1
BI25M1
CARBOHYDRATES
AND
INTERMEDIARY
METABOLISM
LECTURE 1
AIM:
to review carbohydrate structures
and functions.
[Lehninger
Instant Notes
Chapters 7, 14, 15
Section J]
2
FUNCTIONS
‘Carbohydrates’ includes molecules that:
1 are highly oxidizable,
e.g.
sugars,
starch.
They have many ‘high energy’ H atomassociated electrons,
[Energy Transformations Lecture 3]
and are major energy-providing food
materials.
Carbohydrate catabolism is thus a
major metabolic process for most
organisms.
2 function to store potential energy,
e.g.
starch
in plants,
glycogen in animals.
3
3 have
structural
functions,
and
protective
e.g. in cell walls
of plants
pericellular matrices of animals.
4 contribute to cell-cell communication.
The formula of simple carbohydrates is
(CH2O) n
hence the name.
There is much (complicated) nomenclature
attached to their structures.
A note of some (for reference only) is in the
Hand-out, pp.106-108.
4
MONOSACCHARIDES
There are 3 important hexoses (6-C sugars)
in human biochemistry:
glucose
(Glc)
galactose (Gal)
fructose
(Fru)
[For structures, see Hand-out, p.107 Figs. 5, 7, 8.]
5
DISACCHARIDES
Monomers are linked by glycosidic bonds
(elimination of water).
The 3 important disaccharides in human
biochemistry are:
1 Lactose
main sugar of milk.
[For structure, see Hand-out, p.108]
Gal  1
4 Glc
6
2 Sucrose
common (table) sugar;
only made by plants;
may be 25% of all dietary carbohydrate
in man;
sweetener in most processed food.
[For structure, see Hand-out, p.108]
Glc  1 2 Fru
7
3 Maltose
not much in diet directly,
but it’s a break-down product of
starch;
in beer (from starch in barley);
in ‘malt extract’ baby foods
(Roo’s strengthening medicine).
[For structure, see Hand-out, p.108]
8
9
POLYSACCHARIDES
1 Starch
storage in plants;
major food material in man;
consists of 2 components:
(i)
amylose
~20% of total;
103-106 Glc, linked  1
4;
forms a helix.
(I2 inserts in centre, and gives the
‘starch test’.)
10
See the structure of starch (amylose) in
Lehninger, Edition 4 p.252, Fig.7-21(a,b)
Edition 5 p.249, Fig.7-20(a,b)
11
(ii)
amylopectin
~80% of total;
same structure as amylose,
but with
1
6 branches every ~25 monomers;
~106 monomers.
12
So the structure looks like this:
lots of
‘non-reducing ends’
just one
‘reducing end’
[See Hand-out, p.106 note 8 for ‘reducing’ sugars.]
In 3D, a compact, globular structure is
formed,
rather like a KOOSH ball.
13
2 Glycogen
storage function in animals
(‘animal starch’);
90% is in
liver (acts to replenish blood Glc)
skeletal muscle (its catabolism
produces ATP for contraction);
same structure as amylopectin,
but has branches every ~ 10 monomers.
14
Why do organisms store starch and
glycogen,
(rather than the equivalent number of free
Glc molecules)?
1 Compactness.
2 Amylopectin and glycogen have many
non-reducing ends;
it is to/from these that monomers are
added/removed,
so fast synthesis/degradation is possible.
15
3 The polymers form hydrated gels
and are not really ‘in solution’.
This means that they are osmotically
inactive.
If free Glc were stored in cells, then
[Glc]in would be > > [Glc]out ,
and either Glc would move out,
or the cell would use a lot of energy
keeping it in.
16
GLYCOPROTEINS
= proteins to which carbohydrate is
covalently attached.
includes most extracellular, eukaryotic
proteins;
the carbohydrate content varies
(1-80% by weight)
and usually consists of
2- to 10-monomer-long chains.
The carbohydrate part may:
1 increase solubility of the protein;
2 influence how it folds when it’s being
made;
3 protect it from degradation.
17
Glycoproteins also function in cell-cell
communication:
isolated normal cells stop growing when
they touch each other:
‘contact inhibition’.
Isolated cancer cells continue to grow
(mirroring their behaviour in the body).
Normal and cancer cells have different
glycoproteins on their surfaces:
they must be involved in the cell-cell
recognition process.
