1. Carbohydrates are the largest source of dietary
calories.
The major cabohydrates are starch, lactose and
sucrose
The main carbihydrates in body metabolism is
glucose
transported to muscle (and other tissues) via blood
stored in liver and muscle as glycogen
ATP produced more quickly from CHO than from fats
or proteins
CHO stores can be depleted
Carbohydrates (Fuels)
2. Digestion
Starch digestion begins in mouth by the
salivary α-amylase which converts starch to
smaller polysaccharides called α-dextrins
Pancreatic α-amylase continue the digestion of
α-dextrins into maltose, maltotriose &
oligosaccharides called limited dextrins
Digestion of maltose, maltotriose, sucrose &
lactose continues by disaccharidases attached
to the membrane of surface of the brush
border (microvilli)of intestinal epithelial cells
The monosaccharides produced are
transported by into the intestinal cells
3. Disaccharidases
Glucoamylase : Glycoprotein
attached to the luminal
membrane with two catalytic
activities. An exoglucosidase
specific for α-1-4 glycosidic
bonds, acts at the non-reducing
ends of limited dextrins releasing
glucose up to isomaltose.
4. Disaccharidases
Sucrase-Isomaltase Complex: Similar to
glusamylase attached to membrane with two
polypeptides protrudes into the intestinal lumina,
an intestinal protease clips it into two separate
subunits that remain attached to each other.
Each subunit site has a catalytic site that differs
in substrate specificity. The sucrose maltase site
accounts for 100% of intestinal ability to
hydrolyze sucrose in addition to maltase activity.
The isomaltase-maltase site accounts for almost
all activity to hydrolyze α 1-6 bonds in addition to
maltase activity.
5. Disaccharidases
Trehalase: one catalytic site and
hydrolyzes the glycosidic bond in
trehalose (disaccharide of two glycosyl
units attached by their anomeric
carbons)
Lactase-Glucosyl Ceramidase Complex:
glycoprotein found in the brush border
that has two calaytic sites extending in
the intestinal lumin. It hydrolyzes lactose
and the B-bonds betweeen glucose or
galactose and ceramide in glycolipids.
6. Dietary Fiber
Composed principally of polysaccharides which
can not be digested by human enzymes of
intestinal tract
Derivatives of lignan ( cellulose, hemicellulose,
lignin, pectin s, mucilages & gums)
Bacterial flora of the colon metabolize the
fibers to gases(H2,CO2,CH4) & short chain fatty
acids
Fatty acids are absorbed by colonic cells
Fibers is seen to soften the stool, thereby
reducing pressure on colonic wall & enhancing
expulsion of feces
7. Absorption of carbohydrates
Sugars are transported
across the intestinal
epithelial cells into the
blood
Not all complex sugars
are digested in the
same rate within the
intestine
Glycemic index of the
food is an indication of
how rapidly blood
glucose level rises after
consumption
Whole weat 100
pasta 67
cornflakes 121
potatoes 120
Ice cream 69
fruits 52
8. Glucose Absorption
Two types of glucose transport
proteins are present in the intestinal
cells
– Na+ dependent: an active transport
which depends on cotransport of sodium
and glucose
– Facilitated : passive transport known as
Glut 1 -4
9. Cell membranes are not inherently permeable to
glucose. There are many glucose transporters.
GLUT- 1 enables basal non- insulin stimulated
glucose uptake (erythrocytes)
GLUT- 2 transport glucose into beta cells, a pre-
requisite for glucose sensing.
GLUT- 3 enables non- insulin mediated glucose
uptake into brain.
GLUT- 4 enables much of the peripheral action of
insulin (muscles, adipose)
Glucose transporters
13. Glycogenolysis
Occurs mainly in liver & muscles
Both pathways in the liver & muscle
are the same
End product in liver is glucose, while in
muscle is glucose 6 phosphate
23. uses only CHO
occurs in sarcoplasm
first step is glucose transport into tissues
after entry, 2 ATP are used (with glucose)
glucose (C6) is split into two C3 molecules
final product is pyruvate
4 ATP are synthesized
pyruvate forms either lactate or enters
mitochondria
Glycolysis
27. Electron Transport Chain (ETC)
Oxidative Phosphorylation
Oxidation
NADH and FADH2 transfer electrons to ETC
final acceptor of electrons is O2
Phosphorylation
energy generated by oxidation used to
resynthesize ATP
– 3 ATP from each NADH
– 2 ATP from each FADH2
30. Hexokinase
Tissue specific isoenzyme, low Km
value, inhibited by G-6-P, in
muscles, brain, RBC and adipose
tissue.
