Biochemistry of Carbohydrates: Structure, Reactions, Digestion, and Metabolism

Comprehensive vocabulary flashcards covering the structural characteristics, chemical classifications, optical activity, diagnostic reactions, digestion, and metabolic pathways of carbohydrates as outlined in the lecture notes.

Carbohydrates

Polyhydroxyaldehydes or ketones, or compounds that produce them on hydrolysis; they are the most abundant biomolecules on Earth, primarily composed of carbon, hydrogen, and oxygen, and yield $4\,\text{Cal/g}$ of energy.

Monosaccharides

The simplest group of carbohydrates (simple sugars) with the empirical formula $(CH_2O)_n$ (where $n \ge 3$), which cannot be further hydrolyzed into simpler units.

Aldoses

Monosaccharides containing an aldehyde group ($-\text{CHO}$) located at the end of the carbon chain (carbon No. 1), such as glyceraldehyde and glucose.

Ketoses

Monosaccharides containing a keto functional group ($C=O$) that does not occur at the end of the carbon chain (typically at carbon No. 2), such as dihydroxyacetone and fructose.

Reference Carbohydrate

Glyceraldehyde (glycerose), the simplest aldose triose containing one asymmetric carbon atom, used as the benchmark to define the structural configurations ($\text{D}$ and $\text{L}$) of all carbohydrates.

Stereoisomers

Compounds that have the same chemical formula and structural bonding sequence but differ in their spatial arrangement of atoms.

Asymmetric Carbon Atom

A chiral carbon atom bonded to four entirely different atoms or functional groups; the number of possible optical isomers of a compound with $n$ asymmetric carbons is given by $2^n$.

Enantiomers

Pairs of stereoisomers that are non-superimposable mirror images of each other; in sugars, designated as $\text{D}$ or $\text{L}$ based on the orientation of the $-OH$ group on the penultimate asymmetric carbon atom.

Penultimate Carbon Atom

The asymmetric carbon atom farthest from the potential aldehyde or ketone group (carbon No. 5 in hexoses like glucose), which determines whether a sugar belongs to the $\text{D}$ or $\text{L}$ series.

Epimers

Two isomers that differ in configuration around only one specific asymmetric carbon atom other than the anomeric/carbonyl carbon (e.g., glucose and galactose at C-4; glucose and mannose at C-2).

Anomers

Diastereomeric cyclic forms of a sugar that differ only in configuration around the new asymmetric stereocenter created by cyclic hemiacetal/hemiketal formation (anomeric carbon: C-1 in aldoses, C-2 in ketoses), yielding $\alpha$ and $\beta$ forms.

Mutarotation

The spontaneous change in specific optical rotation observed over time when a pure $\alpha$ or $\beta$ anomer of a sugar is dissolved in water, progressing until a fixed equilibrium mixture is reached (for glucose, ending at $+52.7^\circ$).

Racemic Mixture

An equimolar mixture of dextrorotatory ($+$- or $d$-) and levorotatory ($-$ or $l$-) isomers showing zero net optical rotation due to external compensation.

Meso Compounds

Molecules that contain asymmetric carbon atoms but are optically inactive due to internal compensation, where one half of the molecule rotates plane-polarized light equally and oppositely to the other half (e.g., meso-tartaric acid).

Pyranose and Furanose

Cyclic ring representations of sugars resembling heterocyclic rings; pyranoses are six-membered rings (5 carbons, 1 oxygen) derived from pyran, while furanoses are five-membered rings (4 carbons, 1 oxygen) derived from furan.

Chair Conformation

The most scientifically accurate and thermodynamically stable puckered conformational representation of the six-membered pyranose ring.

D-Glucose

An aldohexose also known as dextrose or grape sugar; it represents nearly $100\%$ of circulating monosaccharide in human blood, with a normal fasting plasma level of $70\text{--}110\,\text{mg/dL}$.

D-Fructose

A ketohexose known as levulose or fruit sugar; it is the sweetest naturally occurring sugar ($173\%$ the sweetness of sucrose), abundant in honey and semen, and exists in a furanose form in sucrose.

2-Deoxy-D-ribose

A deoxy pentose sugar lacking a hydroxyl oxygen at C-2 that forms the structural sugar constituent of DNA, identifiable in tissues using Feulgen's reaction with Schiff's reagent.

Lobry de Bruyn-von Ekenstein Transformation

The base-catalyzed keto-enol tautomerization of monosaccharides yielding an enediol intermediate through which glucose, fructose, and mannose interconvert in alkaline solution.

Reducing Sugars

Sugars containing a free, unattached hemiacetal or hemiketal anomeric carbon that can open into a reactive linear carbonyl form and reduce oxidizing ions such as $\text{Cu}^{2+}$, $\text{Fe}^{3+}$, and $\text{Ag}^+$.

