Namespaces Page Discussion. Views Read View source View history. This page was last modified on 10 January , at This page has been accessed 49, times. Imteyaz Alam , M. Ali Haider. Mechanistic insights into the ring-opening of biomass derived lactones. RSC Advances , 6 16 , ADP-Ribosylargininyl reaction of cholix toxin is mediated through diffusible intermediates.
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Carbohydrate Research , 13 , The Journal of Physical Chemistry B , 8 , Mechanisms for nucleophilic aliphatic substitution at glycosides. Journal of Biological Chemistry , 3 , M Chagit Denekamp , Yana Sandlers. Formation and stability of oxocarbenium ions from glycosides. Journal of Mass Spectrometry , 40 8 , Thus, its two glucose molecules must be linked in such a way as to leave one anomeric carbon that can open to form an aldehyde group.
Lactose is known as milk sugar because it occurs in the milk of humans, cows, and other mammals. In fact, the natural synthesis of lactose occurs only in mammary tissue, whereas most other carbohydrates are plant products. Human milk contains about 7.
This sugar is one of the lowest ranking in terms of sweetness, being about one-sixth as sweet as sucrose. Lactose is produced commercially from whey, a by-product in the manufacture of cheese. It is important as an infant food and in the production of penicillin.
The two monosaccharides are obtained from lactose by acid hydrolysis or the catalytic action of the enzyme lactase :. Many adults and some children suffer from a deficiency of lactase. These individuals are said to be lactose intolerant because they cannot digest the lactose found in milk.
A more serious problem is the genetic disease galactosemia , which results from the absence of an enzyme needed to convert galactose to glucose. Certain bacteria can metabolize lactose, forming lactic acid as one of the products.
As a result, many adults experience a reduction in the ability to hydrolyze lactose to galactose and glucose in their small intestine. For some people the inability to synthesize sufficient enzyme increases with age. In people with lactose intolerance, some of the unhydrolyzed lactose passes into the colon, where it tends to draw water from the interstitial fluid into the intestinal lumen by osmosis. At the same time, intestinal bacteria may act on the lactose to produce organic acids and gases.
The buildup of water and bacterial decay products leads to abdominal distention, cramps, and diarrhea, which are symptoms of the condition. The symptoms disappear if milk or other sources of lactose are excluded from the diet or consumed only sparingly. Alternatively, many food stores now carry special brands of milk that have been pretreated with lactase to hydrolyze the lactose. Cooking or fermenting milk causes at least partial hydrolysis of the lactose, so some people with lactose intolerance are still able to enjoy cheese, yogurt, or cooked foods containing milk.
Many other glycosides have important physiological functions. Enzymes that break glycosidic bonds i. Before monosaccharide units are incorporated into glycoproteins, polysaccharides, or lipids in living organisms, they are typically first "activated" by being joined via a glycosidic bond to the phosphate group of a nucleotide such as uridine diphosphate UDP , guanosine diphosphate GDP , thymidine diphosphate TDP , or cytosine monophosphate CMP.
Sometimes mono- or oligosaccharides are also activated by being linked to lipids through a phosphate or diphosphate group. These activated species are known as sugar donor substrates. Then enzymes known as glycosyltransferases transfer the sugar unit from the activated glycosyl donor to an accepting nucleophile the acceptor substrate. Categories: Glycosides Carbohydrates. Read what you need to know about our industry portal bionity. My watch list my. My watch list My saved searches My saved topics My newsletter Register free of charge.
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