Carbohydrate metabolism.ppt

30 May 2023 Dr. Mohamed Z Gad 1
Dr. Mohamed Z Gad 1
Clinical Biochemistry 6th semester Spring 2016
Week Theoretical Lecturer Tutorial Practical
1 [28 Jan] CHO metabolism Prof. Gad --- ----
2 [31 Jan- 4 Feb] CHO metabolism Prof. Gad/Dr. Ingy Glycolysis •Creatinine and urea assay
3 [7-11 Feb] CHO metabolism Prof. Gad/Dr. Ingy Aerobic metabolic and
explanation of student
presentations
Urine analysis
4 [14-18 Feb] CHO metabolism Prof. Gad/Dr. Ingy glycogen & gluconeogenesis •Fragility test
•Effect of different of haemolytic agents
•Determination of RBCs count
• First quiz: Will be informed
5 [21-25 Feb] Disorders of CHO metab. Prof. Gad/Dr. Ingy HMP shunt and Cells adapted
to function
Hb assay, ESR & hematocrit
6 [28 Feb- 3 Mar] Disorders of CHO metab. Prof. Gad/Dr . Ingy Clinical cases on diabetes Oral glucose tolerance test
Midterm exams [5-14 Mar]
7 [12-17 Mar] Oxidative phosphorylation
(Compensation)
Prof. Hans --------- Glycated Hb
8 [20-24 Mar] Lipid metabolism Dr. Sahar Q & A on oxid. phosphorylation Extraction of PC from egg yolk
Extraction of cholesterol from egg yolk
Performing solubility tests on different lipids
(compensation on Sat 26 Mar)
9 [27-31 Mar]
(27 Mar off)
Lipid metabolism
(compensation)
Dr. Sahar Student Presentation
(compensation on Sat 26 Mar)
Determination of TG, Determination of
cholesterol and HDL –C (Combined lab)
10 [3-7 Apr] Lipid metabolism Dr. Sahar Student Presentation • Revision
• Second quiz: Will be informed
11 [10-14 Apr] Disorders of lipid metab. Dr. Sahar Lipid metabolism  Practical exam days
12 [17-21Apr] Integration of metabolism Prof. Hans Clinical cases on lipid disorders  Practical exam days

Email: mohamed.gad@guc.edu.eg
Day off: Mostly Thursday

A Map of The Major Metabolic
Pathways in A Typical Cell
The Diagram was taken from “Biochemistry” by Voet & Voet (2nd edition, 1995, John Wiley &
Sons, pg. 413)

Carbohydrate
Metabolism
(Glycolysis)
Lecture 1 PHBC621

Learning Objectives
By the end of this session you should be able to:
• Outline the three stages of glycolysis.
• Describe the steps of glycolysis between glucose and
pyruvate and recognize all the intermediates and
enzymes and the cofactors that participate in the
reactions.
• Mention ATP-generating reactions.
• Illustrate the regulation of glycolysis.

Major Pathways of CHO Metabolism
CHO metabolism in mammalian cells can be
classified into:
1. Glycolysis: Oxidation of glucose to
pyruvate (aerobic state) or lactate
(anaerobic state)
2. Krebs cycle: After oxidation of pyruvate
to acetyl CoA, acetyl CoA enters the Krebs
cycle for the aim of production of ATP.
3. Hexose monophosphate shunt: Enables
cells to produce ribose-5-phosphate and
NADPH.
4. Glycogenesis: Synthesis of glycogen from
glucose, when glucose levels are high
5. Glycogenolysis: Degradation of glycogen
to glucose when glucose in short supply.
6. Gluconeogenesis: Formation of glucose
from noncarbohydrate sources.
Glucose is the major fuel of most
organisms. The major pathways of CHO
metabolism either begin or end with
glucose.

