Carbohydrates

A non-reducing sugar lacks a free carbonyl group, so it cannot participate in non-enzymatic glycation (Maillard reaction) with amino groups. This stability is crucial in the hemolymph, where high concentrations of trehalose could otherwise react with and damage circulating proteins.

Fructose is phosphorylated to fructose-1-phosphate, which is cleaved to glyceraldehyde and dihydroxyacetone phosphate, entering glycolysis downstream of PFK-1. This bypasses a critical regulatory checkpoint, allowing a rapid, unregulated influx of carbon that can overload the TCA cycle and lead to increased de novo lipogenesis.

Peptidoglycan (murein) is unique to bacteria. Its carbohydrate backbone is a linear chain of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds. Lysozyme hydrolyzes this specific bond.

The "sweetness" is a function of binding affinity to the receptor. Fructose exists in solution in several forms, including a significant proportion of a sweet furanose form, which interacts more optimally with the receptor's binding site than the pyranose forms of glucose.

The enzyme lactase (a β-galactosidase) is specific for the β-configuration of the galactose residue and the 1,4 linkage to glucose. Sucrase acts on the α,β-1,2 linkage of sucrose, and maltase acts on the α-1,4 linkage of maltose.

The anomeric carbon is derived from the carbonyl carbon of the open-chain form. In glucose, this is C-1. The nucleophilic attack by the C-5 hydroxyl group on this planar carbonyl carbon can occur from either face, creating the two possible anomeric configurations: α (-OH down) or β (-OH up).

Hyaluronic acid is a linear, unbranched polymer with repeating disaccharide units containing glucuronic acid (negatively charged). The polymer chains are long and rigid, and they trap a large volume of water, forming a viscous solution that acts as an excellent shock absorber and lubricant.

Endoglycosidases cleave glycosidic bonds internally within a polysaccharide chain, producing shorter oligosaccharides (dextrins). In contrast, exoglycosidases cleave sugar residues one at a time from the non-reducing end of the chain.

The number of possible stereoisomers for a molecule with 'n' chiral centers is 2ⁿ. A sugar with 4 chiral centers has 2⁴ = 16 possible stereoisomers. These are divided into 8 D-sugars and their 8 L-enantiomeric counterparts.

Starch and glycogen are composed of α-D-glucose units, where the -OH on the anomeric carbon (C1) is below the plane of the ring. Cellulose consists of β-D-glucose, where the anomeric -OH is above the plane of the ring. This simple difference makes starch digestible and cellulose indigestible to humans.

The synthesis of sucrose involves UDP-glucose as the glucosyl donor. The reaction is: UDP-glucose + Fructose-6-phosphate → Sucrose-6-phosphate + UDP. ADP-glucose is the activated monomer for starch synthesis.

Inulin is a polysaccharide of D-fructose linked by β-2,1 glycosidic bonds. It is not metabolized and is freely filtered at the glomerulus without being reabsorbed or secreted, making it an ideal marker for measuring the glomerular filtration rate (GFR).

Cellulose's linear, unbranched chains allow them to lie parallel and form extremely stable, regular intermolecular hydrogen bonds. These strong lateral interactions create crystalline microfibrils that exclude water and are highly resistant to hydrolysis.

The free aldehyde or ketone group of a reducing sugar reduces the blue Cu²⁺ ions (cupric) in Benedict's reagent to Cu⁺ ions (cuprous). These precipitate out of the alkaline solution as an insoluble brick-red solid, copper(I) oxide (Cu₂O).

The formation of a glycosidic bond between two monosaccharides is a dehydration synthesis (condensation). The hydroxyl group from one sugar's anomeric carbon combines with a hydrogen from a hydroxyl of another sugar, releasing one water molecule and forming a new covalent bond.

Sucrose is a disaccharide of glucose and fructose. Hydrolysis of the glycosidic bond (e.g., by sucrase/invertase) yields one molecule of D-glucose and one molecule of D-fructose. This 1:1 mixture is known as invert sugar.

Cellulose is a homopolymer of β-D-glucose and constitutes the majority of plant biomass. Its annual production is estimated to be over 10¹¹ tons, making it the most abundant organic compound on Earth. Starch and glycogen are storage, not structural, polymers.

The D/L classification is based on the configuration of the chiral carbon farthest from the most oxidized end (the carbonyl group). If the -OH on this carbon is on the right in a Fischer projection, it is D; if on the left, it is L. For glucose, this is C-5.

In lactose intolerance, lactase (β-galactosidase) is deficient. Undigested lactose passes to the colon, where gut microbiota ferment it. This produces gases like H₂, CO₂, and methane (causing bloating) and short-chain fatty acids, which draw water into the bowel (osmotic diarrhea).