Carbohydrates from basics to graduate level
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Carbohydrates: From Basics to Graduate Level
A complete reference covering structure, classification, digestion, transport, and all major metabolic pathways - drawn from Lippincott's Biochemistry, Basic Medical Biochemistry (6th ed.), Harper's Illustrated Biochemistry, and Goldman-Cecil Medicine.
1. What Are Carbohydrates?
Carbohydrates are organic molecules with the empirical formula (CH₂O)n, meaning they are literally "hydrates of carbon." They serve as:
- The primary energy currency of the cell
- Structural scaffolds (cellulose, chitin)
- Information molecules (glycoproteins, glycolipids, blood group antigens)
- Precursors for nucleic acid synthesis (ribose, deoxyribose)
2. Classification
2.1 Monosaccharides (Simple Sugars)
The irreducible unit. Classified by:
- Carbon number: trioses (3C), pentoses (5C), hexoses (6C)
- Functional group: aldoses (aldehyde) vs ketoses (ketone)
| Sugar | Type | Significance |
|---|---|---|
| Glucose | Aldohexose | Primary blood sugar, brain fuel |
| Fructose | Ketohexose | Fruit sugar, enters glycolysis via fructose-1-P |
| Galactose | Aldohexose | From lactose digestion, UDP-galactose donor |
| Ribose | Aldopentose | RNA backbone |
| Deoxyribose | Aldopentose | DNA backbone |
| Glyceraldehyde | Aldotriose | Glycolytic intermediate |
| Dihydroxyacetone | Ketotriose | Glycolytic intermediate (DHAP) |
Stereochemistry: Monosaccharides are chiral. The D/L designation is determined by the configuration of the asymmetric carbon farthest from the carbonyl group (by convention relative to glyceraldehyde). All naturally occurring monosaccharides in humans are D-configuration. Enantiomers (mirror images) are rarely metabolized.
Ring forms (Haworth projections): In aqueous solution, glucose spontaneously cyclizes. The C1 aldehyde attacks the C5 hydroxyl, creating a pyranose (6-membered) ring via hemiacetal formation. This generates two anomers:
- α-D-glucose: OH at C1 is axial (below the ring in chair form)
- β-D-glucose: OH at C1 is equatorial (above the ring) - the more stable form
Fructose forms either furanose (5-membered, as in sucrose) or pyranose rings.
2.2 Disaccharides
Two monosaccharides joined by a glycosidic bond (condensation reaction with loss of water):
| Disaccharide | Components | Bond | Notes |
|---|---|---|---|
| Sucrose | Glucose + Fructose | α(1→2) | Table sugar; non-reducing (both anomeric C bonded) |
| Lactose | Galactose + Glucose | β(1→4) | Milk sugar; reducing |
| Maltose | Glucose + Glucose | α(1→4) | Starch digest product; reducing |
| Trehalose | Glucose + Glucose | α(1→1) | Insect hemolymph energy source; non-reducing |
| Cellobiose | Glucose + Glucose | β(1→4) | Cellulose subunit |
A reducing sugar has a free anomeric carbon (C1-OH), which can reduce cupric ions in Benedict's/Fehling's tests. Sucrose and trehalose are non-reducing because both anomeric carbons are involved in the bond.
2.3 Oligosaccharides
Short chains (3-10 monosaccharides). Functionally important as:
- Cell surface recognition molecules (blood groups, lectins)
- N- and O-linked chains on glycoproteins
- Fructooligosaccharides (prebiotics feeding gut flora)
2.4 Polysaccharides
Long chains with structural or storage roles:
| Polysaccharide | Monomer | Linkage | Location/Function |
|---|---|---|---|
| Starch (amylose) | Glucose | α(1→4) | Plant storage; linear |
| Starch (amylopectin) | Glucose | α(1→4) + α(1→6) branches | Plant storage; branched every 24-30 residues |
| Glycogen | Glucose | α(1→4) + α(1→6) branches | Animal storage; more branched than amylopectin (every 8-14 residues) |
| Cellulose | Glucose | β(1→4) | Plant cell walls; structural; humans cannot digest |
| Chitin | N-acetylglucosamine | β(1→4) | Arthropod exoskeletons, fungal walls |
| Hyaluronic acid | GlcUA + GlcNAc | β(1→3)/β(1→4) | Connective tissue ECM, joint fluid |
The critical difference between starch/glycogen (digestible) and cellulose (indigestible) is the α vs β linkage. Humans lack cellulase (the enzyme needed to cleave β-1,4 bonds).
