Chem*4520 Metabolic Processes

Fall Semester 2000

Modified August 2000

schematic view of the enzyme citrate synthase,
with bound acetyl-CoA analog in green

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Lecture 7:	Monosaccharide interconversions.
Mon Sept 25	Utilization of fructose, mannose and galactose; synthesis of N-acetyl glucosamine, glucuronic acid, rhamnose, fucose and N-acetyl-neuraminidate.
	Voet Chapter 16, pp.600-606,, 657-658. Mathews, Van Holde: Chapter 16 pp. 561-572. Stryer Chapter 19, pp. 491-493. None of the texts deal with synthesis of the modified monosaccharides, although these compounds are important components of glycoproteins, glycolipids and heteropolysaccharides.

Utilization of fructose, mannose and galactose.

Fructose is the most widely used alternative to glucose. Sucrose is abundant in the diet, and breaks down into equal parts of glucose and fructose in the digestive tract. Fructose has about 30 fold sweeter taste than glucose; "calorie reduced" foods are often reduced in sugar content by using fructose as the sweetener.

The hexokinase of muscle and adipose tissue can accept fructose as a substrate, but K_M is much higher for the furanose sugar than the usual pyranoses.

The fructose-6-P simple enter the glycolysis sequence at phosphofructokinase.

Liver contains the enzyme fructokinase:

Fructose-1-P is not a substrate for phosphofructokinase, and instead a variant fructose-1-P aldolase breaks fructose-1-P down to dihydroxyacetone-3-P and unphosphorylated glyceraldehyde (since the 6-P is missing).

Glyceraldehyde kinase phosphorylates most of the glyceraldehyde:

Some glyceraldehyde may be reduced to glycerol by the abundant liver alcohol dehydrogenase and is then converted to dihydroxyacetone-3-P; this is really opportunistic use of enzymes normally used for other purposes,and not a major pathway in its own right.

glyceraldehyde

NADH

NAD⁺

glycerol

ATP

ADP

glycerol-3-P

NAD⁺

NADH

DHA-3-P

Mannose is phosphorylated by hexokinase; a specific mannosephosphate isomerase yields fructose-6-P.

Pentoses such as ribose, ribulose, xylose and xylulose can be phosphorylated and are converted to triose phosphates by the non-oxidative rearrangements of the pentose phosphate pathway.

Galactose is derived from breakdown of lactose in the digestive tract. There is a specific galactokinase, which phosphorylates on C-1 rather than C-6. This allows for activation to UDP-galactose.

galactokinase

UDP-galactose and UDP-glucose can be interconverted by the enzyme 4-epimerase. This enzyme contains a tightly bound NAD⁺, and the 4-ketopyranose is formed as a intermediate which is then reduced without ever being released from the active site. The new hydroxyl is randomly generated with either glucose or galactose configuration.

4-epimerase

Nucleoside diphosphate sugars

Nucleoside diphosphate sugars, e.g. UDP-Glc or UDP-Gal contain an energy rich phosphoanhydride bond, which is formed by using UTP as a donor to couple UMP to glucose-1-phosphate, and with release of PPi. The reaction is driven by the low [PPi] in the cell due to the action of pyrophosphatase.

UDP glucose

= -30.5 kJ/mol for hydrolysis of the glycosyl-phosphate bond in UDP-sugars. This makes them suitable as glycosyl group donors. A variety of target-specific enzymes can transfer the glucosyl unit of UDP-sugars to an acceptor with a nucleophilic -OH, -NH or -SH group. (Some enzymes use ADP, GDP or even dTDP as the activating nucleotide, e.g. ADP-glucose in starch synthesis.)

glucosyl transferase

for transfer to O is about -10 to -15 kJ/mol. Acceptors may include the non-reducing end of an existing polymer such as glycogen, and the transfer of glucose, e.g. by glycogen synthase, elongates the polymer by one unit, releasing UDP.

Glycoproteins, glycolipids and heteropolysaccharides often contain modified sugars in the chain, such as uronic acids, 6-deoxysugars, amino-sugars (usually as N-acetyl amides), N-acetylneuraminic acid (NANA, also known as sialic acid) in animals and N-acetylmuramic acid in bacterial cell walls.

Uronic acids

Uronic acids result from oxidation of C-6 of UDP-hexose to a carboxylate.

UDP-galacturonate is formed by the action of 4-epimerase on UDP-glucuronate. A 5-epimerase converts UDP-D-glucuronate into UDP-L-iduronate, incorporated into the heteropolysaccharide dermatan sulfate.

6-Deoxysugars

The 6-deoxysugars rhamnose (found on bacterial O-antigens) and fucose (in ABO blood-group antigens) are formed by a mechanism involving oxidation, dehydration and reduction.

dTDP-rhamnose

Bacteria contain a variety of deoxy- and dideoxy sugars made by similar processes. In animal blood group oligosaccharides, GDP-fucose is made by a similar process starting from GDP-mannose.