CARBOHYDRATE METABOLISM By DR. MARYJANE. TRANSPORT OF GLUCOSE Glucose cannot diffuse directly into...
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CARBOHYDRATE METABOLISM By DR. MARYJANE. TRANSPORT OF GLUCOSE Glucose cannot diffuse directly into cells, but enters by either a Na⁺-independent facilitated
TRANSPORT OF GLUCOSE Glucose cannot diffuse directly into
cells, but enters by either a Na-independent facilitated diffusion
transport system OR a Na-monosaccharide co-transporter system. They
are designated GLUT-1 to GLUT-14 (glucose transporter 1 to 14)via
the Na-independent facilitated diffusion transport system. GLUT-1:
abundant in erythrocytes and brain GLUT-2: liver, kidney and cells
of the pancreas GLUT-3: in neurons GLUT-4: adipose tissue and
skeletal muscle GLUT-5: small intestine and testes, it is the
primary transporter of fructose. GLUT-7: liver
Slide 3
OVERVIEW OF GLYCOLYTIC PATHWAY Also known as embden-meyerhof
pathway. SITE: cytoplasm
Slide 4
ALL CELLS CARRY OUT GLYCOLYSIS Glycolysis is the ONLY source of
ATPs in: Cornea and lens of the eye Renal medulla RBCs Skin
Cancerous cells.
Slide 5
OVERVIEW OF GLYCOLYSIS The glycolytic pathway is employed by
all tissues for the breakdown of glucose to provide energy (in the
form of ATP) and intermediates for other metabolic pathways.
Slide 6
Two types of Glycolysis: A.Aerobic Glycolysis : formation of
Pyruvate as end product with production of ATP and NADH when oxygen
is available B.Anaerobic Glycolysis : formation of lactate as end
product with production of only ATP in the absence of oxygen.
Allows continuous production of ATPs in cells without mitochondria
or cells deprived of oxygen
Slide 7
REACTIONS OF GLYCOLYSIS a. energy investment phase b. energy
production phase
Slide 8
Energy investment phaseEnergy production phase Glycolysis
Slide 9
Glucose Glucose -6-PO4 Fructose -6-PO4 Fructose
-1,6-bisphosphate Glucokinase /Hexokinase Phosphofructokinase-1 ATP
ADP ATP ADP Energy consuming phase Irreversible step -1
Irreversible step -2 Rate limiting step Phosphohexose isomerase
Reversible but driven forward because of a low concentration of
F6P, which is constantly consumed during the next step of
glycolysis.
Slide 10
Glycolysis Splitting phase into molecules of 3 carbons each
Fructose -1,6-bisphosphate Glyceraldehyde-3-PO4 Dihydroxyacetone
phosphate Aldolase 6C 3C Isomerase Glycerol -3-po4 Glycerol -3-po4
dehydrogenase Fatty acid synthesis
Slide 11
Energy yielding phase Glyceraldehyde-3-PO4 1,3 bis
phosphoglycerate NAD NADH Glyceraldehyde-3-PO4 dehydrogenase
3-phosphoglycerate 2-phosphoglycerate Phosphoenolpyruvate Pyruvate
ADP ATP Pyruvate Kinase Irreversible step -3 Pathway repeats twice
because of 2 molecules of Glyceraldehye 3-PO4 formed ADP ATP
Phosphoglycerate kinase Enolase (-) Fluoride Substrate level
phosphorylation Phosphoglycerate mutase
Slide 12
Glycolysis in Erythrocytes: 1,3 Bis phosphoglycerate
3-phosphoglycerate 2,3 Bis phosphoglycerate (2,3BPG) 2,3 Bis
phosphoglycerate (2,3BPG) Mutase Phosphatase Phosphoglycerate
kinase ADP ATP Net ATP production during production of 2,3 BPG in
RBCs = 0 ATPs Increase in 2,3 BPG shifts the oxygen dissociation
curve to the right
Slide 13
Difference between Hexokinase and Glucokinase
HexokinaseGlucokinase Substrate specificity All hexosesMainly
Glucose Km Low (high affinity) Works at normal glucose
concentration High (low affinity) works only when glucose levels
are elevated LocationUniversal Mainly liver and Beta cells of
pancreas Vmax (rate of reaction)LowHigh Glucose-6-PO4 (Allosteric
inhibition)Inhibits the enzymeNo inhibition InsulinNo
regulationPositive regulation
Slide 14
The irreversible phosphorylation reaction involving
fructose-6-phosphate fructose- 1,6-bisphosphate catalyzed by
phosphofructokinase (PFK) is the rate limiting step of
glycolysis
Slide 15
SUBSTRATE LEVEL PHOSPHORYLATION This means phosphorylation of
ADP to form ATP. In glycolysis, there are 2 examples: 1,3
biphosphoglycerate 3-phosphoglycerate Phosphoenolpyruvate
pyruvate
Slide 16
REVERSIBILTY OF GLYCOLYSIS Reversible reaction means that a
same enzyme can catalyze the reaction in both directions. In
glycolysis, all reactions except 3 are reversible.