3. Digestion and Absorption
3.1 Digestion
Dietary carbohydrates are primarily starch (50-60% of intake), plus sucrose and lactose.
- Salivary α-amylase begins starch hydrolysis in the mouth (cleaves internal α-1,4 bonds → dextrins, maltose)
- Chewing, then gastric acid inactivates amylase - digestion pauses
- Pancreatic α-amylase in the small intestine resumes hydrolysis - major step
- Brush border enzymes of the intestinal mucosa complete digestion:
- Maltase-glucoamylase: cleaves maltose and short chains
- Sucrase-isomaltase: cleaves sucrose (and α-1,6 branch points)
- Lactase (β-galactosidase): cleaves lactose → glucose + galactose
Undigested carbohydrates (fiber, resistant starch) pass to the colon, where gut bacteria ferment them to short-chain fatty acids (acetate, propionate, butyrate) - an important energy source for colonocytes.
3.2 Intestinal Absorption
Two systems carry glucose and galactose across the brush border:
1. SGLT1 (Na+/glucose cotransporter) - active, secondary active transport. Uses the Na+ gradient (maintained by Na+/K+-ATPase) to carry glucose against its concentration gradient. ATP is used indirectly. Also transports galactose.
2. GLUT5 - passive facilitated diffusion, transports fructose (not glucose) across the apical membrane.
At the basolateral side, all three monosaccharides exit via GLUT2 into the portal blood. GLUT2 has a high Km (~15-20 mM), meaning it is a low-affinity, high-capacity transporter - ideal for removing large post-meal glucose loads.
4. Glucose Transport into Cells (GLUTs)
Once in the bloodstream, glucose enters cells via a family of 14 GLUT isoforms (facilitated diffusion, passive, no energy required):
(From Lippincott's Biochemistry, 8th ed.)
| GLUT | Location | Km (mM) | Special Notes |
|---|---|---|---|
| GLUT-1 | Most tissues, RBCs, BBB | 1 | Basal glucose uptake; constitutively expressed |
| GLUT-2 | Liver, kidney, β-cells | 15-20 | Bidirectional; glucose sensor in β-cells |
| GLUT-3 | Neurons, most tissues | 1 | High affinity ensures brain supply |
| GLUT-4 | Muscle, adipose | 5 | Insulin-responsive; translocates from intracellular vesicles to plasma membrane |
| GLUT-5 | Small intestine, testes | 10 | Fructose transporter |
Insulin does not alter GLUT expression directly; it triggers vesicle translocation of GLUT4 to the cell surface, rapidly increasing muscle and fat glucose uptake. This is the key mechanism defective in type 2 diabetes.
5. Glycolysis
The central pathway of carbohydrate catabolism. Ten enzymatic reactions convert 1 glucose (6C) → 2 pyruvate (3C), occurring in the cytosol of all cells.
(From Basic Medical Biochemistry, 6th ed. and Lippincott's Biochemistry, 8th ed.)
