Metabolism of pentoses, glycogen, fructose and galactose

Jana Novotna

1. The Pentose Phosphate Pathway

The pentose phosphate pathway (PPP):(hexose monophosphate or 6-phosphogluconate patway)

• Process that generates NADPH and pentoses (5-carbon sugars).

• Enzymes are located in the cytosol.

• Rapidly dividing cells (bone marrow, skin, intestinal mucosa, tumors) ribose 5-phosphate RNA, DNA.

• Other tissues NADPH electron donor for reductive biosynthetic reactions– fatty acids synthesis (liver, adipose tissue),– cholesterol and steroid hormones synthesis (liver, adrenal glands,

gonads) – elimination of oxygen radicals effects (erythrocytes).

An overview:

Two stages:

1) Oxidative (irreversible)• products:

→ ribose 5-phosphate (nucleotide synthesis)

→ NADPH (fatty acid synthesis, detoxification, reduction of glutathion)

2) Nonoxidative (reversible)• conversion of ribose 5-phosphate to intermediates of

glycolysis• production of ribose 5-phosphate from intermediates of

glycolysis

Regulation:

Glucose 6-phosphate dehydrogenase

• inhibition - by NADPH

• induction - by insulin/gluckagon ↑

1. The oxidative phase of PPP:

Some concepts

• Isomers - molecules with the same molecular formula but different chemical structures (glucose and fructose)

• Epimeres - differ at only one chiral center, not the anomeric carbon.

• Enantiomers - chiral molecules that are mirror images of one another.

Epimers Enantiomers

2. The nonoxidative phase of PPP:

Pathways that require NADPH:

Detoxification• reduction of oxidized glutathione• cytochrome P450 monooxygenases

Reductive synthesis• fatty acid synthesis• fatty acid chain elongation• cholesterol synthesis• steroid hormon synthesis• neurotransmitter synthesis• deoxynucleotide synthesis

The role of PPP in maintenance of the erythrocyte membrane integrity:

Clinical correlations:

Treatment by certain drugs (i.e. sulfonamides) people with glucose 6-phosphate dehydrogenase deficiency (7% of the

world population) increased production of free radicals reduced protection of erythrocytes against FR hemolysis, hemoglobinuria, hemolytic anemia

Summary:

The pentose phosphate pathway A shunt from glycolysis Production of NADPH (reductive syntheses, detoxifications),

ribose 5-phospate Conversion to intermediates of glycolysis Isomerases, epimerases, transketolases, transaldolases Glucose 6-phosphate dehydrogenase deficiency

2. Metabolism of glycogen

Glycogen

• The glycogen – a storage form of glucose• Required as a ready source of energy• The liver – tremendous capacity for storing glycogen –

10% of the wet weight• Muscle – max.1 – 2% of the wet weight• Muscle and liver glycogen stores serve completely

different roles:– muscle glycogen – fuel reserve for ATP synthesis– liver glycogen – glucose reserve for the maintenance of blood

glucose concentration

Glucosyl units of α-D-glucose linked by α-1,4 and α-1,6 link (branching every 8-10 units)

source of energy in animals (liver, muscles)highly branched structure (rapid degradation and synthesis, better solubility)

Nonreducing end

glycogenin

The glycogen metabolism in the muscles and the liver:

Decrease in glucose in the blood

→ glycogen degradation

→ release of glucose to the blood

Glucose 6-phosphatase (only in liver)

High ATP demand

→ glycogen degradation

→ anaerobic glycolysis

Glycogen metabolism - an overview:

Synthesis and degradation of glycogen:

→ different enzymes (regulation!)

UDP-glucose – the substrate for glycogen synthesis and UDP is released as a reaction product glucose-1-phosphate + UTP UDP-glucose + PPi PPi + H2O 2 Pi Overall: glucose-1-phosphate + UTP UDP-glucose + 2 Pi Cleavage of PPi is the only energy cost for glycogen synthesis (one ~P bond per glucose residue).

Glycogenin - (enzyme) initiates glycogen synthesis.

