Comprehensive Bullet-Point Notes: Carbohydrate Metabolism, Aerobic/Anaerobic Strategies & Sterol Biology Across Taxa

Distribution of Carbohydrates in Vertebrates & Invertebrates

• Sugars occur free or as parts of complex molecules; major classes and examples:
  • Pentoses: D-ribose, 2-deoxy-D-ribose (DNA sugar)
  • Hexoses: D-glucose (ubiquitous blood sugar), D-galactose, D-mannose, D-fructose, L-fucose
  • Uronic acids: D-glucuronic, L-iduronic acids (detoxification, GAG structure)
  • Hexosamines: D-glucosamine, D-galactosamine (chitin, cartilage)
  • Pentose-phosphate–pathway intermediates appear as phosphates: D-erythrose-4-P, D-ribulose-5-P, D-xylulose-5-P, D-sedoheptulose-7-P
• Invertebrate-specific sugars:
  • L-arabinose & D-xylose (pentoses) and L-rhamnose (deoxyhexose) in many polysaccharides
  • D-xylose links glycosaminoglycan (GAG) chain to core protein in mucopolysaccharides
  • D-arabinose in unusual sponge nucleosides
  • L-galactose restricted to snail galactans; D-galacturonic acid seldom in animals (brain, elasmobranch & gastropod secretions)
• Ascarylose (3,6-dideoxy-L-mannose) unique to bacteria & nematode Ascaris; forms glycolipid “ascaryl alcohol” within egg-shell ascarosides → confers extreme chemical resistance
• Inositols (polyalcohols):
  • Myo-inositol universal in membranes; free in mammalian seminal fluid; phosphate esters modulate Hb O₂-affinity in bird/reptile RBCs
  • Scyllo-inositol widespread; L-chiro-inositol only in cockroach fat-body & haemolymph
• Sialic acids (N-acetyl/ N-glycolyl-neuraminic acids; >30 variants):
  • Substitutions possible at C-4,7,8,9 (acetyl, glycolyl, lactyl, methyl, sulphate, phosphate)
  • Occur mainly bound in gangliosides & glycoproteins; present in mammalian milk/urine, many bacteria, protozoa (e.g., Trypanosoma cruzi) but absent from fungi, algae, higher plants, sponges, arthropods, molluscs, Plasmodium

Extracellular Carbohydrates & Environmental Adaptation

• Insects: extracellular glycerol, sorbitol, trehalose, erythritol, threitol, mannitol; glucose, fructose; amino acids (proline, alanine) & antifreeze proteins accumulate for cold tolerance
• Trehalose = major haemolymph sugar (<glucose except in honey-bee where glucose≈fructose≈trehalose)
• Nematodes: extracellular sugars enable extreme desiccation tolerance
• Vertebrate blood glucose concentrations (mmol L⁻¹): mammals $3–11$, birds $7–12$, reptiles $2–9$, teleosts $1–5$, amphibians $0.5–1.6$; invertebrate haemolymph <$2$
• Mammalian seminal plasma: fructose primary sperm nutrient ± sorbitol, myo-inositol; glucose mostly absent (exception: human, rat, mouse, guinea-pig)

Overview of Glycolysis (Embden-Meyerhof-Parnas)

• Universal cytosolic pathway; only a few anaerobic bacteria/archaea lacking phosphofructokinase-1 cannot complete pathway
• Two phases:
  1. Preparatory (investment): glucose → fructose-1,6-bisP; consumes $2$ ATP
  2. Pay-off: cleavage → two triose-P → pyruvate; produces $4$ ATP + $2$ NADH → net $\Delta ATP = +2$
• Key enzymes (Mg²⁺ or K⁺ cofactors): Hexokinase, PFK-1, Aldolase, Triose-P isomerase, G3P dehydrogenase, PGK, PGM, Enolase, Pyruvate kinase

Glycolysis in Kinetoplastids (e.g., Trypanosoma, Leishmania)

