Carbohydrates

Epimers are a subclass of diastereomers that differ in configuration at exactly one chiral center. For example, D-glucose and D-galactose are C-4 epimers, and D-glucose and D-mannose are C-2 epimers. Anomers are epimers specifically at the hemiacetal/hemiketal carbon.

Benedict's reagent is an alkaline solution of copper(II) sulfate and sodium carbonate. In a hot alkaline environment, glucose's carbonyl group reduces Cu²⁺ to Cu⁺, forming a colored precipitate. The alkalinity is crucial for the reaction to proceed.

O-linked glycosylation involves the formation of a glycosidic bond between the anomeric carbon of a sugar (often N-acetylgalactosamine) and the hydroxyl group of a serine or threonine residue in the protein. N-linked glycosylation links to the amide nitrogen of asparagine.

Saccharification is the step where the polysaccharide starch is chemically or enzymatically hydrolyzed into simple sugars (glucose and maltose). These sugars then serve as substrates for the subsequent fermentation by microorganisms to produce ethanol.

In β-D-glucose, the anomeric -OH on C1 is equatorial. In the chair form, bulky substituents in equatorial positions have more space and experience less steric strain (1,3-diaxial interactions), making this conformation thermodynamically more stable than the α-anomer (axial -OH).

While providing no calories, the insoluble cellulose fibers absorb water, increasing fecal bulk. This bulk stimulates stretch receptors in the gut wall, promoting peristaltic contractions and helping prevent constipation and related disorders.

The Lobry de Bruyn–Alberta van Ekenstein transformation proceeds through the removal of a proton from C-2, forming an enediol (or enolate) intermediate. This intermediate can reprotonate to give either the original aldose (glucose), its C-2 epimer (mannose), or the ketose (fructose).

Rumen microbes ferment cellulose and other carbohydrates to volatile fatty acids (VFAs) like acetate, propionate, and butyrate. These are absorbed through the rumen wall and serve as the primary energy source for the ruminant, not the glucose monomers of cellulose.

Branching enzyme is a transglycosylase. It cuts a short α-1,4-linked chain of 6-7 glucose units and transfers it to the 6-OH position of a glucose residue in the same or a nearby chain. This creates the α-1,6 branch points characteristic of amylopectin and glycogen.

Both have the same molecular formula (C₆H₁₂O₆), but different functional groups—glucose is an aldehyde (aldose) and fructose is a ketone (ketose). This difference in the connectivity of atoms makes them structural (constitutional) isomers.

Agar melts at ~85°C and solidifies at ~32-40°C. Once gelled, it remains solid at typical incubation temperatures (e.g., 37°C) and is resistant to degradation by most microorganisms, making it an ideal, inert solidifying agent for culture media.

Resistant starch is physically inaccessible or structurally resistant to pancreatic amylases. It passes to the colon, where it acts similarly to soluble fiber, being fermented by gut microbiota, producing beneficial short-chain fatty acids.

Amylopectin is a homopolymer of D-glucose. Complete hydrolysis with strong acid will break all the α-1,4 and α-1,6 glycosidic bonds, yielding only D-glucose monomers as the final product.

The D/L designation is fixed by the configuration of the highest-numbered chiral center (C-5 in hexoses). The α and β anomers are defined specifically by the orientation of the hydroxyl group on the newly formed chiral center, the anomeric carbon (C-1 in aldoses), relative to the ring.

Each monosaccharide has several polar -OH groups that can participate in hydrogen bonding with water molecules. This strong interaction (adhesion) overcomes the sugar-sugar interactions and allows the sugars to dissolve readily in water.

The iodine test requires a helix of sufficient length to stabilize the polyiodide chain. Glycogen's highly branched structure means its α-1,4 helical segments are very short. With iodine, it yields a reddish-brown color (not blue-black), which can be easily confused with a negative test if not careful.

S. mutans produces glucosyltransferases that specifically use sucrose to synthesize sticky, water-insoluble glucan polymers (dental plaque). The sucrose is then fermented to lactic acid within this plaque, causing localized demineralization of tooth enamel.

The anomeric carbon in a glycoside is part of an acetal (or ketal) functional group, which is stable and cannot open to the free carbonyl form in a neutral/basic aqueous solution. Since no free carbonyl can form, the sugar cannot act as a reducing agent.

Oxidation of the aldehyde group (C-1) of an aldose yields an aldonic acid (e.g., gluconic acid from glucose). Oxidation of the primary alcohol group (C-6) yields a uronic acid. Reduction yields a sugar alcohol (alditol).