Slide 17
SHUTTLES Two types: Malate shuttle: cytoplasmic NADH oxidized
using this shuttle produces a mitochondrial NADH and yields
2.5ATPs. Glycerophosphate shuttle: cytoplasmic NADH oxidized by
this shuttle produces a mitochondrial FADH and yields 1.5ATPs.
Slide 18
ENERGY GAIN OF GLYCOLYSIS Energy gain of glycolysis = ATP
produced ATP lost. In the absence of O: ATP produced; 2 ATP from
1,3 biphosphoglycerate 2 ATP from phosphoenolpyruvate Total ATP
produced: 4 ATP lost: 1 ATP from glucose to glucose-6-phosphate 1
ATP from fructose-6-phosphate to fructose-1,6- biphosphate. Total
ATP lost =2 Net result = 4 2 = 2 ATP
Slide 19
In the presence of O: ATP produced; 2 ATP from
1,3-biphosphoglycerate 2 ATP from phosphoenolpyruvate 5 or 3 ATP
from oxidation of 2NADH + H Total ATP produced: 7 or 9 ATPs ATP
lost: 1 ATP from glucose to glucose-6-phosphate 1 ATP from
fructose-6-phosphate to fructose-1,6- biphosphate Total ATP lost: 2
ATPs Net result: 9 2 = 7 ATPs 7 2 = 5 ATPs
Slide 20
ALLOSTERIC REGULATION OF GLYCOLYSIS a) ATP and AMP: AMP
activates phosphofructokinase enzyme while ATP inhibits both
phosphofructokinase 1 b) glucose-6-phosphate: inhibits hexokinase
c) citrate: inhibits phosphofructokinase 1 d)
fructose-1,6-bisphosphate: activates pyruvate kinase
Slide 21
Hormonal regulation
Slide 22
Insulin: activates glucokinase, phosphofructokinase-1 and
pyruvate kinase Glucagon: inhibits glucokinase,
phosphofructokinase-1 and pyruvate kinase
Slide 23
IMPORTANCE OF LACTATE PRODUCTION In the absence of O, lactate
is the end product of glycolysis. This reaction reoxidizes NADH + H
into NAD. This helps in the continuity of glycolysis as the
generated NAD will be used in the reaction glyceraldehyde-3-P to
1,3 diphosphoglycerate once more which helps with the continued
production of ATP in tissues that lack mitochondria or those
deprived of sufficient oxygen.
Slide 24
LACTIC ACIDOSIS Refers to elevated concentrations of lactate in
the plasma. CAUSES: 1. Physiological: severe muscular exercises. 2.
pathological: in cases of anoxia (absence of oxygen in the blood)
e.g., pulmonary embolism, shock, hemorrhage, Lactic acidosis can
result in death from coma.
Slide 25
CLINICAL SIGNIFICANCE OF PYRUVATE KINASE DEFICIENCY It is the 2
nd most common genetic deficiency that causes hemolytic anemia.
G-6PDH deficiency is the most common genetic deficiency of
hemolytic anemia It is Autosomal recessive Absence of Heinz
bodies
Slide 26
Diabetes Mellitus : Insulin dependent Diabetes Mellitus (IDDM)
def of insulin due to autoantibodies against Beta cells Non insulin
dependent Diabetes mellitus (NIDDM) insulin receptor resistance
Maturity onset diabetes of the young (MODY) mutation in the
Glucokinase gene.