• Possess defective mitochondria; early steps housed in microbody-like organelles = glycosomes
• Glycosomal contents: first 7 glycolytic enzymes + glycerol-3-P kinase & glycerol kinase
• Only G3P-DH & PGK have cytosolic isoforms
• Compartment confers separate redox & ATP balance:
  • Under anaerobiosis: glycerol-3-P → glycerol + ATP (reverse GK)
  • Net products per glucose: 1 glycerol + 1 pyruvate (↓ATP yield)
• Regulatory divergence: Hexokinase & PFK unresponsive to typical modulators → potential drug targets

Carbohydrate-Metabolizing Insects (RQ = $1.0$)

• Orders: Diptera (flies), Hymenoptera (bees, wasps), Dictyoptera (cockroaches)
• Traits:
  • Short flight bursts, rely on trehalose → glucose (via trehalase) then glycolysis
  • High glycolytic/gluconeogenic enzyme activity; low lipid-metabolic activity; phosphocreatine system absent
• Muscle types:
  • Non-fibrillar (locust, roach): neural frequency = contraction frequency
  • Fibrillar (flies, bees): myogenic oscillation >> nerve signal rate; nerve impulse initiates cycle
• Additional substrates:
  • Sorbitol → fructose-6-P
  • UDP-galactose → UDP-glucose → G-1-P → G-6-P
  • Mannose → mannose-6-P → fructose-6-P
• Long flight: glycogen mobilisation → G-1-P → G-6-P
• Anaerobic glycolysis absent (no lactate DH activity)

Bees

• Nectar (sucrose) split by α-glucosidase; contaminant starch by amylase; glucose → gluconic acid + H₂O₂ via glucose oxidase (antiseptic honey)

Insects Using Both Carbohydrates & Lipids (RQ drop $1.0\to0.7$)

• Orthoptera, Lepidoptera, Coleoptera, Hemiptera
• Trehalose/glycogen fuel take-off; prolonged flight switches to fatty acids
• Adipokinetic hormone triggers fat-body DAG release; DAG + HDLp + apoLp-III → LDLp → muscle → DAG hydrolysis → FFA + glycerol (glycerol used in trehalose synthesis)
• FFA transported as carnitine esters into mitochondria → β-oxidation
• Analogous to vertebrate VLDL system

Amino-Acid Fuelling (Proline)

• Bees, wasps, tsetse flies: proline oxidation in mitochondria replenishes TCA intermediates (pyruvate-fumarate shuttle) enabling high carbohydrate flux during flight onset

Carbohydrate Metabolism in Fish

• Muscle: $\sim92\%$ white (anaerobic); glycogen phosphorylase A activated during burst → high glycolytic flux
• $80–90\%$ blood lactate oxidised to $CO<em>2 + H</em>2O$ (cori cycle minor); oxidation mainly in heart & red muscle; limited gluconeogenesis
• Fibre types: I (red, aerobic), IIa (intermediate), IIb (white, anaerobic)

Carbohydrate Metabolism in Plants

Calvin Cycle (C₃ pathway)

1. Fixation: $CO_2 + \text{RuBP} \xrightarrow{\text{Rubisco}} 2\,3\text{-PGA}$
2. Reduction: $3\text{-PGA} \to Glyceraldehyde\,3\text{-P}$ using $ATP$ & $NADPH$
3. Regeneration: triose-P → RuBP

• Overall: $3\,CO<em>2 + 9\,ATP + 6\,NADPH → Glyceraldehyde\,3\text{-P} + 9\,ADP + 8\,P</em>i + 6\,NADP^+$

C₄ Pathway

• Spatial separation: Mesophyll (PEP carboxylase → oxaloacetate/malate) vs bundle-sheath (Calvin)
• Costs extra $2\,ATP/CO_2$ but suppresses photorespiration; favoured in hot, high-light habitats (e.g., maize, sugarcane)

CAM Pathway

• Temporal separation: stomata open at night (PEPC → malate stored in vacuole); daylight: malate decarboxylation provides $CO_2$ for Calvin with closed stomata → extreme water use efficiency (cacti, pineapples)