Glycolysis (Embden-Meyerhof Pathway)
[glycolysis: from the Greek glyk-, sweet, and lysis, splitting]
Glycolysis occurs in all human cells. Glycolysis is believed to be
among the oldest of all the biochemical pathways.
Aerobic: Glucose  Pyruvate
Anaerobic: Glucose  Lactate (or ethanol & acetic
acid)
Glycolysis (10 reactions in 3 stages, all in cytoplasm)
1) Priming stage: D-Glucose + 2ATP  D-fructose 1,6-biphosphate + 2ADP + 2H+
2) Splitting stage : D-Fructose 1,6-biphosphate  2 D-Glyceraldehyde 3-phosphate
3) Oxidoreduction – Phosphorylation stage:
2 D-Glyceraldehyde 3-phosphate + 4ADP + 2Pi + 2H+  2Lactate + 4ATP
-----------------------------------------------------------------------------
Sum:
Glucose + 2ADP + 2Pi ----- 2 Lactate + 2ATP + 2H2O (Anaerobic)
Glucose + 2ADP + 2Pi + 2NAD+ ---- 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H2O
(Aerobic)

Common Abbreviations & Alternative Names

1) Priming Stage
• Glucose (and other hexoses) are
phosphorylated immediately upon
entry into the cell. Phosphorylation
prevents transport of glucose out of
the cell and increases the reactivity
of oxygen in the resulting phosphate
ester.
• Several isoenzymes of hexokinase
with different Km values for glucose
are located in different tissues. Brain
hexokinase has a particularly low
Km for glucose.
• The major enzyme for
phosphorylating glucose in liver is
glucokinase.
• Steps catalyzed by hexokinase &
PFK-1 are irreversible.

Differences between Glucokinase & Hexokinase
Hexokinase Glucokinase
 Present in all tissues  Liver only
 Low Km for glucose (<0.1
mM)
 Higher Km for glucose
 Strongly inhibited by G6P  Not inhibited by G6P
 Non-inducible enzyme, not
affected by diabetes or
insulin
 Inducible, synthesis induced by
insulin & repressed in diabetes
 Level of enzyme is not
affected by fasting or high
CHO diet
 Depends on glucose concentration
 Act on glucose, fructose and
galactose
 Glucose only

2) Splitting Stage
• The reaction catalysed by aldolase is
the reverse of aldol condensation.
• Although the cleavage of F1,6BP is
energetically unfavourable, rapid
removal of the product drives the
reaction forward.
• Of the two products of the aldolase
reaction, only GAP (or G3P) serves as a
substrate for the next reaction in
glycolysis.
• To prevent the loss of the other three-
carbon unit, triose phosphate isomerase
catalyses the interconversion of DHAP
& G3P. Because of this reaction, the
original molecule of glucose has now
been converted to two molecules of
G3P.

3) Oxidoreduction –
Phosphorylation Stage
• G-3-P dehydrogenase is a tetramer,
each subunit contains 1 binding site
for G3P & another for NAD+ (NAD+
is permanently bound to the enzyme).
• G3P  1,3-BPG  3PG and PEP 
Pyruvate are examples of “substrate–
level” phosphorylation.
• 3-PG  2-PG is mediated by an
intermediate [2,3-BPG]. Most cells
have low amounts of 2,3-BPG except
in RBCs, which act as allosteric
modifier of Hb-O2 binding.
• PEP  Pyruvate is an “irreversible
reaction” due to free energy loss
associated with tautomerization of the
enol to the more stable keto form.

ADP is converted to ATP by the direct
transfer of a phosphoryl group from a high
energy compound.
Study Question ?????
What is meant by “substrate–level
phosphorylation” ?

Why glycolysis under anaerobic
conditions proceed to lactate and
not just stop at pyruvate formation ?
The reaction of lactate dehydrogenase is essential in
anaerobic glycolysis , as it is the mean for reoxidizing
NADH formed in the G-3-P dehydrogenase step to re-
enter into the glycolysis cycle. In aerobic glycolysis
reoxidation takes place in mitochondria by the respiratory
chain.