5.1 Overview
Glycolysis has two phases:
- Preparatory (investment) phase (reactions 1-5): 2 ATP consumed, glucose is phosphorylated and split
- Payoff (yield) phase (reactions 6-10): 4 ATP and 2 NADH generated
Net yield per glucose (aerobic): 2 ATP + 2 NADH + 2 pyruvate
5.2 The 10 Reactions
| Step | Enzyme | Reaction | Key Feature |
|---|---|---|---|
| 1 | Hexokinase (tissue) / Glucokinase (liver, β-cells) | Glucose → Glucose-6-P | Irreversible; traps glucose in cell |
| 2 | Phosphoglucose isomerase | G-6-P → F-6-P | Isomerization |
| 3 | Phosphofructokinase-1 (PFK-1) | F-6-P + ATP → F-1,6-bisP | Rate-limiting step; major regulation site |
| 4 | Aldolase | F-1,6-bisP → DHAP + G-3-P | Splits the 6C into two 3C |
| 5 | Triose phosphate isomerase | DHAP → G-3-P | Allows both 3C fragments to proceed |
| 6 | Glyceraldehyde-3-P dehydrogenase | G-3-P + NAD+ + Pi → 1,3-bisphosphoglycerate + NADH | Oxidation; generates high-energy intermediate |
| 7 | Phosphoglycerate kinase | 1,3-bisP-glycerate + ADP → 3-phosphoglycerate + ATP | First ATP synthesis (substrate-level) |
| 8 | Phosphoglycerate mutase | 3-PG → 2-PG | |
| 9 | Enolase | 2-PG → PEP + H₂O | |
| 10 | Pyruvate kinase | PEP + ADP → Pyruvate + ATP | Irreversible; second ATP synthesis |
5.3 Regulation of Glycolysis
PFK-1 is the master regulator. It is:
- Activated by: AMP, ADP, fructose-2,6-bisphosphate (F-2,6-BP), inorganic phosphate
- Inhibited by: ATP (high energy charge), citrate
Fructose-2,6-bisphosphate is a key regulatory molecule made by PFK-2 (a bifunctional enzyme that is itself regulated by insulin/glucagon). Insulin stimulates F-2,6-BP production → activates PFK-1 → promotes glycolysis. Glucagon does the opposite.
Hexokinase is inhibited by its product (G-6-P) - a feedback mechanism. Glucokinase (liver) is NOT inhibited by G-6-P and has a high Km - it acts as a glucose sensor, active only when blood glucose is high.
5.4 Fates of Pyruvate
| Condition | Fate | Product |
|---|---|---|
| Aerobic (O₂ available) | Enters TCA cycle (via pyruvate dehydrogenase complex) | CO₂ + H₂O + ATP |
| Anaerobic (no O₂) | Reduced to lactate (regenerates NAD+) | Lactate (Cori cycle) |
| Yeast (fermentation) | Decarboxylated → acetaldehyde → ethanol | CO₂ + ethanol |
| Liver/Adipose | Precursor for fatty acid synthesis | Acetyl-CoA |
6. Pyruvate Dehydrogenase Complex (PDC) and the TCA Cycle
Pyruvate dehydrogenase complex (PDC) in the mitochondrial matrix is the irreversible bridge from glycolysis to the TCA cycle:
Pyruvate + CoA + NAD+ → Acetyl-CoA + CO₂ + NADH
This complex requires five cofactors: TPP (thiamine/B1), lipoic acid, CoA (pantothenate/B5), FAD (riboflavin/B2), NAD+ (niacin/B3).
PDC is inhibited by its products (acetyl-CoA, NADH) and by ATP. It is activated when AMP, CoA, NAD+, and calcium are elevated.
Wernicke's encephalopathy is caused by thiamine (B1) deficiency, which impairs PDC and α-ketoglutarate dehydrogenase. Glucose infusion without thiamine in malnourished patients can precipitate this.
The TCA (Krebs) cycle then oxidizes acetyl-CoA completely:
1 Acetyl-CoA → 2 CO₂ + 3 NADH + 1 FADH₂ + 1 GTP
The NADH and FADH₂ feed the electron transport chain (ETC) and oxidative phosphorylation, generating approximately 34 ATP per glucose aerobically (total yield ~36-38 ATP per glucose, depending on shuttle used).
7. Glycogen Metabolism
(From Lippincott's Biochemistry, 8th ed., and Goldman-Cecil Medicine)
7.1 Structure of Glycogen
Glycogen is a branched-chain polysaccharide made exclusively of α-D-glucose:
- Primary linkage: α(1→4) (linear chain)
- Branch points: α(1→6) every 8-14 glucosyl residues
- Can contain up to 55,000 glucosyl residues per molecule
- Stored as large cytoplasmic granules with associated enzymes
Stores: ~400 g in muscle (for local use), ~100 g in liver (for blood glucose maintenance). Muscle mass > liver mass, so most total body glycogen is in muscle.