Aerobic & Anaerobic Metabolism in Animals

Aerobic Respiration

1. Glycolysis (cytosol): $Glucose \to 2\,Pyruvate + 2\,ATP + 2\,NADH$
2. TCA cycle (matrix): $2\,Acetyl\,CoA \to 2\,ATP + 6\,NADH + 2\,FADH_2$
3. ETC (inner membrane): $\approx26–28\,ATP + H_2O$

Aerobic Glycolysis (Warburg Effect)

• Rapidly proliferating cells (tumours, activated lymphocytes) ferment glucose → lactate even with O₂
• Advantages:
  1. Lactic acid export acidifies micro-environment → invasion, immune evasion
  2. Glucose-6-P feeds pentose-P pathway (NADPH, ribose)
  3. Less O₂ dependency during hypoxia
• Biochemical shifts: mitochondrial-bound hexokinase isoform (no feedback by G-6-P); slow pyruvate kinase M (PK-M) diverts intermediates to biosynthesis
• HIF-1 activated by hypoxia or intense anaerobic training → up-regulates glycolytic enzymes, GLUT1/3 & VEGF → angiogenesis

Exercise Metabolism

• Aerobic (distance): gradual switch glycogen → FFA; RQ falls $1.0\to0.7$; AMP-activated kinase + ↓malonyl-CoA activate CPT-I (fat oxidation)
• Anaerobic (sprint): rely on muscle glycogen & phosphocreatine; lactate accumulates; training increases mitochondrial biogenesis → delays lactate reliance

Vertebrate Anaerobic Glycolysis

• $Glucose/Glycogen + 2\,ADP + 2\,P_i \to 2\,Lactate + 2\,ATP$
• Requires high lactate dehydrogenase to re-oxidise $NADH$
• Lactic acid removal (when O₂ returns):
  1. $Lactate \to Pyruvate \to CO<em>2 + H</em>2O$ (oxidation)
  2. $Lactate \to Glucose/Glycogen$ (gluconeogenesis; prominent in fish, amphibians, reptiles)

Phosphagens

• Creatine-P (vertebrates) / Arginine-P (most invertebrates): $\text{Phosphagen} + ADP \leftrightarrow \text{Phosphagen}_{deP} + ATP$ via creatine/arginine kinase; serves as immediate ATP buffer during burst activity

Exercise Interplay Examples

• Fish: steady cruise (red muscle, aerobic) vs escape burst (white muscle, anaerobic)
• Crustacean tail-flip: initial ATP from arginine-P, prolonged from anaerobic glycolysis
• Insect flight muscle: entirely aerobic; up to $\sim50\%$ volume mitochondria, supplied by tracheal O₂
• Molluscs (squid, scallop): pyruvate → octopine via octopine dehydrogenase
• Amphibians: frogs (anaerobic jump burst, high LDH) vs toads (aerobic sustained)

Environmental (Habitat-Dependent) Anaerobiosis

• Oxygen regulation (constant $\dot{V}<em>{O2}$) until critical $P</em>{O2}$ → oxygen conformity
• Aquatic invertebrates (bivalves, annelids) & some fish tolerate prolonged anoxia by metabolic depression (down to $\le10\%$ basal rate)
• Goldfish & crucian carp convert lactate → ethanol + $CO_2$ in muscle via modified alcohol DH; ethanol diffuses across gills → avoids acidosis & allows months-long survival under ice
• End-product patterns:
  • Activity-dependent: lactate or opines (octopine, etc.)
  • Habitat-dependent: succinate, acetate, propionate, alanine (yield up to $7\,ATP/glucose$)
• Excretion of end products or conversion to neutral compounds (ethanol) mitigates acidosis

Sterols & Steroids Across Taxa

General Features

• Sterols rigidify & condense eukaryotic membranes; parent molecule: cholesterol (animals), β-sitosterol/stigmasterol (plants), ergosterol (fungi)
• Many taxa produce steroidal glycosides, saponins, alkaloids for defense