Liver isoenzyme (glucokinase) has
high Km value, not inhibited by G-
6-P
31.
32.
33. Phosphofructokinase(PFK-1)
Allosteric enzyme, catalyze rate
limiting step
Has six binding sites
– 2 for substrates (ATP & F-6-P)
– 4 allosteric regulatory sites
ATP & Citrates are inhibitors
AMP & F2,6bp are stimulatory
34. PFK-2
Not glycolytic enzyme, synthesizes F-2,6 bp,
has two domains, a kinase & phosphatase
In skeletal muscles, F-6-P activates the kinase
domain and this increases F2-6-bp
concentration
In liver, PFK-2 regulated by phosphorylation,
which when near amino terminal decreases the
activity of kinase & increases activity of
phosphatase (CAMP)
Cardiac isoenzyme, phosphorylation near the
carboxylic terminal in response to adrenergic
activator or AMP increases the kinase activity
35.
36.
37. Pyruvate Kinase
Tissue specific isoenzyme
Brain & muscle contains no allosteric
site and thus do not contribute toward
regulation of glycolysis.
In liver, it is regulated by
phosphorylation & dephosphorylation
by CAMP-dependent kinases. It is also
inhibited by allosteric regulators (F-1-
6bp & ATP).
38. Lactate Dehydrogenase
Tetramer composed of A-subunits (M-
skeletal) and also B subunits (H-heart).
Different tissues produce different
amounts of the two subunits (M4, M3H
……….H4)
M4 facilitates conversion of pyruvate into
lactate in skeletal muscles
M4 facilitates conversion of lactate into
pyruvate in the heart.
39. lactate formation
enters mitochondria (i.e. Kreb’s cycle)
formation of Kreb’s cycle intermediates
Pyruvate goes in one of three directions
Metabolic Fate of Pyruvate
40.
41.
42. primary function is to reduce NAD+ and FAD
acetyl CoA (C2) combines with a C4
molecule forming a C6 molecule
C6 molecule is partially degraded back to a
C4 molecule
each loss of C gives off a CO2
Kreb’s Cycle
(Citric Acid Cycle)
48. Citrate Synthetase
Has no alosteric regulators
Oxaloacetate concentration
activates the enzyme
Citrate concentration
inhibits the enzyme
49. Isocitrate Dehydrogenase
Rate limiting step
Allosteric, activated by ADP & inhibited
by NADH
Binding of ADP to one subunit causes
the activation of other subunits
(positive cooperativity)
Binding of ADP shift Km to a lower
value
Small change in ADP brings about
large change in the rate.
50. α-ketoglutrate Dehydrogenase
No allosteric regulators
Product inhibition by NADH and
succinyl CoA
It may also be inhibited by GTP
In contracting heart muscle, the
release of Ca2+ may provide activation
of both enzymes when ATP is rapidly
hydrolyzed.
51. NADH
Plays a central role in Regulation
Rate of oxidation of NADH depends
on oxidative phosphorylation
It inhibits all dehydrogenases in TCA
& pyruvate dehydrogenase and thus
regulates entry point of acetyl CoA
57. Sites include liver & kidney while
substrates include, amino acids,
lactate, pyruvate, glycerol.
Cori & alanine cycles for transport of
intermediates between site of
production of these metabolites and
site of synthesis.
Many enzymes of glycolysis are
common to gluconeogenesis.
Gluconeogenesis(1)
58.
59. Irrversible enzymes of glycolysis are
replaced by gluconeogenesis
enzymes.
1. Pyruvate kinase – pyruvate
carboxylase & PEPCK
2. PFK ---- F 2,6 diphosphatase
3. Glucokinase --- Glu 6 phosphatase
Gluconeogenesis(2)
65. The plasma glucose concentration reflects the
balance between intake, tissue utilization &
endogeneous production .
Insulin promotes up take of glucose thus decreasing
plasma glucose while glucagon stimulates both the
release of glucose from glycogen stores & its denovo
synthesis, thus causing an increase in plasma
glucose.
Glucose stimulates the secretion of insulin &
suppresses the secretion of glucagon.
Glucose Homeostasis
66. Insulin acts on three main targets, liver, adipose &
muscles.
In liver insulin stimulates, glycolysis, glycogenesis &
lipogenesis & suppresses lipolysis.
In peripheral tissues, insulin induces lipoprotein
lipase activity & thus stimulates triglyceride synthesis.
In muscles, insulin increases glucose & amino acid
transport & glycogen synthesis.