18
GLYCOSAMINOGLYCANS (GAGs)
and PROTEOGLYCANS
GAGs:
old name: mucopolysaccharides;
polymers in which, usually, monomers of
a hexuronic acid and an aminosugar
alternate;
[See Hand-out, p.106 note 10].
Proteoglycans:
molecules formed when GAGs
covalently attached to protein.
are
[Note: glycoproteins have lower carbohydrate
content than proteoglycans, and the carbohydrate
part doesn’t have the alternating structure noted
above.]
19
[For reference only, the 6 major GAGs are shown
in the Hand-out, p.108.]
Note:
names;
heparin, the
anticoagulant.
widely-used
Many proteoglycans
connective tissues,
are
clinical
found
in
forming a gel-like ‘ground substance’,
which holds cells of a tissue together,
and in which are embedded fibrous
proteins: collagen, elastin.
GAGs are turned over by being taken into
cells and degraded in lysosomes.
Mucopolysaccharidoses
are genetic disorders, of varying clinical
severity, in which one or other of the
degradative enzymes does not function.
20
BI25M1
CARBOHYDRATES
AND
INTERMEDIARY
METABOLISM
LECTURE 2
AIMS:
to review:
carbohydrate digestion and
absorption;
the initial fate of absorbed Glc.
[Lehninger
Instant Notes
Chapters 7, 14, 15
Section J]
21
Look back at Lecture 1:
[Carbohydrates
are highly oxidizable,
e.g.
sugars,
starch.
They have many ‘high energy’ H atomassociated electrons,
and are major energy-providing food
materials.
Carbohydrate catabolism is thus a major
metabolic process in many organisms.]
22
CARBOHYDRATES IN OUR DIET
1 starch
cereals, potatoes, rice.
2 glycogen
meat (but post-mortem
enzyme activity degrades
much).
3 cellulose and hemicelluloses
plant cell walls;
not digested.
4 oligosaccharides containing
 1 6 -linked Gal
peas, beans, lentils;
not digested.
5 lactose
sucrose
maltose
6 Glc
Fru
fruit, honey.
23
DIGESTION OF CARBOHYDRATES
This is hydrolytic:
H-OH
-X-O-Y-
-X - OH + HO - Y -
catalysed by glycosidases.
In the mouth:
salivary amylase breaks
 1 4 bonds in starch.
In the stomach:
no carbohydrate digestion.
In the duodenum:
pancreatic amylase works
as above.
24
In the jejunum:
final digestion
by mucosal cell-surface
enzymes:
isomaltase
breaks  1
6 bonds;
glucoamylase removes Glc sequentially
from non-reducing ends;
sucrase
hydrolyses sucrose;
lactase
hydrolyses lactose.
The main products are:
Glc,
Gal,
Fru.
25
ABSORPTION OF MONOSACCHARIDES
Occurs mostly in duodenum and jejunum.
[Na+] is low inside intestinal mucosal cells,
and high in the gut lumen,
so Na+ is transported into the cells.
Its carrier also transports in one Glc for
every one Na+ moved.
An ATP-powered pump moves Na+ back
out, so keeping [Na+] inside cells low, and
allowing Glc to be continuously moved in.
So Glc is absorbed,
if necessary against a concentration
gradient,
by a process indirectly powered by ATP.
26
Gal absorption is similar.
Fru binds to a carrier, but simply moves in
along its concentration gradient.
27
A word about Roughage
28
Undigested cellulose and hemicelluloses
from plant matter in the diet
increase faecal bulk and decrease transit
time.
If avoided, as they often are in western
diets rich in white flour and meat,
this may lead to
constipation,
and hence increased exposure of the large
intestine to potential carcinogens.
Although the polymers aren’t digested,
they are broken by gut bacteria,
to gases, including CH4 and H2.
Beans,
although a good source of protein,
also contain undigested oligosaccharides
(see the earlier diet list).
They, too, are broken by gut bacteria,
with similar results.
29
DISACCHARIDASE DEFICIENCIES
[Look back to p.24 to see where disaccharidases
work.]
Deficiencies may be genetic,
or result from:
severe
intestinal
infection
inflammation of the gut lining;
causing
drugs injuring the gut lining;
surgical removal of the small intestine.
30
The most common genetic condition is
lactose intolerance.