7.2 Glycogen Synthesis
- Glucose → Glucose-6-P (glucokinase/hexokinase + ATP)
- Glucose-6-P → Glucose-1-P (phosphoglucomutase)
- Glucose-1-P + UTP → UDP-glucose + PPi (UDP-glucose pyrophosphorylase)
- Glycogenin (a self-glucosylating protein) forms the oligosaccharide primer
- Glycogen synthase extends chains via α(1→4) bonds (primary regulatory enzyme)
- Branching enzyme (amylo-4,6-glucosyltransferase) creates α(1→6) branch points by transferring terminal 6-7 residue segments
Glycogen synthase is inactivated by phosphorylation (by protein kinase A, triggered by glucagon/epinephrine) and activated by dephosphorylation (insulin activates phosphatase). It is also allosterically activated by glucose-6-P.
7.3 Glycogen Breakdown (Glycogenolysis)
- Glycogen phosphorylase cleaves α(1→4) bonds phosphorolytically → Glucose-1-P (stops 4 residues from branch point)
- Debranching enzyme (bifunctional):
- 4α-glucanotransferase activity: transfers 3 of 4 remaining residues to another chain
- α(1→6)-glucosidase activity: cleaves the branch point → free glucose
- Glucose-1-P → Glucose-6-P (phosphoglucomutase)
- In liver: Glucose-6-phosphatase releases free glucose into blood In muscle: No glucose-6-phosphatase - G-6-P enters glycolysis directly
Phosphorylase is activated by phosphorylation (glucagon/epinephrine → cAMP → PKA → phosphorylase kinase → phosphorylase). Calcium also activates phosphorylase kinase directly (crucial during muscle contraction).
7.4 Glycogen Storage Diseases (GSDs)
More than 15 enzymatic defects correspond to different GSDs (most autosomal recessive):
| Type | Enzyme Deficiency | Organs | Key Features |
|---|---|---|---|
| I (von Gierke) | Glucose-6-phosphatase | Liver, kidney | Severe fasting hypoglycemia, hepatomegaly |
| II (Pompe) | Lysosomal acid α-glucosidase | Heart, muscle, liver | Cardiomyopathy, hypotonia; lysosomal storage disease |
| III (Cori/Forbes) | Debranching enzyme | Liver, muscle | Hepatomegaly, myopathy, less severe than type I |
| V (McArdle) | Muscle phosphorylase | Muscle | Exercise intolerance, cramps, myoglobinuria; no rise in lactate with exercise |
| VI (Hers) | Liver phosphorylase | Liver | Mild hepatomegaly |
8. Gluconeogenesis
Synthesis of new glucose from non-carbohydrate precursors. Occurs primarily in liver, and during prolonged fasting, kidney cortex.
Precursors: Lactate (Cori cycle), glycerol (from fat), amino acids (especially alanine - glucose-alanine cycle), pyruvate.
The Three Bypass Reactions
Gluconeogenesis is not simply the reverse of glycolysis. Three irreversible glycolytic steps must be bypassed:
Bypass 1: Pyruvate → PEP (bypasses pyruvate kinase)
- Pyruvate + CO₂ + ATP → Oxaloacetate (pyruvate carboxylase, mitochondria; requires biotin; activated by acetyl-CoA)
- OAA → PEP + CO₂ (PEPCK; uses GTP)
- OAA cannot cross inner mitochondrial membrane, so it is first reduced to malate, transported, then reoxidized to OAA in cytosol
Bypass 2: F-1,6-bisphosphate → F-6-P (bypasses PFK-1)
- Fructose-1,6-bisphosphatase (F-1,6-BPase)
- Inhibited by AMP and F-2,6-BP (signals low energy - don't make glucose now)
Bypass 3: G-6-P → Glucose (bypasses hexokinase)
- Glucose-6-phosphatase in endoplasmic reticulum membrane
- Present in liver and kidney; absent in muscle and brain (why these tissues cannot release glucose)
Energy Cost
Gluconeogenesis is energetically expensive: 6 ATP equivalents per glucose synthesized (vs 2 ATP generated in glycolysis). The extra energy comes from fatty acid oxidation during fasting.
