bouncers at the glucose door"

• Hexokinase: Found everywhere. Always working even at LOW glucose levels. As soon as cells get a little glucose, it traps it immediately. Gets shut off when glucose-6-phosphate builds up (self-regulating).
• Glucokinase: Only in liver and pancreas. Works only at HIGH glucose levels (like after a big meal). Doesn't get saturated easily - the more glucose comes in, the harder it works. Doesn't shut off easily. The liver uses it to store glucose as glycogen after meals.

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PAGE 74: GLYCOLYSIS REGULATION + PYRUVATE DEHYDROGENASE

Glycolysis - The Big Equation

Glucose + 2 Pi + 2 ADP + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

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Key Glycolysis Enzymes:

1. Glucose → Glucose-6-P via Hexokinase/Glucokinase (uses 1 ATP)
2. Fructose-6-P → Fructose-1,6-bisphosphate via Phosphofructokinase-1/PFK-1 (rate-limiting! uses 1 ATP)
   • PFK-1 is the "gatekeeper" of glycolysis
   • ACTIVATED by: AMP (+), Fructose-2,6-bisphosphate (+)
   • INHIBITED by: ATP (-), Citrate (-)

1. 1,3-BPG → 3-PG via Phosphoglycerate kinase
2. Phosphoenolpyruvate → Pyruvate via Pyruvate kinase (another key regulated step!)
   • ACTIVATED by: Fructose-1,6-bisphosphate
   • INHIBITED by: ATP (-), Alanine (-), Glucagon (-), Epinephrine (-)

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Regulation by Fructose-2,6-Bisphosphate (FBP)

• As PFK-2: Makes fructose-2,6-bisphosphate → PROMOTES GLYCOLYSIS
• As FBPase-2: Destroys fructose-2,6-bisphosphate → PROMOTES GLUCONEOGENESIS

• Glucagon rises → cAMP rises → Protein Kinase A activates → phosphorylates the enzyme → FBPase-2 is active → less fructose-2,6-bisphosphate → less glycolysis, MORE gluconeogenesis (liver makes glucose for the blood)
• Memory trick: "FaBian the Peasant (FBP) has to work hard when STARVING" - FBPase is active when starving

• Insulin rises → PFK-2 is active → MORE fructose-2,6-bisphosphate → MORE glycolysis, less gluconeogenesis (burn that glucose you just ate!)
• Memory trick: "Prince FredericK (PFK) works only when FED"

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Pyruvate Dehydrogenase Complex (PDH Complex)

1. Thiamine pyrophosphate (B1) = TPP
2. Lipoic acid = Lipoamide
3. CoA (B5, pantothenic acid) = Coenzyme A
4. FAD (B2, riboflavin) = Flavin
5. NAD+ (B3, niacin) = Nicotinamide

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Arsenic Poisoning and PDH

• Arsenic inhibits lipoic acid
• Memory trick: "Imagine a vampire (pigmented skin changes, skin cancer), vomiting and having diarrhea, running away from a cutie (QT prolongation) with garlic breath" = findings of arsenic poisoning!

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PAGE 75: PDH DEFICIENCY + PYRUVATE METABOLISM + TCA CYCLE

Pyruvate Dehydrogenase Complex Deficiency

• Pyruvate can't become Acetyl-CoA
• So pyruvate piles up
• It gets shunted to:
  • Lactate (via LDH = lactate dehydrogenase)
  • Alanine (via ALT = alanine aminotransferase)
• This is X-linked (gene is on the X chromosome)

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Pyruvate Metabolism - Four Fates of Pyruvate

• Carries amino groups from muscle to liver
• Part of the Cahill cycle (glucose-alanine cycle)
• Think: muscle exports alanine to liver for gluconeogenesis

• Oxaloacetate can replenish TCA cycle OR go into gluconeogenesis
• Location: Mitochondria
• Remember: "OAA = C4 compound, Pyruvate = C3, adding CO2 = +1 carbon"

• This is the transition to TCA cycle
• Location: Mitochondria

• End of anaerobic glycolysis (when no oxygen - like RBCs, WBCs, kidney medulla, lens, cornea, Sertoli cells)
• These cells have NO mitochondria or prefer anaerobic - they make lactate always

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TCA Cycle (Tricarboxylic Acid Cycle = Krebs Cycle)

1. Pyruvate (3C) → Acetyl-CoA (2C) + CO2 [by PDH] → 1 NADH
2. Acetyl-CoA + Oxaloacetate → Citrate (6C) [by Citrate Synthase]
3. Citrate → cis-Aconitate → Isocitrate (6C)
4. Isocitrate → α-Ketoglutarate (α-KG) (5C) + CO2 + NADH [by Isocitrate Dehydrogenase - rate-limiting!]
5. α-KG → Succinyl-CoA (4C) + CO2 + NADH [by α-KG Dehydrogenase - same cofactors as PDH!]
6. Succinyl-CoA → Succinate + GTP [releases energy → 1 GTP made here!]
7. Succinate → Fumarate + FADH2 [by Succinate dehydrogenase = Complex II of ETC!]
8. Fumarate → Malate
9. Malate → Oxaloacetate + NADH [cycle complete!]