Most human adults lose lactase after
weaning.
However, most western whites retain the
enzyme into adulthood,
possibly a trait that developed following
cattle domestication, ~104 y ago.
If lactase is lacking,
then milk products in the diet produce
abdominal pains,
distention,
flatulence,
diarrhoea.
This is because undigested lactose is
broken by gut bacteria,
producing gases and irritant organic acids.
31
Also, unbroken lactose is osmotically
active,
and draws water from the gut into the
lumen,
causing ‘osmotic diarrhoea’.
Generally, then, the symptoms are those
that occur when the gut contains
undigested carbohydrate.
Symptoms are avoided by:
1 avoiding milk products
(as many non-western cultures do);
2 using milk products treated with a
fungal lactase;
3 supplementing diet with lactase.
[Tutorial 2 Question 2 is to do with lactose
intolerance]
32
33
FATE OF THE ABSORBED Glc
[Leave Gal, Fru until later.]
Glc diffuses from the intestinal mucosal
cells into the portal blood, and is
transported to the liver.
Immediately Glc enters the liver (or any)
cell, it is phosphorylated:
Glc
glucose 6-phosphate
(G-6-P)
ATP
ADP
In this form, Glc is unable to diffuse out of
the cell, and is hence trapped.
34
The reaction just seen is catalysed by one
of two isozymes
(different
reaction):
proteins
catalysing
the
same
glucokinase (in liver);
hexokinase (in other tissues).
[Enzymes Lecture 3]
Despite the names, both can work on other
hexoses as well as Glc, in all cases
producing hexose 6-phosphates. Both have
less affinity for the other sugars than for
Glc.
They have different kinetic properties:
Km
Vmax
for Glc
glucokinase
high
high
hexokinase
low
low
35
[Look back at Enzymes Lecture 3:
the higher the Vmax, the more efficient the
enzyme is when saturated with substrate;
the lower the Km, the higher the affinity of the
enzyme for the substrate.]
Glucokinase high Km means that
when [blood Glc] is normal,
liver doesn’t grab the Glc:
it lets other tissues have it.
But after a meal,
when [portal Glc] increases,
it does grab the Glc.
Its high Vmax allows it to phosphorylate all
that Glc.
Result:
Most absorbed Glc is trapped as liver
G-6-P.
36
Hexokinase low Km means
that even when [tissue/blood Glc] is low,
tissues effectively grab Glc.
(They need it as an energy source.)
The low Vmax, however, means that tissues
are ‘easily satisfied’;
they don’t keep grabbing Glc,
and tying up cell Pi
as G-6-P:
it’s needed for other things,
like making ATP from ADP.
Result:
The other tissues efficiently take up and
use Glc.
37
BI25M1
CARBOHYDRATES
AND
INTERMEDIARY
METABOLISM
LECTURE 3
AIMS:
to review: fates of liver glucose 6-phosphate;
glycogen synthesis and
degradation;
glycogen storage diseases.
[Lehninger
Instant Notes
Chapters 7, 14, 15
Section J]
38
FATES OF LIVER GLUCOSE
6-PHOSPHATE (G-6-P)
It may be metabolised in the liver,
or be converted back to Glc,
transported to other tissues,
and metabolised there.
Possible routes of metabolism,
either in liver or other tissues, are:
1 storage as glycogen;
2 catabolism through glycolysis
provide ATP,
and further breakdown,
in the presence of O2,
to provide still more ATP;
to
3 metabolism through the pentose
phosphate pathway,
to provide pentoses and NADPH.
[The following five pages (pp.39-43) illustrate these fates.]
39
40
41
42
43
44
GLYCOGEN
~90% is present in liver, skeletal muscle.
(Lecture 1)
Functions:
In liver:
when [blood Glc] falls,
glycogen
G-6-P
Glc into blood
glucose 6-phosphatase
In skeletal muscle:
there is no glucose 6-phosphatase:
glycolysis
glycogen
G-6-P
lactate
substrate-level phosphorylation
[Energy Transformations Lecture 3]
ATP
for muscle
contraction
45
Synthesis:
1
G-6-P
G-1-P
2
G-1-P
UDPG
UTP PPi (
2Pi)
UTP = uridine 5’-triphosphate
(an analogue of ATP)
3
An enzyme, glycogenin, takes the Glc from
the UDPG and forms a covalent link with
it:
UDPG
Glc-glycogenin
UDP
4
Another Glc is added from UDPG,
forming an  1 4 bond
Glc-glycogenin
UDPG
Glc-Glc-glycogenin
UDP
glycogen synthase
46
5
Step 4 is repeated several times, to give:
6
‘Branching enzyme’ then acts:
to break:
and then add on:
7
Then further action of
glycogen synthase,
‘branching enzyme’
produces glycogen.