Hormonal Regulation
| Condition | Dominant Signal | Effect |
|---|---|---|
| Fasting | Glucagon ↑ | Activates gluconeogenesis, inhibits glycolysis |
| Fed state | Insulin ↑ | Inhibits gluconeogenesis, activates glycolysis |
| Stress | Glucocorticoids ↑ | Increase gluconeogenic enzyme expression (PEPCK), promote amino acid supply from muscle |
Metformin (first-line type 2 diabetes drug) inhibits hepatic gluconeogenesis primarily by activating AMPK, which reduces PEPCK and G6Pase expression.
9. Pentose Phosphate Pathway (PPP)
Also called the hexose monophosphate (HMP) shunt. Occurs in the cytosol. An alternative fate for Glucose-6-P.
(From Basic Medical Biochemistry, 6th ed., and Lippincott's Biochemistry)
Two phases:
9.1 Oxidative Phase (irreversible)
Generates NADPH and ribulose-5-P:
Glucose-6-P + 2 NADP+ + H₂O → Ribulose-5-P + CO₂ + 2 NADPH + 2 H+
Key enzyme: Glucose-6-phosphate dehydrogenase (G6PD) - commits glucose to the PPP. Rate-limiting step. Inhibited by NADPH (product inhibition).
NADPH is essential for:
- Reductive biosynthesis (fatty acid, cholesterol synthesis)
- Maintenance of reduced glutathione (GSH) - protects RBCs from oxidative hemolysis
- Cytochrome P450 reactions
- NADPH oxidase (reactive oxygen species generation in neutrophils)
G6PD deficiency (X-linked, most common enzyme deficiency worldwide): RBCs cannot regenerate GSH → oxidative stress → hemolytic anemia triggered by oxidants (primaquine, dapsone, fava beans, infections). Protective against malaria.
9.2 Non-oxidative Phase (reversible)
Interconverts pentose phosphates and glycolytic intermediates (F-6-P and G-3-P) via transketolase (requires TPP/thiamine) and transaldolase.
Overall equation:
3 Glucose-6-P + 6 NADP+ → 3 CO₂ + 6 NADPH + 2 Fructose-6-P + Glyceraldehyde-3-P
The PPP is most active in tissues with high biosynthetic demand: liver, adipose tissue, adrenal cortex, gonads, lactating mammary gland, activated phagocytes, and RBCs.
10. Fructose and Galactose Metabolism
Fructose
- In liver: Fructokinase phosphorylates fructose → Fructose-1-P, then aldolase B cleaves to DHAP + glyceraldehyde (bypasses PFK-1 regulation → unchecked substrate for fatty acid synthesis)
- Hereditary fructose intolerance: aldolase B deficiency → F-1-P accumulates → inhibits glycogenolysis and gluconeogenesis → severe hypoglycemia after fructose ingestion
- Essential fructosuria: fructokinase deficiency → benign
Galactose (Leloir pathway)
- Galactose → Galactose-1-P (galactokinase)
- Galactose-1-P + UDP-glucose → Glucose-1-P + UDP-galactose (galactose-1-P uridylyltransferase)
- UDP-galactose → UDP-glucose (UDP-galactose-4-epimerase)
Classic galactosemia: deficiency of galactose-1-P uridylyltransferase → galactose-1-P accumulates → liver disease, intellectual disability, cataracts (galactitol accumulates in lens from aldose reductase). Managed with lactose-free diet.
11. Blood Glucose Regulation and Glucose Homeostasis
| State | Blood Glucose | Primary Pathways | Hormones |
|---|---|---|---|
| Postprandial (fed) | ↑ (>5.5 mM) | Glycolysis, glycogen synthesis, lipogenesis | Insulin ↑ |
| Fasting (4-12h) | Normal (3.5-5.5 mM) | Glycogenolysis | Glucagon ↑ |
| Prolonged fasting (>24h) | Slightly ↓ | Gluconeogenesis, ketogenesis | Glucagon, cortisol, GH ↑ |
| Stress/exercise | Variable | Glycogenolysis, gluconeogenesis | Epinephrine, cortisol ↑ |
Insulin acts through receptor tyrosine kinase → PI3K → Akt pathway:
- Promotes GLUT4 translocation
- Activates glycogen synthase (via PP1)
- Inactivates glycogen phosphorylase
- Induces glucokinase, PFK-2, pyruvate kinase
- Inhibits PEPCK and G6Pase (anti-gluconeogenic)
12. Glycoproteins and Glycolipids
Carbohydrates are covalently attached to proteins and lipids, creating complex molecules with critical biological functions.
