What is the difference between alpha and beta glycosidic linkages

Figure 1: Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond between carbon 1 in glucose and carbon 2 in fructose.

In the process, a water molecule is lost. Consequently, the blood galactose level is markedly elevated, and galactose is found in the urine. An infant with galactosemia experiences a lack of appetite, weight loss, diarrhea, and jaundice. The disease may result in impaired liver function, cataracts, mental retardation, and even death. If galactosemia is recognized in early infancy, its effects can be prevented by the exclusion of milk and all other sources of galactose from the diet.

As a child with galactosemia grows older, he or she usually develops an alternate pathway for metabolizing galactose, so the need to restrict milk is not permanent. The incidence of galactosemia in the United States is 1 in every 65, newborn babies. Sucrose, probably the largest-selling pure organic compound in the world, is known as beet sugar , cane sugar , table sugar , or simply sugar.

The dark brown liquid that remains after the recrystallization of sugar is sold as molasses. This linkage gives sucrose certain properties that are quite different from those of maltose and lactose.

Thus, sucrose is incapable of mutarotation and exists in only one form both in the solid state and in solution. In addition, sucrose does not undergo reactions that are typical of aldehydes and ketones. Therefore, sucrose is a nonreducing sugar. The hydrolysis of sucrose in dilute acid or through the action of the enzyme sucrase also known as invertase gives an equimolar mixture of glucose and fructose. This mixture is referred to as invert sugar because it rotates plane-polarized light in the opposite direction than sucrose.

The hydrolysis reaction has several practical applications. Sucrose readily recrystallizes from a solution, but invert sugar has a much greater tendency to remain in solution. In the manufacture of jelly and candy and in the canning of fruit, the recrystallization of sugar is undesirable. Therefore, conditions leading to the hydrolysis of sucrose are employed in these processes. Moreover, because fructose is sweeter than sucrose, the hydrolysis adds to the sweetening effect.

Common disaccharides include maltose grain sugar , lactose milk sugar , and sucrose table sugar. The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be , daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides. Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin both polymers of glucose. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals.

The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. The numbers and refer to the carbon number of the two residues that have joined to form the bond.

Figure 6. Amylose and amylopectin are two different forms of starch. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen not shown is similar in structure to amylopectin but more highly branched. Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells.

Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis. Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Figure 7. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.

As shown in Figure 7, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells.

In these animals, certain species of bacteria and protists reside in the rumen part of the digestive system of herbivores and secrete the enzyme cellulase. In a 1,4-glycosidic bond a C1-O-C4 bond is made involving the C1 of one sugar molecule and C4 of the other; likewise a C1-O-C6 bond is called a 1,6-glycosidic bond. Important examples in biochemistry include DNA or RNA , where deoxyribose or ribose sugar units are joined to nucleobases via N-glycosidic bonds.

The polysaccharides often used for energy storage were already mentioned above. Organisms also often form glycoproteins by attaching sugars to proteins via O- or N-glycosidic bonds in a process known as glycosylation.

Animals and pharmacists often join substances to glucuronic acid via glycosidic bonds in order to increase their water solubility ; this is known as glucuronidation. Many other glycosides have important physiological functions. Enzymes that form or 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.

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