47
Degradation (mobilisation)
1
Monomers are removed,
one at a time,
from non-reducing ends,
as G-1-P.
Pi
glycogen
(Glc)n-1 + G-1-P
(Glc)n
glycogen phosphorylase
[note: NOT phosphatase]
This continues until about 4 Glc from a
branch.
48
2
‘De-branching enzyme’ then acts:
to break:
and then add on, allowing further phosphorylase action:
So, products are: mostly G-1-P,
and a little free Glc.
49
Then, as we saw (in part) earlier,
G-1-P
G-6-P
LIVER
SKELETAL
MUSCLE
glucose 6-phosphatase
glycolysis
substrate-level
phosphorylation
Glc
ATP
to blood
for contraction
lactate
50
GLYCOGEN ‘STORAGE’ DISEASES
several,
rare,
genetic deficiencies in one or other of the
enzymes of glycogen metabolism.
51
von Gierke’s disease
liver (and kidney, intestine)
glucose 6-phosphatase deficiency.
Symptoms:
1 high [liver
structure;
2
glycogen]
of
normal
fasting hypoglycaemia (low [blood Glc]);
(as can’t use glycogen, or anything else except
dietary carbohydrate as source of blood Glc).
3 lacticacidaemia (high [blood lactate]);
(lactate produced by skeletal muscle and
elsewhere can’t be reconverted to Glc in the
liver: this process needs glucose 6phosphatase.)
[See the Cori cycle later.]
Treatment:
regular carbohydrate feeding.
52
McArdle’s disease
skeletal muscle phosphorylase deficiency.
Symptoms:
1 high [muscle glycogen] of normal
structure;
2 temporary
exercise;
weakness,
cramp
after
3 no increase in [blood lactate] after
exercise.
Symptoms are not apparent in resting
state, when muscles are using other energy
sources – Glc and fatty acids from blood.
Usually first apparent in 20- to 30-yearolds.
(Do children rely less on muscle glycogen?)
Treatment:
avoid strenuous activity.
[Tutorial 2 Question 4 asks about Glycogen
Storage Diseases]
53
BI25M1
CARBOHYDRATES
AND
INTERMEDIARY
METABOLISM
LECTURE 4
AIMS:
to review: glycolysis;
some fates of lactate and pyruvate;
the Cori cycle.
[Lehninger
Instant Notes
Chapters 7, 14, 15
Section J]
54
Refer back to the start of Lecture 3
(pp.39-43):
[FATES OF
PHOSPHATE
LIVER
GLUCOSE
6-
It may be metabolised in the liver,
or converted back to Glc,
transported to other tissues,
and metabolised there.
Possible routes of metabolism,
either in liver or other tissues, are:
1
storage as glycogen;
2
catabolism through glycolysis to provide
ATP,
and further breakdown,
in the presence of O2,
to provide still more ATP;
3
metabolism
through
the
pentose
phosphate pathway,
to provide pentoses and NADPH].
55
GLYCOLYSIS
means ‘sugar splitting’;
is a catabolic pathway concerned with
saving some of the potential energy of
glucose/G-6-P
by forming ATP
through substrate-level phosphorylation.
[Energy Transformations Lecture 3]
It’s essentially the only way that ATP can
be made in our bodies from catabolism of
fuel molecules
when cells lack oxygen
(e.g. in vigorously exercising skeletal muscle)
or lack mitochondria
(e.g. RBCs).
So, it’s a crucial process.
56
It’s also an ancient process:
evolved before O2 became abundant in
atmosphere;
occurs in most cells of most organisms;
occurs in cytosol – not in complex
organelles.
In human cells with O2, and having
mitochondria, glycolysis acts as a prelude
to more extensive breakdown of
glucose/G-6-P, to CO2 and H2O, and the
generation
of
ATP
by
oxidative
phosphorylation.