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PAGE 76: ELECTRON TRANSPORT CHAIN + GLUCONEOGENESIS

Electron Transport Chain (ETC) and Oxidative Phosphorylation

1. NADH donates electrons to Complex I
2. FADH2 donates electrons to Complex II (at a LOWER energy level than NADH, so less ATP)
3. Electrons flow through CoQ (Coenzyme Q = Ubiquinone) → Complex III → Cytochrome C → Complex IV
4. At Complex IV, electrons combine with O2 + H+ → H2O
5. As electrons flow, H+ (protons) are pumped OUT of the mitochondrial matrix → creates a proton gradient
6. H+ flows BACK IN through Complex V (ATP Synthase) → drives ATP production

• Via malate-aspartate shuttle (heart and liver): 32 net ATP
• Via glycerol-3-phosphate shuttle (muscle): 30 net ATP
• Anaerobic glycolysis: Only 2 net ATP

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Inhibitors of the ETC:

• Cyanide (CN-) and CO block Complex IV (block electrons reaching oxygen → no ATP, cells die fast!)
• These are the classic "rapid death" poisons

• Found in brown fat (babies and hibernating animals have lots of it)
• They make the inner membrane LEAKY to H+
• H+ flows back WITHOUT going through ATP synthase → energy is released as HEAT, not ATP
• Brown fat "wastes" energy as heat to keep warm
• More mitochondria + uncoupling = MORE heat production

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Gluconeogenesis - Making New Glucose

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The 4 Unique Gluconeogenesis Enzymes:

• Pyruvate → Oxaloacetate
• Requires: Biotin + ATP
• Activated by: Acetyl-CoA
• Why start here? Because you need OAA to eventually make PEP for gluconeogenesis

• Oxaloacetate → PEP (phosphoenolpyruvate)
• Requires: GTP
• This bypasses the pyruvate kinase step of glycolysis

• Fructose-1,6-bisphosphate → Fructose-6-phosphate
• Activated by: Citrate
• Inhibited by: AMP, Fructose-2,6-bisphosphate
• This bypasses PFK-1, the rate-limiting step of glycolysis

• Glucose-6-phosphate → Glucose (free glucose, can leave the cell!)
• This is why muscle CANNOT do gluconeogenesis - it has NO glucose-6-phosphatase!
• Liver has it, so liver glucose can go into blood

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Odd-chain vs Even-chain Fatty Acids for Gluconeogenesis

• Even-chain fatty acids (most fatty acids) → break down to only Acetyl-CoA → enter TCA → make energy but CANNOT make new glucose (acetyl-CoA can't become glucose - it's a dead end for gluconeogenesis)
• Odd-chain fatty acids → break down to Acetyl-CoA + Propionyl-CoA → Propionyl-CoA → Succinyl-CoA (via vitamin B12) → enters TCA → CAN feed into gluconeogenesis!
• Memory trick: "It's ODD for fatty acids to make glucose" = only ODD-chain can do it

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PAGE 77: PENTOSE PHOSPHATE PATHWAY (HMP SHUNT) + G6PD DEFICIENCY

Pentose Phosphate Pathway (also called HMP Shunt)

1. NADPH - needed for reductive reactions (making fat, cholesterol, steroid hormones, and protecting RBCs from oxidative damage)
2. Ribose-5-phosphate - needed for making nucleotides (DNA and RNA building blocks)

• Make a lot of fat/cholesterol (liver, lactating mammary glands, adrenal cortex for steroids)
• Need nucleotides
• RBCs (to maintain glutathione for protection)

1. Oxidative phase (IRREVERSIBLE): Glucose-6-P → 6-Phosphogluconate → Ribulose-5-P + 2 NADPH (via G6PD enzyme)
2. Nonoxidative phase (REVERSIBLE): Ribulose-5-P can become various sugars including ribose-5-P (for nucleotides) or re-enter glycolysis as F6P or G3P

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G6PD Deficiency - Glucose-6-Phosphate Dehydrogenase Deficiency

1. Glucose-6-P + NADP+ → 6-phosphogluconate + NADPH (via G6PD)
2. NADPH + oxidized glutathione (GSSG) → reduced glutathione (2 GSH) (via Glutathione reductase)
3. 2 GSH + H2O2 → GSSG + 2 H2O (via Glutathione peroxidase)

• No G6PD → No NADPH → Glutathione becomes oxidized → H2O2 builds up → Hemoglobin gets denatured → Heinz bodies form (these are clumps of denatured hemoglobin inside RBCs)
• Splenic macrophages try to eat these Heinz bodies out of RBCs → results in Bite cells (RBCs with a bite taken out!)
• The RBCs are fragile → Hemolytic anemia

• Fava beans (hence also called Favism)
• Drugs: Primaquine (malaria drug), dapsone, sulfonamides, nitrofurantoin
• Infections (most common cause of hemolysis - inflammatory response makes free radicals)

