Biochemistry

Department
Biochemistry II
(PB315/PBC315)
Eman Amer, MD
Assoc. Prof. of Medical Biochemistry &
Molecular Biology
Faculty of Pharmacy
Ahram Canadian University

Ahram Canadian University - Faculty of Pharmacy

 Interactive learning
• Brainstorming
• Class discussion
• Self directed learning
• Flipped class room
Uronic acid pathway
• This is a minor pathway for the
conversion of glucose to glucuronic
acid.
Location of the pathway:
• Intracellular location: cytoplasm
• Organ location: mainly liver
Functions (importance) of uronic acid pathway:
- This pathway produces glucuronic acid, which is important for:
1- Synthesis of substrates:
a- Glycosaminoglycans e.g. chondrotin sulfate.
b- Vitamin C (not in humans).
2- Conjugation reactions:
- UDP-glucuronic acid is used for conjugation with many compounds to
make them more soluble before excretion e.g. steroid hormones and
bilirubin.
3- Detoxification reactions:
- UDP-glucuronic acid is used for conjugation with toxic compounds to make
them less toxic e.g. phenols.
Fate of glucuronic acid:
- UDP-glucuronate is converted to glucuronate then → L-xylulose → D-xylilol
→ D-xylulose → which then join HMP pathway to be completely oxidized.
Defects_of uronic acid pathway (Essential pentosuria):
- It is hereditary disease due to failure of conversion of L-xylulose into D-
xylulose (due to genetic deficiency of L-xylulose reductase).
- L-xylulose will accumulate and excreted in urine.
 GLYCOGEN METABOLISM
• Glycogen is a highly branched
polymer of α, D-glucose. The
glucose residues are united by α 1-4
glucosidic linkages within the
branches. At the branching points,
the linkages are α 1-6. The branches
contain from 8-12 glucose residues.
• Glycogen is the main storage form
of carbohydrates in animals. It is
present mainly in the liver (forms 8 -
10% of its wet weight) and in
muscles (forms 2% of its wet
weight). However, due to the
greater mass of muscles, its
glycogen store is three to four times
more than that of liver.
• The main function of muscle glycogen is
to supply glucose within muscles during
contraction. Liver glycogen is mainly
concerned with the maintenance of
blood glucose especially between
meals. After 12-18 hours fasting, liver
glycogen is depleted, whereas muscle
glycogen is depleted after muscular
exercise.
Glycogen metabolism includes the
following: -
1. Glycogenosis: It is the synthesis of
glycogen from glucose.
2. Glycogenolysis: It is the breakdown
of glycogen.
 GLYCOGENESIS
• Synthesis of glycogen from glucose
occurs mainly in the cytosol of liver
cells and muscles.
• The synthesis of glycogen from UDP-
Glc requires the presence of the two
enzymes, glycogen synthase and
branching enzyme.
1- Glycogen Synthase
• This is the key enzyme for glycogen synthesis. It
catalyzes the transfer of the glucosyl group of UDP-Glc
to the glycogen primer (an already existing chain of
glucose molecules) or to Glycogenin which is a small
protein (replaces glycogen primer), then successive
glucose residues are added from UDP-Glc by glycogen
synthase. Sometimes, it remains in the center of
glycogen molecules.
• Glycogen synthase forms successive α 1-4 glucosidic
linkage. This produces elongation of the branches of
glycogen primer up to a minimum of 11 glucose
residues
2- Branching Enzyme
• If catalyzes the synthesis of α 1-6
glucosidic linkage in glycogen. It
transfers parts of the elongated chains
(contain 6 to 8 glucose residues) to the
nearest chain forming a new branching
point.
• The new branches are elongated by the
glycogen synthase and the process is
repeated.
Regulation of activity of glycogen
synthase
Glycogen synthase is present in two forms:
 Dephosphorylated or a-form: It is the active form.
It is allosterically inhibited by accumulated
glycogen.
 Phosphorylated or b-form: It is the inactive form,
but becomes active in the presence of high
concentration of G-6-phosphate (allosteric
activator). Accumulation of G-6-P means that the
cell is not in need of more oxidation and ATP
production. Conversion of a to b is catalyzed by the
enzyme protein kinase A, which is activated by
cAMP. The conversion of b to a is catalyzed by
protein phosphatase-1.
• Insulin produces activation of
phosphodiesterase, which decreases
cAMP and produces inactivation of
protein kinase A. At the same time,
insulin activates the protein
phosphatase-1. These effects lead to
the conversion of b to a form of
glycogen synthase and activation of
glycogenesis in both liver and muscles.
• Glucagon (in liver) and epinephrine (in
liver and muscles) produce activation of
adenylyl cyclase, which increase cAMP
level. This produces activation of cAMP
dependent protein kinase A and
inhibition of protein phosphatase-1, that
lead to inactivation of glycogen
synthase. Accordingly, glucagon and
epinephrine decrease the rate of
glycogenesis.
• Growth hormone and
glucocorticoids may stimulate
glycogenesis in liver cells by
allosteric activation of glycogen
synthase b by elevated levels of
glucose-6-phosphate. The latter
is produced by stimulation of
gluconeogenesis.
 GLYCOGENOLYSIS
• It is the breakdown of glycogen
to form glucose-1-phosphate,
which is converted to glucose-6-