12.1 N-linked vs O-linked Glycoproteins
| Feature | N-linked | O-linked |
|---|---|---|
| Attachment site | Asparagine (Asn-X-Ser/Thr) | Serine or Threonine |
| Core sugar | N-acetylglucosamine | GalNAc or GlcNAc |
| Assembly | ER (on dolichol-P precursor) → Golgi | Golgi (added sequentially) |
| Examples | IgG antibodies, MHC molecules, viral coat proteins | Mucins, blood group antigens |
12.2 Glycolipids
Carbohydrates attached to lipids (ceramide base):
- Cerebrosides: galactose or glucose + ceramide (myelin, brain)
- Gangliosides: ceramide + oligosaccharide containing sialic acid (NANA)
- Tay-Sachs: GM2 gangliosidosis - deficiency of hexosaminidase A → progressive neurological deterioration
- Blood group antigens (A, B, O) are carbohydrate modifications on glycolipids and glycoproteins on RBC surfaces
12.3 Proteoglycans and Glycosaminoglycans (GAGs)
Highly sulfated, repeating disaccharide units attached to a protein core:
| GAG | Disaccharide Unit | Location | Function |
|---|---|---|---|
| Hyaluronic acid | GlcUA + GlcNAc | Synovial fluid, vitreous humor, ECM | Lubrication, hydration |
| Heparin/Heparan sulfate | GlcUA + GlcNS | Mast cells, basement membranes | Anticoagulant; binds antithrombin III |
| Chondroitin sulfate | GlcUA + GalNAc | Cartilage | Compressive load resistance |
| Keratan sulfate | Gal + GlcNAc | Cornea, cartilage | |
| Dermatan sulfate | IdoUA + GalNAc | Skin, blood vessels |
Mucopolysaccharidoses (e.g., Hurler, Hunter, Morquio syndromes) are lysosomal storage diseases caused by deficiencies of GAG-degrading enzymes → accumulation in tissues → organomegaly, skeletal deformity, corneal clouding, intellectual disability.
13. Advanced Topics (Graduate Level)
13.1 Warburg Effect (Aerobic Glycolysis)
Cancer cells preferentially use glycolysis even in the presence of oxygen, producing lactate. This seems wasteful (only 2 ATP vs ~36-38 ATP aerobically), but it provides:
- Rapid ATP production
- Carbon skeletons for biosynthesis (nucleotides, amino acids, lipids)
- NADPH via PPP for reductive biosynthesis
- Acidic microenvironment (from lactate) that suppresses immune surveillance
PET scanning exploits this: tumors avidly take up 18F-FDG (non-metabolizable glucose analog), giving high signal.
13.2 Hexosamine Biosynthetic Pathway
When glucose flux is very high, some G-6-P enters this pathway → UDP-N-acetylglucosamine (UDP-GlcNAc), which is the donor for O-GlcNAc modification of serine/threonine residues on nuclear/cytoplasmic proteins. O-GlcNAc competes with phosphorylation at the same sites on many proteins (including Akt, tau, β-catenin). This pathway is a nutrient sensor and is dysregulated in diabetes (contributes to insulin resistance) and neurodegeneration (tau hyperphosphorylation in Alzheimer's is linked to reduced O-GlcNAcylation).