Aldonic Acids

Sugar acids produced by mild oxidation selectively at the aldehyde group (C-1) converting it to a carboxyl group ($-\text{COOH}$), exemplified by gluconic acid.

Uronic Acids

Sugar acids formed by selective oxidation of the terminal primary alcohol group (C-6 in hexoses) to a carboxyl group while leaving the C-1 carbonyl intact, exemplified by glucuronic acid.

Saccharic Acids (Glucaric Acids)

Dicarboxylic sugar acids formed when both the C-1 aldehyde and the terminal C-6 alcohol groups of an aldose are simultaneously oxidized to carboxyl groups (e.g., glucaric acid from glucose, mucic acid from galactose).

Barfoed's Test

A diagnostic chemical test using cupric acetate in acetic acid (slightly acidic medium) that selectively oxidizes monosaccharides within a short heating time while leaving reducing disaccharides unreacted.

Osazones

Yellow, crystalline phenylhydrazone derivatives formed by boiling reducing sugars with three equivalents of phenylhydrazine, reacting exclusively at C-1 and C-2 (glucose, fructose, and mannose yield identical glucosazone crystals).

Molisch's Test

A general test for all carbohydrates based on dehydration by concentrated $\text{H}_2\text{SO}_4$ to form furfurals or hydroxymethylfurfurals, which condense with $\alpha$-naphthol to produce a characteristic purple ring.

Glycosides

Compounds formed when the anomeric hydroxyl group of a cyclic sugar condenses with a hydroxyl or amino group of another compound (an aglycone or another sugar) through an acetal or ketal linkage.

Maltose

A reducing disaccharide produced during starch hydrolysis composed of two $\alpha\text{-D-glucose}$ units joined by an $\alpha(1\rightarrow 4)$ glycosidic bond.

Sucrose

Common table sugar; a non-reducing disaccharide composed of $\alpha\text{-D-glucopyranose}$ and $\beta\text{-D-fructofuranose}$ linked by an $\alpha 1\rightarrow\beta 2$ glycosidic bond that undergoes inversion upon hydrolysis.

Invert Sugar

The equimolar mixture of glucose and fructose produced by hydrolysis of dextrorotatory sucrose ($+66.5^\circ$), which results in a net levorotatory optical rotation ($-19.8^\circ$ to $-28.2^\circ$) because the levorotatory activity of fructose exceeds the dextrorotatory activity of glucose.

Lactose

Milk sugar; a reducing disaccharide synthesized in the lactating mammary gland consisting of $\beta\text{-D-galactose}$ joined to $\text{D-glucose}$ via a $\beta(1\rightarrow 4)$ glycosidic linkage.

Lactulose

A synthetic, non-absorbable disaccharide of galactose and fructose that functions as an osmolar laxative and traps ammonium ions ($\text{NH}_4^+$) in the bowel lumen to treat hepatic encephalopathy.

Amylose

A water-soluble component of starch ($15\text{--}20\%$ by mass) consisting of unbranched, helical chains of 50 to 5,000 $\alpha\text{-D-glucose}$ residues joined exclusively by $\alpha(1\rightarrow 4)$ glycosidic bonds that stains blue with iodine.

Amylopectin

The major water-insoluble component of starch ($80\text{--}85\%$) composed of branched chains of $\alpha\text{-D-glucose}$ with $\alpha(1\rightarrow 4)$ linkages along straight chains and $\alpha(1\rightarrow 6)$ bonds at branch points spaced every 24 to 30 residues.

Dextrins

Intermediate breakdown products of starch digestion generated by acid or amylase hydrolysis, proceeding sequentially through soluble starch, amylodextrin (violet), erythrodextrin (red), and achroodextrin (colorless).

Dextrans

Highly branched, viscous polymers of $\alpha\text{-D-glucopyranose}$ linked predominantly by $\alpha(1\rightarrow 6)$ bonds, synthesized by Leuconostoc mesenteroides and employed clinically as plasma volume expanders in hypovolemic shock.

Inulin

A low-molecular-weight fructosan (fructose polymer) found in dahlia bulbs and garlic that is filtered freely by the glomerulus and neither reabsorbed nor secreted, making it the gold standard for measuring glomerular filtration rate (GFR).

Cellulose

The most abundant biopolymer on Earth, consisting of unbranched linear chains of up to 15,000 $\beta\text{-D-glucose}$ units held together by $\beta(1\rightarrow 4)$ glycosidic bonds, which cannot be digested by humans due to the absence of cellulase.

Glycogen

The primary storage polysaccharide in animals (animal starch), stored in the liver ($6\text{--}8\%$) and skeletal muscle ($1\text{--}2\%$), with a highly branched structure containing $\alpha(1\rightarrow 4)$ linkages ($92\%$) and $\alpha(1\rightarrow 6)$ branches ($8\%$) every 12 to 14 residues.