phosphate. Glycogenolysis are
catalyzed by two enzymes,
glycogen phosphorylase and
debranching enzyme.
1- Glycogen Phosphorylase
• It catalyzes removal of glucose residues
from the outer branches of glycogen by
adding inorganic phosphate and
producing glucose-1- phosphate. The
action of the enzyme stops when there
is only four glucose residues on each
side of the branching point.
2- Debranching Enzyme
Glucosyl transferase activity of debranching enzyme
• It catalyzes transfer of the outer three residues of
glucose at a branching point to the near non-
reducing end of the glycogen molecule, leaving only
one glucose residue at the branching point.
Glucosidase activity of debranching enzyme
• It catalyzes the removal of the last glucose residue
at the branching point by adding water and
producing free glucose, leaving a chain that is
further hydrolyzed by the phosphorylase enzyme.
• Glucose-1-phosphate is converted by
phosphoglucomutase to glucose-6- phosphate.
• In muscles: Glucose-6-phosphate is oxidized by
glycolysis to provide energy. Glucose - 6-
phosphate cannot be converted to free glucose
due to deficiency of Glucose -6-Pase.
• In liver: Glucose - 6 - phosphate is mainly
converted to glucose due to the presence of G-6-
Pase. Free glucose formed is released to the
blood, as the main function of liver glycogen is
to maintain the blood glucose level especially
during fasting or carbohydrate deficiency.
Regulation of Activity of Glycogen Phosphorylase
 Glycogen phosphorylase is the key enzyme of
glycogenolysis. It is present in two forms. The active
form (a) is phosphorylated, while the inactive form
(b) is dephosphorylated. Conversion of the (b) to (a)
form is catalyzed by the active phosphorylase
kinase. The conversion of (a) to (b) form is catalyzed
by the protein phosphatase-1.
 Phosphorylase kinase is also present in two forms.
The active form (a) is phosphorylated and the
inactive form (b) is dephosphorylated. Conversion
of the inactive (b) to active (a) form is catalyzed by
the cAMP dependent protein kinase A, this reaction
is reversed by the protein phosphatase-1.
 Insulin produces activation of phosphodiesterase, which
decreases cAMP level and leads to inhibition of the protein
kinase A. At the same time, insulin activates the protein
phosphatase-1, which produces inactivation of
phosphorylase kinase and glycogen phosphorylase
(conversion of a to b). So, insulin decreases glycogenolysis in
liver and muscles.
 Glucagon (in liver) and epinephrine (in liver and muscles)
produce activation of adenylyl cyclase, increase cAMP level,
activate protein kinase A, activate phosphorylase kinase,
which activates the glycogen phosphorylase, this enzyme
cascade increases glycogenolysis markedly (produces signal
amplification). ATP and G-6-P act as allosteric inhibitors for
glycogen phosphorylase a, because their elevated levels
indicate that the cell is not in need of more energy and there
is no need to breakdown glycogen.
Muscle phosphorylase
It is a dimer and each monomer contains one mole of pyridoxal
phosphate.
The increase of intracellular calcium ions alone during muscle
contraction help to supply the contracting muscles with glucose and
increase the rate of glucose oxidation for energy production through
the following effects:
1- Direct activation of the phosphorylase kinase b (without
phosphorylation) by binding with the calmodulin subunit of the
enzyme.
2- Calcium ions bind with troponin C (TPC), which is a specific muscle
protein. TPC-Ca + complex produces more activation of
phosphorylase kinase a.
3- Calcium ions activate pyruvate dehydrogenase that converts
pyruvate to active acetate and increases activity of citric acid
cycle enzymes i.e. citrate synthase, isocitrate dehydrogenase and
α-KG dehydrogenase.
Type I (Von Gierk's Disease): The most common and is due to genetic
deficiency of
glucose-6-phosphatase enzyme leading to accumulation of glucose-6-
phosphate and glycogen in liver and kidneys. Its manifestations include the
following:
1. Fasting hypoglycemia.
2. Hyperlipidemia and ketosis due to enhanced lipolysis in adipose tissues.
3. Hyperuricemia or gout (high plasma uric acid level), accumulation of G-6-
P results in stimulation of HMP and increases synthesis of PRPP and purine
nucleotides. The latter produce uric acid by catabolism.
Type V (McArdle's Syndrome): It is due to genetic deficiency of muscle
phosphorylase resulting in accumulation of glycogen in muscles and painful
muscle cramps. Accumulation of glycogen and decreased ATP production
produce damage of muscle cells, that result in increased serum levels of
muscle enzymes e.g. creatine phosphokinase (CK) and lactate
dehydrogenase (LDH).
 LIVER GLYCOGEN MUSCLE GLYCOGEN
Source  Blood glucose.  Blood glucose only
 Other hexoses e.g.
fructose
 Noncarbohydrate
sources e.g. lactate
Concentration 6 % of liver weight 2% of muscle weight
Function Supply all body cells with Private source of energy
glucose for muscles only
End product Glucose Lactate
Effect of hormones
 Insulin Stimulates glycogenesis Same
 Epinephrine Stimulate glycogenolysis Same
 glucagons Stimulate glycogenolysis No effect