Fructose Metabolism Fructose can enter glycolysis and gluconeogenesis. Glucose is a main metabolic fuel in most organisms. Other sugars convert to glycolytic intermediates. Fructose metabolism is faster than glucose in blood. Hexokinase can phosphorylate fructose: Fructose + ATP  Fructose 6-P + ADP K m for fructose >> K m for glucose, thus important only if [frucose] is high. Most of fructose metabolized to fructose 1-P by fructokinase. Fructose + ATP  Fructose 6-P + ADP Adolase B cleaves the molecule of fructose into two 3-Carbon compounds. dihydroxyaceton-P + glycealdehyde glycogenesis/gluconeogenesis

17-19 after dietary fructose consumption low blood glucose level

Excess fructose is toxic. Accumulation of fructose 1-P causes damage to liver. fructosekinase > aldolase B in activity Metabolism of (fructose by fructokinase) >> (glucokinase for glucose) in liver  Generated fructose 1-P stimulates pyruvate kinase.  Hypertriglyceridemia  improper substitute of glucose for diabete patient

Disorder of fructose metabolism Essential fructosuria: deficiency of fructokinase Hereditary fructose intolerance: deficiency of aldolase B -Accumulation of fructose 1-P: inhibits aldolase, phosphohexose isomerase and glycogen phosphorylase stimulates glucokinase -Tying up P i in the form of fructose 1-P makes it impossible for liver mitochondria to generate ATP by oxidative phosphorylation. fructose + ATP  fructose 1-P + ATP ADP + P i + “energy provided by electron transport chain”  ATP Net: P i + fructose  fructose 1-P The ATP levels fall precipitously inside cells. Cells cannot perform normal work functions. Deficiency of fructose 1,6-bisphophatase causes similar effect.

Galactose metabolism Galactose can enter glycolysis and gluconeogenesis Phosphorylation of galactose by galactokinase: Galactose 1-P UDP-galactose is an epimer of UDP-glucose recycle reversible: internal sources for other biosynthesis Galactosemia Deficiency of galactose 1-P uridyl transferase Accumulation of galactose (cataract) or galactose 1-P (damage to liver)

17-20 Recycle

17-23Polyol pathway Galactose Galactitol no reaction

Other pathways Pentose phosphate pathway Produces ribose 5-P and NADPH Oxidative branch: irreversible, high [NADPH]/[NADP + ] NADPH is a stronger reductant than NADH in cells. Non-oxidative branch: irreversible

17-21 Oxidative branch of pentose phosphate pathway 3 glucose 6-P + 6 NADP + 2 fructose 6-P + glyceraldehyde 3-P + 6 NADPH + 6H CO 2

17-22 Non-oxidative branch of pentose phosphate pathway

Thiamine pyrophosphate

17-21 Oxidative branch of pentose P pathway 3 glucose 6-P + 6 NADP + 2 fructose 6-P + glyceraldehyde 3-P + 6 NADPH + 6H CO 2

Use of oxidative and nonoxidative branches is dependent on nee d of NADPH and ribose 5-P in cells 1. When cells need ribose 5-P more than NADPH Generating ribose 5-P from oxidative branch, reverse reaction in Non-oxidative branch Used in muscle, where glucose 6-P dehydrogenase level is low and nucleotides are stored. 2. Need both ribose 5-P and NADPH Predominantly oxidative branch and phosphate pentose isomerase reaction. 3. need NADPH more than ribose 5-P Generating fructose 5-P and glyceraldehyde 3-P by both branches Changed to glucose 6-P through gluconeogenesis Thus, theoretically all glucose can be converted to CO 2 and NADPH.

Activity of pentose phosphate pathway The cell keeps the ratio of [NADPH]/[NADP + ] at above 100 to favor reductive biosynthesis. In some tissues such as adrenal cortex, lactating mammary gland and liver, where fatty acid and cholesterol synthesis are rapid, as much as 30% of glucose is metabolized by the pentose phosphate shunt. (weak in brain and muscle) NADPH as an antioxidant: important to tissues exposed to high oxygen pressure such as the cornea Oxidative branch produces NADPH, The first step in oxidative branch is oxidation of glucose 6-P via glucose 6-P dehydrogenase

Deficiency of glucose 6-P causes hemolytic anemia. The pentose phosphate pathway supplies the RBC with NADPH to maintain the reduced state of glutathione. - Oxidation of glucose 6-P via glucose 6-P dehydrogenase to produce NADPH. The inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H 2 O 2, that in turn results in a weakening of the cell wall and concomitant hemolysis. The pentose phosphate pathway in erythrocytes is essentially the only pathway for these cells to produce NADPH. Any defect in the production of NADPH could, therefore, have profound effects on erythrocyte survival. Oxidant drugs: increase the oxidation of glutathione Many anti-malarial drugs, etc. -Plasmodium requires the reducing power of NADPH for their life cycle. -Favism -Viral hepatitis, pneumonia, and typhoid fever

     Glu Gly Cys

Box 17-1,2,3  -Glu  Cys  SH +  Gly  -Glu  SH  Cys  Gly  -Glu  Cys  S   Gly  -Glu  S  Cys  Gly  -Glu  2 Cys  SH +  Gly H2O2H2O2  -Glu  Cys  S   Gly  -Glu  S  Cys + 2 H 2 O  Gly  -Glu  2 Cys  SH +  Gly NADP +  -Glu  Cys  S   Gly  -Glu  S  Cys + NADPH + H +  Gly Glutathione peroxidase Glutathione reductase

Fructose is a major sugar in semen Advantage over bacteria Polyol pathway is present in the seminal vesicles for fructose synthesis for seminal fluid (energy source for spermatozoa) Amino sugar synthesis from glucose ( 표 17-3 참고 ) Essential pentosuria

17-24 Synthesis of amino sugars

17-25 essential pentosuria Uronic acid pathway