13.3 Carbohydrate Signaling Roles
- Selectins are lectins that bind sulfated sialyl-Lewis-x antigen on leukocytes → rolling adhesion during inflammation
- Mannose-6-phosphate tag on newly synthesized lysosomal enzymes directs them to lysosomes (defect → I-cell disease, mucolipidosis II)
- Galectin family (β-galactoside-binding lectins) regulate T-cell apoptosis, fibrosis, and cancer progression
- N-linked glycosylation is required for proper protein folding in the ER (calnexin/calreticulin chaperone system)
13.4 Regulation of Carbohydrate Metabolism by AMPK
AMP-activated protein kinase (AMPK) is the cellular energy sensor:
- Activated when AMP:ATP ratio rises (exercise, fasting, hypoxia)
- Phosphorylates and inactivates ACC (fatty acid synthesis) and glycogen synthase
- Phosphorylates and activates PFK-2 (→ ↑F-2,6-BP → ↑glycolysis)
- Transcriptionally activates GLUT4 gene expression
- In liver: inhibits SREBP-1c, PEPCK, G6Pase (anti-lipogenic, anti-gluconeogenic)
- Metformin and exercise both activate AMPK
13.5 Congenital Disorders of Glycosylation (CDG Syndromes)
A growing family of >130 rare disorders caused by defects in glycan biosynthesis or remodeling. Present with multi-system disease in infancy: coagulopathy (clotting factors are glycoproteins), liver disease, cerebellar ataxia, intellectual disability, and dysmorphic features. Transferrin isoelectric focusing is a useful screening test (abnormal glycoforms of transferrin).
13.6 Dietary Fiber and Gut Microbiome
Non-digestible carbohydrates (cellulose, pectin, inulin, β-glucan) are fermented by colonic bacteria to short-chain fatty acids:
- Butyrate: primary fuel for colonocytes; anti-inflammatory; histone deacetylase (HDAC) inhibitor → epigenetic effects; protective against colorectal cancer
- Propionate: gluconeogenic substrate in liver; regulates cholesterol synthesis
- Acetate: peripheral fuel; signals through GPR43 on immune cells
The gut microbiome diversity correlates with the range of fermentable carbohydrates in the diet - this is the mechanistic basis of prebiotics.
Summary: Key Energy Yields
| Pathway | Net ATP (per glucose) | Key Organ | Aerobic/Anaerobic |
|---|---|---|---|
| Glycolysis only (anaerobic) | 2 ATP | RBC, hypoxic tissue | Anaerobic |
| Glycolysis + TCA + oxidative phosphorylation | ~36-38 ATP | Most cells | Aerobic |
| Glycogen → Glycolysis | 3 ATP (extra 1 from phosphorolytic cleavage) | Muscle | Either |
| Gluconeogenesis | -6 ATP (cost) | Liver, kidney | Aerobic |
| Pentose phosphate pathway | 2 NADPH + ribose-5-P | Liver, RBC, adrenals | Aerobic |
Clinical Pearls Recap
- Lactase deficiency → lactose malabsorption → osmotic diarrhea, bloating from bacterial fermentation; treat with lactase supplements or lactose-free diet
- G6PD deficiency → hemolytic crisis with oxidant drugs, infections, fava beans; X-linked; most common globally
- Galactosemia → avoid all lactose/galactose; presents as jaundice + liver failure in neonates after first milk feeds
- McArdle disease → exercise-induced muscle cramps + myoglobinuria; forearm ischemic exercise test shows no lactate rise
- Pompe disease (GSD II) → infantile form: cardiomegaly + hypotonia → death by 2 years; treated with alglucosidase alfa (enzyme replacement)
- Diabetic hyperglycemia → glucose shunted to polyol pathway (aldose reductase: glucose → sorbitol) and AGE (advanced glycation end-product) formation → cataracts, neuropathy, nephropathy, retinopathy
- Metformin mechanism: AMPK activation → ↓hepatic gluconeogenesis (PEPCK, G6Pase) + ↑peripheral glucose uptake
- SGLT2 inhibitors (dapagliflozin, empagliflozin): block glucose reabsorption in proximal tubule → glucosuria → lower blood glucose AND blood pressure AND weight; cardioprotective in heart failure
Sources: Lippincott's Illustrated Reviews: Biochemistry, 8th ed. | Basic Medical Biochemistry: A Clinical Approach, 6th ed. | Harper's Illustrated Biochemistry, 32nd ed. | Goldman-Cecil Medicine | Medical Physiology (Boron & Boulpaep)
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