• X-linked RECESSIVE (so males affected more, females can be carriers)
• Most COMMON human enzyme deficiency in the world
• More common in people from malaria-endemic regions (sub-Saharan Africa, Southeast Asia, Mediterranean)
• Why? Because G6PD deficiency actually PROTECTS against malaria (similar to sickle cell - the parasite can't survive well in the abnormal RBC)

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PAGE 78: DISORDERS OF FRUCTOSE AND GALACTOSE METABOLISM

Fructose Metabolism

1. Fructose → Fructose-1-phosphate (by Fructokinase)
2. Fructose-1-P → DHAP + Glyceraldehyde (by Aldolase B)
3. These enter glycolysis normally

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Essential Fructosuria (Fructokinase deficiency)

• Enzyme missing: Fructokinase (autosomal recessive)
• What happens: Fructose can't be converted to Fructose-1-P, so fructose builds up in blood and urine
• Symptoms: NONE! Completely BENIGN and asymptomatic
• Key finding: Fructose appears in blood and urine
• Urine dipstick will be NEGATIVE for glucose (it's not glucose-specific) but a reducing sugar test (like Clinitest) will be positive - detecting fructose
• The "kinase" is missing, so fructose just hangs around harmlessly

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Hereditary Fructose Intolerance (Aldolase B deficiency)

• Enzyme missing: Aldolase B (autosomal recessive)
• What happens: Fructose-1-phosphate accumulates!
  • Fructose-1-P is TOXIC to liver cells
  • Also, it TRAPS phosphate (Pi), so less Pi is available → less ATP production
  • Fructose-1-P inhibits glycogenolysis AND gluconeogenesis → severe hypoglycemia!
• Symptoms:
  • Hypoglycemia (low blood sugar)
  • Jaundice (liver damage)
  • Cirrhosis (liver scarring)
  • Vomiting
  • Symptoms ONLY appear after fructose/sucrose/honey intake
• Treatment: Eliminate fructose, sucrose, sorbitol from diet

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Galactose Metabolism

1. Galactose → Galactose-1-phosphate (by Galactokinase)
2. Galactose-1-P + UDP-Glucose → Glucose-1-P + UDP-Galactose (by Galactose-1-P Uridyltransferase)
3. UDP-Galactose can also be recycled or go to glycolysis

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Galactokinase Deficiency

• Enzyme missing: Galactokinase (autosomal recessive)
• What accumulates: Galactitol (when galactose can't be phosphorylated, it gets reduced to galactitol)
• Symptoms: Mild! Mostly CATARACTS (in infants) and galactosemia/galactosuria
• May present as failure to track objects or develop social smile (because of early cataracts)
• Treatment: Exclude galactose and lactose (found in milk) from diet

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Classic Galactosemia (Galactose-1-Phosphate Uridyltransferase Deficiency)

• Enzyme missing: Galactose-1-phosphate uridyltransferase (autosomal recessive)
• What accumulates: Galactose-1-phosphate (TOXIC!) + galactitol
• This is the SEVERE form!
• Symptoms start when infant is fed formula or breast milk (both contain lactose = glucose + galactose):
  • Failure to thrive
  • Jaundice
  • Hepatomegaly (big liver)
  • Infantile cataracts
  • Intellectual disability
  • Can predispose neonates to E. coli sepsis (classic association!)
• Treatment: Exclude galactose and lactose from diet

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QUICK SUMMARY OF KEY CONCEPTS

Things to Never Forget:

1. Alcohol metabolism: High NADH → blocks TCA → fatty liver, ketoacidosis, lactic acidosis, hypoglycemia. Women more affected. Disulfiram blocks aldehyde dehydrogenase. Fomepizole blocks alcohol dehydrogenase.
2. Rate-limiting enzymes: PFK-1 (glycolysis), FBPase-1 (gluconeogenesis), Isocitrate DH (TCA), G6PD (HMP shunt), HMG-CoA reductase (cholesterol), HMG-CoA synthase (ketones)
3. PDH complex: B1, B2, B3, B5, lipoic acid. Deficiency = lactic acidosis + high alanine. Arsenic poisons it. X-linked.
4. Glucokinase vs Hexokinase: Glucokinase = liver/pancreas, high Km, induced by insulin. Hexokinase = everywhere, low Km.
5. Gluconeogenesis: Only liver and kidney. 4 unique enzymes. Muscles CANNOT make glucose (no G6Pase). Even-chain FAs cannot make glucose. Odd-chain CAN.
6. G6PD deficiency: X-linked recessive, hemolytic anemia triggered by oxidizing agents, Heinz bodies → Bite cells, protective against malaria, most common enzyme deficiency in humans.
7. Fructosuria (Fructokinase deficiency): BENIGN - just fructose in urine, no symptoms
8. Hereditary Fructose Intolerance (Aldolase B deficiency): SEVERE - hypoglycemia, liver damage, avoid fructose
9. Galactokinase deficiency: Cataracts only
10. Classic Galactosemia (Uridyltransferase deficiency): Severe - liver, brain, cataracts, E. coli sepsis

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