Slide 27
IMPORTANCE OF GLYCOLYSIS 1. ENERGY PRODUCTION: it is the only
source of energy to the contracting muscles during muscular
exercise due to lack of O and to the RBCs, kidneys, cornea, lens
and testes due to the absence of mitochondria.
Slide 28
ALTERNATE FATES OF PYRUVATE Oxidative decarboxylation of
pyruvate: is by pyruvate dehydrogenase which converts pyruvate, the
end product of glycolysis into acetyl CoA, a major fuel for
tricarboxylic acid cycle and building block for fatty acid
synthesis. Carboxylation of pyruvate to oxaloacetate: is by
pyruvate carboxylase is a biotin-dependent reaction, which
replenishes the tricarboxylic acid cycle intermediates and provides
substrate for gluconeogenesis
Slide 29
Various fates of Pyruvate:
Slide 30
Slide 31
Under conditions of anaerobic glycolysis, the NAD+ required by
glyceraldehyde-3- phosphate dehydrogenase is supplied by a reaction
catalyzed by which of the following enzymes? Glycerol-3-phosphate
dehydrogenase Alpha-ketoglutarate dehydrogenase Lactate
dehydrogenase Malate dehydrogenase PDH
Slide 32
SUMMARY OF GLYCOLYSIS The rate limiting step of glycolysis The
transporter of glucose in various tissues The substrate level
phosphorylation reactions The energy gain of glycolysis The
irreversible reactions of glycolysis and the enzymes that catalyze
those reactions The rate limiting enzyme of glycolysis Regulations
of glycolysis Effects of deficiency of certain glycolytic enzymes
Fates of pyruvate End product of glycolysis (aerobic &
anaerobic) Tissues that depend on aerobic as well as anaerobic
glycolysis Importance of lactate and effect of excess
Slide 33
MITOCHONDRIA PATHWAY FOR GLUCOSE OXIDATION. Complete oxidation
of glucose molecule occurs in both cytoplasm (glycolysis) and
mitochondria (krebs cycle). Pyruvate is transported into the
mitochondria by a transporter where it is converted to acetylCoA
for entry into the krebs cycle. This reaction that converts
pyruvate to acetyl CoA is irreversible.
Slide 34
The oxidation of pyruvate occurs in 2 stages: First stage:
oxidative decarboxylation of pyruvate to acetyl CoA. Second stage:
krebs cycle (citric acid or tricarboxylic acid cycle)
Slide 35
OXIDATIVE DECARBOXYLATION OF PYRUVATE TO ACETYL CoA. Before the
entry of pyruvate in the citric acid cycle, it must be oxidatively
decarboxylated to acetyl CoA and catalyzed by pyruvate
dehydrogenase It utilizes 5 coenzymes: thiamine, lipoic acid,
coenzyme A, FAD and NAD
Slide 36
Inhibition of oxidative decarboxylation of pyruvate: 1.
arsenic: is a poisonous substance that inhibits lipoic acid.
Symptoms: vomiting, rice water stool and garlic scented breath
2.thiamine deficiency: leads to accumulation of pyruvate as seen in
alcoholics in Wernicke- korsakoff syndrome leading to lactic
acidosis
Slide 37
CITRIC ACID CYCLE Also known as krebs cycle or tricarboxylic
acid cycle (TCA). The citric acid cycle is a series of reactions in
mitochondria where acetyl CoA is oxidized into CO, HO and energy(in
the form of ATP).
Slide 38
Slide 39
ENERGETICS OF THE CITRIC ACID CYCLE On oxidation of one
molecule of acetyl CoA in the citric acid cycle, 10 ATP molecules
are produced, but the entry of one acetyl CoA into one round of the
kreb cycle does not lead to the net production or consumption of
intermediates. Therefore for there to be a net production or
consumption of intermediates, acetyl CoA has to enter into the kreb
cycle twice leading to the production of another 10 ATPs giving a
total of 20 ATPs in the entire kreb cycle.
