Vocabulary:
Triose (3 Carbons), Tetrose (4 Carbons), Pentose (5 Carbons), Hexose (6 Carbons), Heptose (7 Carbons).
Epimer--different configuration at only one carbon. (epimerase).
Isomer--switch between aldehyde and a ketone. (isomerase)
Polyol--reduced carbohydrate (hexitol or pentitol as generic names, glycerol, mannitol or galactitol as specific names).
Structures to know;
[1] D-glyceraldehyde; [2] dihydroxyacetone; [3] glycerol; [4] D-ribose; [5] 2-deoxy-D-ribose.
L-glucose is different at every assymetric carbon, not just C5.
More obscure nomenclature.
Aldoses like glucose can be oxidized on the C1 carbon, the C6 carbon or both, producing the corresponding carboxylic acids. An aldose oxidized on the first carbon is termed as an -onic acid, like gluconic acid. An aldose oxidized on the highest numbered carbon is a -uronic acid, like glucoronic acid. Oxidation of an aldose on both the lowest and highest numbered carbons produces an -aric acid, like glucaric acid. Of course, these carboxylic acids are deprotonated at most biological pH values, so the molecules would be termed as some salt of gluconate, glucuronate or glucarate, respectively.
The above structures are called open-chain or Fischer structures. In reality, aldohexoses, ketohexoses and aldopentoses exist in rings due to to intramolecular bonding (hemiacetal or hemiketal bond is formed).
Aldohexoses usually form a six membered ring (pyranose structure with C-5 hydroxyl bonded to C-1) whereas ketohexoses and aldopentoses form five-membered rings (furanose structures).
The hemiacetal has -OH and -H on anomeric carbon No. 1. Hemiketals have -OH and -CH2OH on anomeric carbon No. 2. In these structures, the hydroxyl on the carbon participating in the hemiacetal bond will be below the plane of the ring (alpha anomer) or above the plane of the ring (beta anomer). The process of a carbohydrate of one anomer opening up to the open chain form and then forming the other cyclic anomer is called mutarotation.
If the hemiacetal or hemiketal of a hexose involves C-5, then C-6 is above the ring in the Haworth projection for D-sugars and below the ring for L-sugars.
Hemiacetals and hemiketals can involve hydroxyl oxygens from the last carbon of ketohexoses (e.g. D-fructose) or aldopentoses (D-ribose). This will permit a 6-membered pyranose ring which can be more stable than the 5-membered furanose.
You must practice drawing these structures for the above carbohydrates. They are termed Haworth Projections.
An additional complication is that some carbohydrates are modified with an amino group. Examples are glucosamine and galactosamine. Notice that in biochemistry it is common to abbreviate the hydroxyl groups with a line and omit the hydrogen atoms.
These amino sugars can be further modified forming carbohydrates with more than six carbons such as muramic acid, neuraminic acid and the sialic acids. Such carbohydrates play structural roles rather than participating in primary metabolism and energy production.
Carbohydrates can bond together to form dimers (disaccharides), trimers (trisaccharides), and larger structures (oligosaccharides and polysaccharides). The bonds are called glucoside (specific for glucose) or glycoside (generic). Other specific glycosides include galactosides (see lactose) and ribosides (see ATP).
Lactose [D-galactose-(beta-1->4)-D-glucose]
The natural anomer of lactose is the beta form. When ice-cream thaws, there tends to be anomerization of the beta to the alpha form, which is less soluble and crystallizes. With time in the freezer, the insoluble alpha anomer gradually accumulates and gives the ice-cream a granular or gritty feel in the mouth. The enzyme that hydrolyzes lactose is lactase, present in the gut. This activity is present at birth and drops to less than 10% of the initial level by age 5 in most people. The exception seems to be people of European origin in which it remains high for most of the life. Consumption of lactose without the lactase enzyme in the gut permits your bacteria to consume it, causing diarrhea and flatulence. One test for lactose intolerance is to measure H2 release in the breath following lactose consumption. The less lactase you have, the more it is consumed by the gut bacteria, producing more H2 which goes into the bloodstream and is released in the breath. The expression of lactase can be subject to evolutionary pressure. Africans in Northern Nigeria have been herding cattle and consuming milk for thousands of years and express the lactase gene as adults. Agricultural people from Western Nigeria who have not traditionally consumed milk do not express lactase as adults and are lactose intolerant.