Chitin

A structural homopolysaccharide composed of repeating $N\text{-acetyl-D-glucosamine}$ units joined by $\beta(1\rightarrow 4)$ glycosidic bonds, abundant in fungal cell walls and arthropod exoskeletons.

Glycosaminoglycans (GAGs)

Negatively charged, linear heteropolysaccharides composed of repeating disaccharide units consisting of an amino sugar and a uronic acid (except keratan sulfate), responsible for structural hydration and lubrication in connective tissue matrices.

Hyaluronic Acid

A non-sulfated, high-molecular-weight GAG composed of alternating $\text{D-glucuronic acid}$ and $N\text{-acetyl-D-glucosamine}$ residues that provides shock absorption and lubrication in synovial joints and vitreous humor.

Hyaluronidase

An enzyme ('spreading factor') found in snake venoms, pathogenic bacteria, and sperm that cleaves $\beta(1\rightarrow 4)$ linkages of hyaluronic acid to facilitate tissue invasion or ovum penetration.

Heparin

A highly sulfated intracellular GAG produced by mast cells that displays the highest negative charge density of any known biological macromolecule and acts as a potent physiological anticoagulant by enhancing antithrombin activity.

Keratan Sulfate

A specialized GAG present in cartilage and the cornea that differs from all other GAGs by having $\text{D-galactose}$ instead of an acidic uronic sugar in its repeating disaccharide unit.

Proteoglycan Complex

A macromolecular assembly consisting of a central hyaluronic acid backbone linked non-covalently via link proteins to core proteins radiating GAG side chains (chondroitin sulfate and keratan sulfate), giving connective tissues compressive resilience.

Glycoproteins

Conjugated proteins covalently linked to short, branched, complex oligosaccharide chains lacking repeating disaccharide motifs (carbohydrate content $1\text{--}90\%$ by weight), functioning as hormones, enzymes, antibodies, and cell-surface recognition antigens.

SGLT-1

The sodium-dependent glucose transporter located at the brush-border apical membrane of enterocytes that transports glucose and galactose into mucosal cells via secondary active symport driven by the $Na^+$ electrochemical gradient.

GLUT-4

The insulin-responsive facilitative glucose transporter isoform localized predominantly in skeletal muscle, cardiac muscle, and adipose tissue membranes.

Lactose Intolerance

A clinical deficiency of brush-border lactase ($\beta\text{-galactosidase}$) resulting in the accumulation of unhydrolyzed lactose in the bowel lumen, provoking osmotic diarrhea and bacterial fermentation into hydrogen, methane, and short-chain fatty acids.

Glycolysis (Embden-Meyerhof Pathway)

A universal cytosolic pathway that oxidizes one mole of glucose into two moles of pyruvate (aerobic) or lactate (anaerobic), generating a net yield of 2 ATP per glucose under anaerobic conditions.

Phosphofructokinase-1 (PFK-1)

The committed, rate-limiting regulatory enzyme of glycolysis that converts fructose 6-phosphate to fructose 1,6-bisphosphate at the expense of ATP; allosterically inhibited by ATP and citrate, and activated by fructose 2,6-bisphosphate and AMP.

Fructose 2,6-Bisphosphate

The most potent allosteric activator of PFK-1 and inhibitor of fructose 1,6-bisphosphatase in the liver, synthesized and degraded by the single bifunctional enzyme PFK-2/FBPase-2.

Substrate-Level Phosphorylation

The direct synthesis of ATP (or GTP) coupled to the cleavage of a high-energy phosphate-donor substrate without involvement of the respiratory electron transport chain (catalyzed in glycolysis by phosphoglycerate kinase and pyruvate kinase).

Rapaport-Leubering Cycle

A glycolytic shunt operating in erythrocytes where 1,3-bisphosphoglycerate is converted to 2,3-bisphosphoglycerate (2,3-BPG) by bisphosphoglycerate mutase, bypassing ATP generation to produce a metabolite that lowers hemoglobin's affinity for oxygen.

Pasteur Effect

The physiological inhibition of glycolysis and reduction of glucose consumption observed when anaerobic cells or yeast are exposed to oxygen, caused by ATP and citrate allosterically suppressing PFK-1.

Pyruvate Dehydrogenase Complex (PDH)

A multienzyme mitochondrial complex that catalyzes the irreversible oxidative decarboxylation of pyruvate to acetyl CoA; requires five cofactors: thiamine pyrophosphate (TPP), lipoamide, FAD, coenzyme A, and $\text{NAD}^+$.