Substrate level phosphorylation : means phosphorylation of GDP
to form GTP. There is only one substrate level phosphorylation
reaction in the kreb cycle Succinyl CoA succinate.
Slide 42
ENERGETICS OF COMPLETE OXIDATION OF ONE MOLECULE OF GLUCOSE
Reaction from glucose to give 2 molecules of pyruvate gives 7 ATPs
or 5 ATPs depending on the shuttle. Acetyl CoA going into the kreb
cycle gives 20 ATPs (i.e., 10 ATPs per molecules of acetyl CoA that
enters into the krebs cycle) The net energetics from glucose to the
entire kreb cycle is 7 OR 5 ATPs + 20 ATPs = 25 or 27ATPs.
Slide 43
REGULATION OF CITRIC ACID CYCLE Citrate synthase: inhibited by
ATP, NADH Isocitrate dehydrogenase: inhibited by ATP and NADH and
stimulated by ADP. NO HORMONAL REGULATION OF CITRIC ACID CYCLE
Slide 44
Aconitase is inhibited by fluoroacetate (non- competitive
inhibition) -ketoglutarate dehydrogenase is inhibited by Arsenite
(non-competitive inhibition of lipoic acid) Succinate dehydrogenase
is inhibited by Malonate (competitive inhibition)
Slide 45
Congenital Lactic acidosis: Deficiency of Pyruvate
Dehydrogenase enzyme. Inability to convert Pyruvate to Acetyl co-A.
Shunted to Lactate Dehydrogenase to form Lactic Acid. Deficient
NADH leading to deficient ATP Lactic acidosis, severe psychomotor
retardation, damage to brain stem, cortex etc.
Slide 46
Type: Reasons: Other causes of lactic acidosis: Mercury
poisoning Arsenic poisoning Pyruvate carboxylase deficiency TPP
deficiency Chronic Alcoholism Binds to SH groups of Lipoic acid and
forms a stable complex. Decreased absorption and poor diet. Severe
exercise excess lactate
Slide 47
Beriberi, Wernickes encephalopathy and Korsakoffs psychosis (WK
syndrome)in Thiamine deficiency is due to failure of TCA cycle (
Pyruvate dehydrogenase and - ketoglutarate dehydrogenase) Symptoms
of wernicke korsakoff syndrome: confabulation, nystagmus
(ophthalmoplegia), ataxia Symptoms of beriberi: dry- peripheral
neuropathy (wrist drop, toe drop) wet- high output cardiac
failure
Slide 48
Congenital deficiency of Pyruvate dehydrogenase Lactic acidosis
and neurodeficit. Treatment: ketogenic diet rich in leucine and
lysine Congenital deficiency of Pyruvate carboxylase OAA is
deficient failure of sparking of TCA severe mental retardation,
lactic acidosis, hypoglycemia TCA cycle enzyme deficiencies are
extremely rare.
Slide 49
Slide 50
IN SUMMARY The total energy gain in the complete oxidation of
glucose Reactions that utilize Tender Loving Care For Nancy The
enzyme and step of substrate level phosphorylation Rate limiting
enzyme of the citric acid cycle
Slide 51
GLUCONEOGENESIS SITE: cytoplasm & mitochondria
Slide 52
Slide 53
Slide 54
gluconeogenesis The major non carbohydrate sources are:
Lactate(from anerobic glycolysis) Glucogenic amino acids (esp
alanine & aspartate) Glycerol (via DHAP)
Glycerol ATP ADP Glycerol Glycerol 3 P This enzyme is absent in
adipose tissue. Glycerol kinase
Slide 57
Conversion of Glycerol to Glucose: Triglycerides Glycerol Fatty
acids Beta oxidation Acetyl Co- A Liver Glycerol 3- PO4
Dihydroxyacetone phosphate Glycerol kinase Glycerol-3-po4
dehydrogenase NAD+ NADH FASTING OR LOW GLUCOSE
Slide 58
CORI CYCLE
Slide 59
Slide 60
Slide 61
The pathway Three nonequilibrium reactions in glycolysis
catalyzed by hexokinase, phosphofructokinase and pyruvate kinase,
prevent simple reversal of glycolysis for glucose synthesis. In
gluconeogenesis, these enzymes have to be bypassed to allow the
reaction to go the other way Four new enzymes are used to bypass
these reactions while the rest of the steps use the same enzyme
just like in glycolysis.