Maltose [D-glucose-(alpha-1->4)-D-glucose]
Sucrose [D-glucose-(alpha-1->beta-2)-D-fructose]
Native populations in the far northern latitudes have not been exposed to sucrose for centuries, unlike like populations in temperate or tropical zones. About 10% of Alaskan and Greenland natives are sucrose intolerant.
Cellobiose [D-glucose-(beta-1->4)-D-glucose]
Some plants contain oligosaccahrides for which we humans don't have the appropriate enzymes to hydrolyze. Examples can include alpha-linked galactosides (e.g. stachyose) present in bean and pea seeds and fructoside polymers (fructans) present in onions. However, even though we humans don't make an enzyme to hydrolyze these glycosides, the anaerobic bacteria in our lower intestines do possess such enzymes. The gas (CO2, methane, H2, ammonia) produced by these bacteria supplying themselves with edible carbohydrates is the source of digestive flatulence.
Love beans, but want to minimize flatulance for social considerations? There are two options. First, these oligosaccharides are polar and soluble in water. When you soak dried beans, much of the offending oligosaccharides is extracted into the water; drain and wash the beans before adding to the recipe. Second, there are over-the-counter solutions of alpha-galactosidases (e.g. Beano®) which you can ingest when you eat the offending legumes. Even more problematic is the Jerusalem artichoke. Beans, after hydration, contain large amounts of protein and oil for the developing plant embryo, and thus only about 5 to 15% carbohydrate. The Jerusalem artichoke is a root and invests most of its energy into carbohydrates and may contain 50% of its dry weight as problematic carbohydrates. There is a relevant account of what happens when one makes a shift in the diet to include such carbohydrates. The Lewis and Clark expedition first crossed the continental divide and had been living on large quantities of meat, but their supplies were low. They met a band of Shoshone natives, who lived largely on roots. The Shoshones saved them from starvation by giving them food, but the results were a digestive culture shock and the Corps of Discovery was incapacitated for over a week.
Starch [D-glucose-(alpha-1->4)-D-glucose] (poly-maltose) Glycogen [D-glucose-(alpha-1->4)-D-glucose] with occasional (alpha-1->6) branches)
Important Structural Polysaccharide:
Cellulose [D-glucose-(beta-1->4)-D-glucose]. Extensive intra- and inter-chain H-bonding gives cellulose polymers physical strength.
Chitin [NAG-(beta-1-->4)-NAG]. NAG is N-acetyl-D-glucosamine. Present in fungal cell walls and arthropod exoskeletons.
Carrageenan is a polysaccharide extracted from red algae. Two major types are lambda-carrageenan [poly-(beta-D-galactose-alpha-D-galactose)] and kappa-carrageenan [poly-(beta-D-galactose-alpha-L-galactose)]. Carrageenans are also substituted to different extents on the galactose residues with sulfate groups. The lambda form is used to thicken foods like salad dressings without gelling. The kappa form is used to gel foods like yogurt and ice cream.
Agar, also extracted from red algae, is a polysaccharide [poly-(D-galactose-3,6-deoxy-L-galactose] with a sulfate group about every 10th galactose unit. It is used to thicken foods rather than carrageenan because it is less sticky and more stable to thermal processing. It is also used to solidify microbial media.
Dextrin [D-glucose-(alpha-1-->6)-D-glucose]. Used for storage of energy as well as structure. When brewing light beers, they add enzymes to hydrolyze the dextrins, producing additional fermentable sugars. The resulting product has 'less body' than beer containing dextrins and is 'less filling'. The alcohol content is usually similar to normal beers.
Sites about carbohydrates