Glycogen synthesis:

A glycogen primer - 4 attached glucose molecules to glycogenin

- not degraded

- synthesis autocatalytic glycosylation, autophosphorylation of glycogenin)

Transfer of 6-8 units Glycogen synthase (regulation) An energy-requiring pathway (UTP)

Chain cleavage (phosphorolysis) –glycogen phosphorylase

- to 4 units from a branch point

- The debrancher enzyme - amylo-16 glukosydase (transfer of 3 units, hydrolysis of 1 glucose)

- two catalytic activities – transferase + a-16-glucosydase

Glycogen phosphorylase (regulation)

Glycogen degradation:

Type Enzyme affected Genetics Organ involved

Manifestations

I (Von Gierke´s disease)

Glucose 6-phosphatase

AR (1/200 000)

Liver Hypoglycemia, lactate acidosis, hyperlipidemia, hyperuricemia.Enlarged liver and kidney.

II (Pompe disease)

Lysosomal α-1,4-glucosidase

AR Organs with lysosomes

Glycogen deposits in lysosomes.Hypotonia, cardiomegaly, cardiomyopathy (Infantile f.).Muscle weakness (Adult f.)

III (Cori´s disease)

The debrancher enzyme

AR Liver, muscle, heart

Hepatomegaly, hypoglycemia

V (McArdles disease)

Muscle glycogen phosphorylase

AR Muscle Exercise-induced muscular pain, cramps, muscle weakness

Glycogen storage diseases:

Regulation of glycogen synthase by covalent modification

Regulation of glycogen phosphorylase by covalent modification

Regulation of glycogen synthesis and degradation in the liver

Activation of muscle glycogen phosphorylase during exercise

Clinical correlations:

Maternal malnutrition in the last trimester of pregnancy (physiologically: glycogen formation and storage during the last

10 weeks of pregnancy by the fetus → reserve for first hours → prevention of hypoglycemia)

reduced or no glycogen reserve in the fetus after birth → hypoglycemia, apathy, coma

State Regulators Response

Liver

Fasting Glucagon ↑, Insulin ↓cAMP ↑

Glycogen degradation ↑Glycogen synthesis ↓

Carbohydrate meal Glu ↑, Glucagon ↓, Insulin ↑cAMP ↓

Glycogen degradation ↓ Glycogen synthesis ↑

Exercise and stress Adrenalin ↑ cAMP ↑, Ca2+-calmodulin ↑

Glycogen degradation ↑ Glycogen synthesis ↓

Muscle

Fasting (rest) Insulin ↓ Glycogen synthesis ↓Glucose transport ↓

Carbohydrate meal (rest) Insulin ↑ Glycogen synthesis ↑Glucose transport ↑

Exercise Epinephrine ↑AMP ↑, Ca2+-calmodulin ↑, cAMP ↑

Glycogen synthesis ↓Glycogen degradation ↑Glycolysis ↑

Regulation of liver and muscle glycogen metabolism:

Summary:

Glycogen metabolism Different role of glycogen stores in the liver and muscles Glycogen synthesis and degradation are separate pathways

(regulation) Glycogen storage diseases

3. Fructose and Galactose metabolism

Principally in the liver (small intestine, kidney)

Aldolase B: low affinity for fructose 1-phosphate (→ accumulation of fructose 1-phosphate in the liver )

Fructose metabolismEssential fructosuria

Hereditary fructose intolerance

The polyol pathway

Seminal vesicles (spermatozoa use fructose)

Accumulation of sorbitol in diabetic patients Lens (diabetic cataract) Muscles, nerves (periferal neuropathy)

Galactose metabolism:

Lens metabolism:

Diabetic cataract :

↑glucose concentration in the lens → ↑aldose reductase activity → sorbitol accumulation → ↑osmolarity, structural changes of proteins

Clinical correlations:

A newborn: failure to thrive, vomiting and diarrhea after milk galactosemia (Galactose 1-phosphate uridylyltransferase

deficiency) genetic disease (AR, 1/60 000) hepatomegaly, jaundice, cataracts, mental retargation, death

Management: early diagnose, elimination of galactose from the diet (artificial milk from soybean hydrolysate)

Summary:

Fructose and Galactose metabolism Conversion to intermediates of glycolysis Genetic abnormalities, accumulation of intermediates, tissue

damage Accumulation of sorbitol in diabetes

Pictures used in the presentation:

Marks´ Basic Medical Biochemistry A Clinical Approach, third edition, 2009 (M. Lieberman, A.D. Marks)

Textbook of Biochemistry with Clinical Correlations, sixth edition, 2006 (T.M. Devlin)