Vertebrate Sterols

• Marine fish: up to $\tfrac13$ sterols as bioactive glycosides
• Human cholesterol balance: ≈50% diet / 50% de novo (liver)
• LCAT esterifies cholesterol in HDL → reverse cholesterol transport to liver via SR-BI; deficiencies (autosomal recessive) cause low HDL, corneal opacities (“fish-eye disease”), normocytic anemia, premature atherosclerosis
• Cholesterol → steroid hormones (C₂₁ corticosteroids/progestins; C₁₉ androgens; C₁₈ estrogens) & bile acids (≈1 g day⁻¹ synthesis)

Insect Sterols

• Cannot synthesise sterols de novo; obtain from diet or symbionts
• Many herbivores 24-dealkylate C₂₈/C₂₉ phytosterols → cholesterol; some insects (beetle, housefly, cockroach) lack this ability
• Silkworm demands cholestane side-chain sterols; sterols serve as membrane components & ecdysteroid precursors

Marine Invertebrates

• Tunicates, annelids, crustaceans, molluscs: cholesterol dominant; bivalves also Δ⁵,⁷-sterols & 4α-methyl sterols; cephalopods produce testosterone
• Echinoderms: Δ⁵ or Δ⁷ C₂₇ sterols; unique C₂₆ sterols, large stores of sterol sulphates, and potent asterosaponins (membrane-toxic)
• Cnidarians: complex Δ⁵ sterols; unique gorgosterol (cyclopropane side-chain) in soft corals w/ symbiotic dinoflagellates

Protozoa

• Tetrahymena synthesises tetrahymanol (hopanoid-like) anaerobically; sterol presence suppresses synthesis
• Amoebae (Acanthamoeba, Naegleria) synthesize cholesterol via cycloartenol pathway

Plant Sterols (Phytosterols)

• Major: campesterol, β-sitosterol, stigmasterol, 24-methylcholesterol
• Regulate membrane fluidity; stigmasterol weaker in ordering effect
• Sterols + glycosphingolipids form lipid rafts influencing protein localisation
• Stress responses: stigmasterol accumulates under heat/salinity/drought
• Campesterol precursor to brassinosteroids (≈70 analogues) – growth hormones; potential anti-cancer agents
• Plants also produce progesterone, testosterone, phytoecdysteroids (defence vs insects)
• Sterol esters store surplus sterol & intermediates in lipid droplets
• Human nutrition: plant sterols/stanols compete with cholesterol absorption, ↓LDL-c; β-sitosterol induces tumour-cell apoptosis via $PI3K/Akt$ inhibition & ROS generation

Fungal Sterols & Antifungals

• Dominant sterol: ergosterol (Ascomycota, Basidiomycota); Oomycetes synthesize fucosterol; lower fungi produce cholesterol & derivatives; some parasites lack sterol synthesis (utilise host cholesterol)
• Roles: membrane stability, stress response, dimorphism, pathogenicity
• Antifungal drug classes & targets:
  1. Azoles (fluconazole, etc.): inhibit lanosterol 14α-demethylase → ↓ergosterol, ↑14-methyl sterols; resistance via efflux pumps, enzyme mutation
  2. Allylamines (terbinafine): block squalene epoxidase
  3. Morpholines (amorolfine): inhibit C-14 sterol reductase & C-8 isomerase → ignosterol accumulation
  4. Polyenes (amphotericin B): bind ergosterol → pores; resistance by altered sterol content
• Balance of biosynthesis/degradation regulated transcriptionally & by feedback to avoid free sterol toxicity

Ecdysteroids (Arthropod Moulting Hormones)

• Cholesterol-derived; retain extended side-chain, highly hydroxylated (↑water-solubility)
• 20-Hydroxyecdysone main active form converted by 20-monooxygenase; other routes: 3-dehydroecdysone reductase, 25-hydroxylation of ponasterone A
• Non-arthropod invertebrates & plants produce ecdysteroid analogues (phytoecdysteroids) – plant defence by disrupting insect development; high concentrations allow eco-friendly pest control

Bacterial Hopanoids

• Hopanoids (e.g., diplopterol) are triterpenoids from squalene; insert into bacterial membranes enhancing rigidity analogous to sterols
• Some pathogens (Mycoplasma) scavenge host sterols for growth