Slide 62
Pyruvate Oxaloacetate Phosphoenol pyruvate ATP GTP ADP GDP
Pyruvate carboxylase Phosphoenolpyruvate carboxykinase Energy
derived from fatty acid oxidation GTP derived from succinate
thiokinase Bypass Step 1: (Mitochondria) (cytosol) CO2 Problem
--Mitochondrial membrane is impermeable to OAA!! USMLE CONCEPT!!!
ABC carboxylase
Slide 63
Oxaloacetate Malate Oxaloacetate Mitochondria cytosol Malate
dehydrogenase NAD NADH NAD NADH
Slide 64
The next few steps are reversal of Glycolysis till Fructose 1,6
bisphosphate is formed.
Bypass Step 3: Conversion to Glucose Glucose Glucose-6-po4
Glucose-6- phosphatase Glucokinase Glycolysis Gluconeogenesis ATP
ADP PO4
Slide 67
Glycolysis and Gluconeogenesis are regulated reciprocally.
Slide 68
Slide 69
Clinical aspects 1. Pyruvate carboxylase deficiency (A.R)- 1 in
25,000 births characterized by Hypoglycemia, lactic
acidosis(metabolic acidosis) and Mental retardation. 2. Fructose
1,6bisphosphatase deficiency lactic acidosis (metabolic acidosis)
and hypoglycemia.Treatment feed high carb. Diets and avoidance of
fasting.
Slide 70
VON GIERKES DISEASE TYPE Ia: due to deficiency of glucose-6-
phosphatase. TYPE Ib: due to deficiency of G-6-Phosphate
translocase. In both cases, the indiv will have a problem making
glucose from G-6-P
Slide 71
SYMPTOMS Affects liver and kidney Hepatomegaly Renomegaly
Fasting hypoglycemia Hyperlacticacidemia, hyperlipidemia,
hyperuricemia Growth retardation Normal glycogen structure.
Treatment: daytime/nocturnal glucose infusion or uncooked
cornstarch.
Slide 72
ALCOHOL DEHYDROGENASE Alcohol Acetaldehyde Acetate Excess NADH
EXCESS LACTATE from PYRUVATE Excess Malate FROM OAA Excess Glycerol
3 P from DHAP No or less Gluconeogenesis!! Hypoglycemia NAD
NADH
Slide 73
Immediately after completing a 25-mile marathon race, a healthy
24-yr old man was extremely dehydrated and thirsty. He quickly
consumed a 6- pack of ice-cold beer and shortly thereafter became
very weak and light-headed and nearly fainted. He complained of
muscle cramping and pain. What is the most probable cause ?
1.Excess lactate in blood 2.Excess Alcohol in blood 3.Excess NADH
4.Dehydration 5.Electrolyte imbalance
Slide 74
GALACTOSE METABOLISM Site: liver, brain and other tissues
Slide 75
Galactose metabolism The major dietary source of galactose is
lactose (milk and milk products) by the enzyme lactase Galactose
can also be gotten from breakdown of glycoproteins and glycolipids.
Entry of galactose into cell is not insulin dependent Galactose and
glucose are C4 epimers.
Slide 76
The enzyme lactase ( -galactosidase) splits dietary lactose
into glucose and galactose.
Slide 77
Galactose Metabolism
Slide 78
UDP-Galactose UDP-galactose is required for biosynthesis of:
Lactose Glycoproteins, Glycolipids Glycosaminoglycans UDP-galactose
can be formed from UDP-glucose by the action of UDP-hexose
4-epimerase in the absence of dietary galactose
Slide 79
Two inherited disorders of galactose metabolism are well-known.
The principal treatment of these disorders is to eliminate lactose
from the diet. Classical galactosemia: Galactose-1-P-
uridyltransferase deficiency Galactokinase deficiency
Non Classical Galactosemia Deficiency of enzyme Galactokinase
Autosomal recessive Less severe or benign compared to classic type.