Overall pathway of Glycolysis & ATP
Formation
Number of ATP generated from
glycolysis
Anaerobic
Aerobic
Enzyme
-1 ATP
-1 ATP
Hexokinase
-1 ATP
-1 ATP
PFK-1
----
+6 ATP
G-3-P
dehydrog.
+2 ATP
+2 ATP
Phospho-
glycerate
kinase
+2 ATP
+2 ATP
Pyruvate
kinase
+2 ATP
+8 ATP
Sum

Louis Pasteur, the great 19th century French chemist and
microbiologist, was the first scientist to observe the
following phenomenon. “Cells that can oxidize glucose
completely to CO2 and H2O utilize glucose more rapidly
in the absence of O2 than in its presence”. It would
appear that O2 inhibits glucose consumption. Thus,
another definition for Pasteur effect is: inhibition of
glucose utilization and lactate accumulation by the
initiation of respiration (O2 consumption)… Can you
explain why ?
Louis Pasteur
(1822 - 1895)
The reason behind this phenomenon is that complete oxidation of glucose under
aerobic conditions yield much more ATP (~38 ATP) than anaerobic glycolysis
(~2ATP). Thus it is anticipated that the rate of glucose consumption will be 19-
20 times faster under anaerobic condition to meet the metabolic demand in a
way equivalent to aerobic conditions.

Regulation of
Glycolysis
Rate of glycolysis is controlled primarily by
allosteric regulation of the 3 key enzymes
(irreversible steps), hexokinase, PFK-1, and
pyruvate kinase.
Inhibitor
Activator
Enzyme
G-6-P
AMP, ADP, Pi
Hexokinase
NADH, H+,
citrate, ATP
F-6-P, AMP,
F-2,6-DP
(liver only)
PFK-1
ATP, acetyl
CoA,
phosphorylation
AMP, F-1,6-
DP
Pyruvate
kinase
• PFK-1 is the major regulatory
enzyme of glycolysis. In the liver
only, PFK-1 is activated by fructose-
2,6-diphosphate (F-2,6-DP).
• PFK-2, the enzyme that synthesize
the activator F-2,6-DP, is itself a
regulatory enzyme. It is inhibited by
citrate & ATP and by
phosphorylation. The reverse reaction
is catalyzed by fructose-2,6-
diphosphatase(F-2,6-DPase).
• Hormones also regulate glycolysis
e.g., glucagon inhibits glycolysis by
repressing the synthesis of F-2,6-DP.
Insulin promotes glycolysis by
stimulating the synthesis of F-2,6-DP.

What effects do fluoride and magnesium
have on glycolysis ?

•
Comments on Glycolysis
 Glycolysis is the only pathway that produce ATP in absence of O2.
 The best known inhibitors of the glycolytic pathway include:
2-Deoxyglucose: causes inhibition of hexokinase.
Sulfhydryl reagents (e.g. Hg-compounds and alkylating agents as
iodoacetate); inhibit glyceraldehydes-3-phosphate dehydrogenase
which has cysteine residue in the active site.
Fluoride: a potent inhibitor of enolase. Thus, fluoride is usually added
to blood samples to inhibit glycolysis before estimation of blood
glucose.
 Magnesium: required for kinase reactions by forming Mg-ATP
complex.
 Accumulation of lactate is responsible for muscle fatigue and cramps
observed under heavy exercise (anaerobic glycolysis).
 In RBCs, glycolysis is the major source of ATP since RBCs lack
mitochondrial oxidation.

Animation of Glycolysis
glycolysis 5.swf

‫كلمات‬
‫باقية‬
‫للعظيم‬
‫قلبان‬
:
‫قلب‬
‫يتألم‬
…
‫و‬
…
‫قلب‬
‫يتأمل‬
.
‫جبران‬
‫خليل‬
‫جبران‬

References for Further Readings:
Latest editions of:
•Biochemistry by L. Stryer, Freeman & Company New York ….
•Harper’s Biochemistry by R.K. Murray, D.K. Granner, P.A. Mayes &
V.W. Rodwell. Biochemistry. Appleton & Lange, New York, Connecticut,
California.
•Biochemistry by T. Mckee & J. Mckee, Wm. C. Brown Publishers,
London, Madrid, Tokyo ….
•Biochemistry by I.D.K. Halkerston, Second Edition, John Wiley & Sons
Inc., Williams E Wilkins, USA.
• Textbook of Biochemistry With clinical Correlations by T.M. Devlin,
Wiley-Liss Publication, New York, Toronto ….
•Biochemistry Illustrated by P.N. Campbell & A.D. Smith, Longman
Group UK Ltd, UK.
•Principles Of Biochemistry by G.L. Zubay, W.W. Parson & D.E. Vance,
Wm. C. Brown Publishers, Iowa, Melbourne, Ox for d.

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