Metabolic effects of insulin
67. Glucagon main effect is the mobilization of the fuel
reserves for the maintenance of the blood glucose
level between meals.
Glucagon inhibits glucose- utilizing pathways and the
storage of metabolic fuels.
It acts on liver, to stimulate glycogenolysis and inhibit
glycogenesis, glycolysis and lipogenesis.
Gluconeogenesis and ketogenesis are then activated
Epinephrine has effects similar to glucagon in the
liver but works through a different receptor. It
promotes an increase in blood glucose in response to
stress
Metabolic effects of Glucagon
68. The glucose level in the vicinity of the B- cell is
sensed by the transporter GLUT- 2. Glucose is carried
into the cell, where it is phophorylated into G-6- P by
glucokinase which also is a part of the glucose
sensing mechanism. Increased G-6- P increases
glucose utilization and ATP production in the B- cell.
This changes the flux of ions across the cell
membrane, depolarizes the cell and increases the
concentration of Ca2+. Hence insulin is exocytosed.
Insulin secretion is biphasic. The first phase occurs
over 10- 15 minutes of stimulation which release the
preformed insulin. The second phase, which lasts up
to 2 hours, is the release of newly synthesized
insulin.
Stimulation of insulin secretion by glucose
69. Insulin secretion is also stimulated by gastrointestinal
hormones (insulinotropic peptide, cholecystokinin)
and amino acids, such as leucine, arginine, and
lysine. Thus, the insulin response to orally
administered glucose is greater than to an
intravenous infusion.
Stimulation of insulin secretion by glucose
70. Hypoglycemia is defined as a blood glucose
concentration below 2.5 mmol/ L ( 45 mg/dl).
Epinephrine and glucagon are released, resulting in a
stress response, the manifestation of which may
include sweating, trembling, increased heart rate and
feeling of hunger. If blood glucose continues to fall,
brain function is compromised (neuroglycopenia).
Hypoglycemia in healthy individuals is usually mild
and may occur during exercise, after a period of
fasting or due to alcohol ingestion.
Hypoglycemia may be caused by a rare insulin
secreting tumor of the beta- cells ( insulinoma) or
overdose of exogenous insulin.
Hypoglycemia
71. Alcohol is metabolized primarily in the liver. Two steps
metabolism of alcohol is relatively unregulated, leading
to a rapid increase in hepatic NADH. This shifts the
equilibrium of LDH catalyzed reaction towards lactate
formation ( lacticacidemia). Also shifts cytosolic
oxaloacetate towards malate formation, reducing
gluconeogenesis from TCA. In addition DHAP is shifted
toward glycerol- 3- phosphate formation and thus
reducing gluconeogenesis from glycerol.
The low blood glucose leads to a stress response (
rapid heart beat, clammy skin), an effort to enhance
stimulation of gluconeogenesis combined action of
glucagon and epinephrine. Rapid breathing is
physiological response to metabolic acidosis.
Alcohol Excess and Hypoglycemia 2
72. Maintaining near normal blood glucose levels
prevents the development of late complications of
diabetes. A recently completed landmark clinical
study, the Diabetes Control Complications trial , has
shown that the development of late complications of
diabetes in type I diabetes is related to long term
glycemia. This study has also shown that in patients
who have complications, good control of glycemic
delays further development of retinopathy,
nephropathy and neuropathy. Similar results were
obtained for type 2 diabetic patients during the UK
Prospective Diabetes Study completed in 1998. Thus
the aim of treatment of diabetes should be the
achievement of blood levels as close to normal as
possible, without precipitating hypoglycemia.
The importance of good glycemic control
73. The entry of glucose into brain, peripheral nerve
tissue, kidney, intestine, lens and RBC does not
depend on insulin action.
During hyperglycemia, the intracellular level of
glucose in these cells is high, which promotes the
non- enzymatic attachment of glucose to protein
molecules.
Glycation involves glucose and alpha- amino terminal
amino acid and ε- amino groups of lysine residues.
Hemoglobin, albumin and collagen become glycated
which effects their function.
For instance, glycation of apolipoprotein B slows
down the rate of receptor dependent metabolism of
LDL.
Protein modification by glucose
74. The measurement of blood glucose remains the most
important laboratory test in diabetes.
As erythrocytes age, there is gradual conversion of a
fraction of native hemoglobin ( HbA) to its glycated
form, HbA1C, so that an older red cell, a greater
fraction of HbA exist as HbA1C.
HbA1C concentration in blood reflects the time –
averaged level of glycemia over the 3-6 weeks
preceding the measurement.
The normal concentration of HbA1C is 4-6% of HbA.