Early onset of cataract in first few months of life.
Galactilol Liver Damage and Cirrhosis due to accumulation of
Gal-1P Gal-1P gets deposited in Renal tubules
Slide 84
Lactose intolerance Deficiency of Lactase enzyme in the GUT.
Loose stools after consuming milk. Seen mostly with new born or
adults. Unabsorbed lactose enters colon. 1.Broken by bacteria
produce gas 2.Unabsorbed lactose causes osmotic diarrhoea. Stool
acidity test - Intestinal biopsy - Breath test hydrogen and
methane
Slide 85
Functions of Galactose in Body Energy Converted to Glucose
Synthesis of Lactose Synthesis of Glycosaminoglycans Glycoproteins
and Proteoglycans
Slide 86
TREATMENT Eliminate sources of galactose from the diet
Slide 87
FRUCTOSE METABOLISM SITE: liver and kidney
Slide 88
INTRODUCTION Fructose is found in honey and fruit and as part
of the disaccharide sucrose. This sucrose is hydrolyzed by sucrase
resulting in glucose & fructose.
Slide 89
Fructose enters into metabolism either as fructose 6-po4 or
fructose 1-po4. Phosphorylation by Hexokinase or fructokinase
Fructokinase found in liver, kidney and small intestine Hexokinase
in skeletal muscle and most organs
Slide 90
Features of Fructose metabolism Entry of fructose into the
cells is not dependent on insulin. Phosphorylation to fructose -1-
phosphate by enzyme fructokinase in liver. 1.Is not dependent on
amount of fructose in plasma 2.Is not dependent on insulin. In
extra hepatic tissues: glucose competes with fructose for
hexokinase.
Slide 91
Fructose metabolism Muscle which contains only hexokinase
phosphorylates fructose to F6P which is a direct glycolytic
intermediate. Hepatic fructose is phosphorylated on C-1 by
fructokinase yielding fructose-1-phosphate.
Slide 92
Aldolase A and B Aldolase B is present in liver, kidney and
small intestine converts fructose 1-P into DHAP and glyceraldehyde.
Aldolase A is a glycolytic enzyme in all other tissues.
Slide 93
Kinetics of fructose metabolism Rate of fructose metabolism
>> rate of glycolysis Mainly because the trioses (DHAP and
Glyceraldehyde)formed from fructose-1- phosphate bypasses PFK-1 the
rate limiting step of glycolysis. PFK-1 step slows metabolism
because of its regulation
Slide 94
Role of FRUCTOSE in body PROVIDES ENERGY SEMINAL PLASMA ENERGY
REQUIRED FOR MOBILITY OF SPERMATOZOA Secreted by Seminal
Vesicle
Slide 95
Fructose Fructose-1-Po4 Glyceraldehyde Dihydroxyacetone
phosphate Fructokinase def Essential Fructosuria Aldolase B def
Hereditary Fructose intolerance ATP ADP
Slide 96
Fructose Metabolism
Slide 97
Fructokinase deficiency: Autosomal recessive benign condition
Excretion of fructose in urine [ no other abnormality Treatment
Avoid fructose.
Slide 98
Hereditary Fructose intolerance Deficiency of aldolase B
Accumulation of fructose-1- phosphate Deficiency of phosphates in
cells. Liver failure Hypoglycemia Hyperuricemia Liver failure
glycogen accumulation. Hyperuricemia
Slide 99
Aldose reductase lens, retina, kidney cells, Schwann cells,
placenta, cells of ovaries and seminal vesicles. Sorbitol
dehydrogenase: liver, ovaries, sperm and seminal vesicles Aldose
reductase Sorbitol dehydrogenase No sorbitol dehydrogenase
Slide 100
Compliations due to increased glucose: Hyperglycemia (as in
diabetes) results in elevated levels of intracellular glucose in
lens, nerve, kidney. This leads to water retention in these tissues
due to osmotic effects of sorbitol swelling, cataract, peripheral
neuropathy and vascular problems nephropathy and retinopathy as
complications of diabetes
Slide 101
Slide 102
TREATMENT: Symptoms are reversed by after removing sucrose and
fructose from diet.