Citric Acid Cycle (Krebs Cycle)

The primary, strictly aerobic amphibolic mitochondrial pathway that oxidizes acetyl CoA into two molecules of $\text{CO}_2$, producing 3 NADH, 1 $\text{FADH}_2$, and 1 GTP (equivalent to 12 ATP, or 10 ATP by revised $\text{P:O}$ ratios) per acetyl CoA turn.

Suicide Substrate

A harmless molecule that is converted by cellular metabolism into an active toxin that irreversibly inactivates an essential enzyme; exemplified by fluoroacetate forming fluorocitrate, which blocks aconitase in the TCA cycle.

Anaplerotic Reactions

Replenishing chemical reactions that synthesize and restore catalytic intermediates of the citric acid cycle that have been siphoned off for anabolic pathways (e.g., pyruvate carboxylase converting pyruvate to oxaloacetate).

Gluconeogenesis

The metabolic synthesis of glucose from non-carbohydrate precursors (lactate, pyruvate, glycerol, and glucogenic amino acids) occurring primarily in the liver ($\sim 1\,\text{kg/day}$) and renal cortex.

Cori Cycle

The inter-organ metabolic cycle in which lactate generated by anaerobic glycolysis in contracting skeletal muscle or erythrocytes travels through the blood to the liver to be reconverted into glucose, which is then released back into the circulation.

Glycogenin

A core protein that acts as an autocatalytic primer for glycogenesis by accepting the initial glucosyl residue from UDP-glucose onto the hydroxyl group of a specific tyrosine residue.

Glycogen Phosphorylase

The key regulatory enzyme of glycogenolysis that sequentially cleaves $\alpha(1\rightarrow 4)$ glycosidic bonds phosphorolytically from non-reducing ends to liberate glucose 1-phosphate, requiring pyridoxal phosphate (PLP) as a cofactor.

Debranching Enzyme

A bifunctional enzyme of glycogenolysis containing glycosyl 4:4 transferase activity (which shifts 3 glucose residues from a branch to an adjacent chain) and amylo-1,6-glucosidase activity (which hydrolyzes the single remaining $\alpha-1,6$ glucose to yield free glucose).

von Gierke's Disease (Type I GSD)

An autosomal recessive glycogen storage disease caused by a deficiency of glucose 6-phosphatase, characterized by severe fasting hypoglycemia, hepatomegaly, lactic acidosis, hyperlipidemia, and hyperuricemia.

Pompe's Disease (Type II GSD)

A fatal glycogen storage disease caused by a defect in lysosomal $\alpha(1\rightarrow 4)\text{-glucosidase}$ (acid maltase), leading to massive lysosomal glycogen accumulation in the heart, skeletal muscles, and liver.

McArdle's Disease (Type V GSD)

A glycogen storage disorder caused by a deficiency of muscle glycogen phosphorylase, leading to painful exercise-induced muscle cramps, fatigue, and lack of blood lactate elevation following strenuous exertion.

Hexose Monophosphate (HMP) Shunt

An alternative, cytosolic oxidative pathway for glucose that does not directly generate or consume ATP, but produces NADPH (for reductive biosyntheses and antioxidant protection) and ribose 5-phosphate (for nucleotide synthesis).

Glucose 6-Phosphate Dehydrogenase (G6PD) Deficiency

An X-linked genetic disorder that impairs NADPH regeneration in erythrocytes, rendering them vulnerable to methemoglobin accumulation, peroxide stress, and hemolytic anemia upon exposure to oxidant drugs (primaquine) or fava beans.

Essential Pentosuria

A benign, asymptomatic inborn error of the uronic acid pathway caused by a deficiency of NADP-dependent xylitol dehydrogenase, resulting in large amounts of L-xylulose excreted in the urine.

Classical Galactosemia

An autosomal recessive disorder caused by a deficiency of galactose 1-phosphate uridyltransferase, resulting in tissue accumulation of galactose 1-phosphate and galactitol (dulcitol), causing early cataract development, hepatosplenomegaly, and mental retardation.

Hereditary Fructose Intolerance

A metabolic disorder caused by the absence of hepatic aldolase B, resulting in toxic intracellular trapping of fructose 1-phosphate, sequestering of inorganic phosphate, and severe hypoglycemia due to glycogen phosphorylase inhibition.

Polyol Pathway

A two-step pathway in which aldose reductase converts glucose to sorbitol using NADPH, followed by sorbitol dehydrogenase converting sorbitol to fructose; sorbitol accumulation in insulin-independent tissues (lens, retina, nerves) drives diabetic cataracts and neuropathy.

Glyoxylate Cycle

A specialized anabolic variant of the citric acid cycle occurring in plant glyoxysomes and microorganisms that converts acetyl CoA derived from fat into succinate via isocitrate lyase and malate synthase, permitting net carbohydrate synthesis from fatty acids.