Levels below 7% indicate acceptable control of
diabetes. Higher levels suggest poor control.
Glycated Hemoglobin (HbA1C)
76. Glucose can be reduced to sorbitol by the action of aldose
reductase. Sorbitol is further oxidized by sorbitol
dehydrogenase to fructose. Since aldose reductase has a
high Km for glucose, the pathway is not very active at
normal glucose level.
In hyperglycemia, however, glucose levels in insulin
independent tissues, such as the RBC, nerve and lens,
increase and consequently there is an increase in the activity
of the polyol pathway with depletion of NADPH.
Like glucose, sorbitol exerts an osmotic effect. This is
thought to play a role in the development of diabetic
cataracts.
In addition, the high level of sorbitol decreases cellular
uptake of another alcohol, myoinositol, which in turn causes
a decrease in the activity of membrane Na+/ K+ ATPase. This
in turn effects nerve function and, along with hypoxia and
reduced nerve blood, contributes to the development of
diabetic neuropathy. Drugs inhibiting aldose reductase
improve nerve function in diabetes.
The Polyol Pathway
80. 80
Biomedical Importance of HMP Shunt
• Various tissues can utilize glucose readily by this shunt when
anaerobic glycolysis is blocked by specific inhibitors as “ lodo-
acetic acid”
• Provides NADPH which is required for various reductive
synthesis in metabolic pathways i.e. Fatty Acids, steroids &
cholesterol.
• Provides pentoses required for nucleic acids (RNA & DNA)
synthesis & nucleotides.
• Deficiency of G6PD enzyme leads to hemolytic anemia, which
has great clinical importance.
• Not meant for energy, but multicyclic process in which 3
molecules of Gl-6-P enter, producing 3 moles of CO2 & 3 moles
of 5-C residues which rearrange to give 2 moles of Fr.6.p & one
mole of Glyceraldehyde -3-P.
81. 81
Difference of EM Pathway and HMP Shunt
HMP Pathway
EM Pathway
1. Occurs in certain spec. tissue
2. Multicyclic process
3. NADPH2 is produced
4. Not meant for energy (ATP)
5. CO2 is produced
1. Occurs almost in all tissues
2. Not a multicycle process
3. NADH2 is produced
4. ATP is required and produced
5. CO2 is never formed
82. The pentose phosphate pathway protects erythrosytes
against hemolysis by asissting the enzymes:
A. Superoxide dismutase
B. Catalase
C. Glutathionic peroxidase
D. Cytochrome oxidase
Deficiency of glucose-6-phosphate dehydrogenase
causes:
A. Cataract
B. Hemolytic anemia
C. Hypoglycemia
D. Mental retardation
MCQ’s
83. Transketolase activity is decreased in the
deficiency of:
A. Thiamine pyrophosphate (TTP)
B. Nicotinamide adenine dinucleotide
C. Flavin adenine dinucleotide
D. Pyridoxal phosphate
Glucose-6-phosphate dehydrogenase enzyme:
A. Is a member of glycolytic pathway
B. Is necessary for production of NADPH
C. Requires the co-enzyme ATP
D. Is inhibited by insulin
84. Transketolase reaction:
A. Takes place between xylulose phosphate and ribose
phosphate
B. Occurs between glyceraldehde phosphate and
fructose phosphate
C. Needs pyridoxal phosphate as co-enzyme
D. Occurs between glyceraldehyde phosphate and
sedoheptulose phosphate
Hemolytic episode after administration of antimalarial
drug is due to the deficiency of enzyme:
A. 6-phosphogluconate dehydrogenase
B. Glucose-6-phosphate dehydrogenase
C. Glucose-6-phosphatase
D. Hexokinase
85. Comments: A number of drugs, particularly primaquine
and related antimalarials, undergo reactions in the cell,
producing large quantities of superoxide and H2O2.
Superoxide dismutase converts superoxide into H2O2,
which is inactivated by glutathione peroxidase using
NADPH.
Glucose 6 phosphate dehydrogenase deficiency
86. Some persons have genetic defect in G6PD,
typically yielding an unstable enzyme that has a
shorter life in the RBC. Therefore insufficient
production of NADPH under stress, the cell
ability to recycle GSSG to GSH is impaired and
drug induced oxidative stress leads to lysis of
RBC’s and hemolytic anemia. If the hemolysis is
severe enough Hb spills over into the urine,
resulting in hematuria and dark colored urine.
Older cells, which can’t synthesize and replace
their enzyme are therefore particularly affected.
Genetically the deficiency is X- linked. Favism is
associated with G6PD deficiency.