[Energy Transformations Lecture 3]
This more extensive breakdown occurs by
a mechanism also used in breakdown of
the other main energy-providing food
materials, fatty acids.
57
A summary
glucose
(6-C molecule)
G-6-P
ATP
(substrate-level
phosphorylation)
pyruvate
(3-C molecule)
lactate
58
CO2
acetyl coenzyme A
(2-C molecule)
citric acid cycle
terminal respiratory
system
ATP
(oxidative
phosphorylation)
CO2 + H2O
(1-C molecule)
59
fatty acids
‘ oxidation’
60
THE PROCESS ITSELF
There are 10 steps between Glc and
pyruvate.
Look at Lehninger Edition 4 p.524; Edition 5 p.529
Instant Notes p.306.
61
So what ought we to concentrate on?
62
Glc
G-6-P
Fru-6-P
Fru-1,6-bisP
dihydroxyacetone-P
glyceraldehyde-P
phosphoenolpyruvate
pyruvate
-
O-C O
C O
CH3
63
Locate the reactions catalysed by
glucokinase/hexokinase;
phosphofructokinase;
aldolase;
pyruvate kinase.
64
ATP
ADP
ATP
ADP
NAD+
NADH + H+
ADP
ATP
ADP
ATP
65
Locate the reactions concerned with:
‘Energy input’ (2);
Sugar splitting;
Oxidation;
‘Energy extraction’ (2).
66
OUTCOMES
(per Glc)
Reaction
(numbers as in Lehninger Edition 4 p.524; Edition 5 p.529.)
1
1
ATP used
3
1
ATP used
6
2
(NADH + H+) made
7
2
ATP made
10
2
ATP made.
So partial catabolism yields:
2
mol ATP/mol Glc/G-6-P.
67
LACTATE PRODUCTION AND FATE
NADH produced in reaction 6 must be reoxidised,
because reaction 6 needs NAD+.
(If all cytosolic NAD ended up reduced, glycolysis
would stop.)
In human cells lacking O2 (e.g. in
vigorously exercising skeletal muscle) or
lacking mitochondria (e.g. RBCs), the
NADH is oxidised in a reaction producing
lactate.
-
pyruvate
NADH + H+
O – C=O
C=O
CH3
lactate
dehydrogenase
NAD+
O – C=O
HCOH
CH3
-
lactate
68
[In the presence of O2 (and in cells with
mitochondria), the NADH is re-oxidised in the
terminal respiratory system and used to make ATP
by oxidative phosphorylation
– see later Term. Resp. and Ox. Phos. lectures.]
So, for every 1 NAD+ used in reaction 6,
1 NAD+ is re-made.
As for the lactate,
it passes through blood to the liver,
where it is re-converted to Glc
(by ‘gluconeogenesis’ – a process we see
later).
69
tissue in absence of O2
(or without mitochondria)
e.g. exercising muscle
G-6-P
ATP
lactate
blood
liver
70
Glc
gluconeogenesis
pyruvate
lactate dehydrogenase
lactate
71
This almost looks like a system of generating
ATP at no cost!
But, the gluconeogenesis in the liver requires
ATP, and this is made by O2-requiring
oxidative phosphorylation.
72
So, when we sprint,
muscles don’t receive O2 fast enough to
make ATP by oxidative phosphorylation.
Instead, ATP is made by substrate-level
phosphorylation, and lactate is formed.
The lactate is reconverted to Glc in the
liver by a process needing O2,
which is why we pant after sprinting.
The liver ‘repays the oxygen debt’ run up
by the muscles.
The interaction between tissues
that produce lactate
and the liver
is called the ‘Cori cycle’.
73
WHAT HAPPENS TO PYRUVATE IN
CELLS WITH ACCESS TO O2,
(AND WITH MITOCHONDRIA)?
Instead of conversion to lactate,
the pyruvate is further broken,
to acetyl coenzyme A (acetyl CoA),
in an oxidation reaction in mitochondria
catalysed by pyruvate dehydrogenase.
(Look back at the summary scheme at the start of
the lecture.)
pyruvate
(3-C)
NAD+
NADH + H+
CO2
acetyl CoA
(2-C)
CH3 - C - S - coenzyme A
O
74
BI25M1
CARBOHYDRATES
AND
INTERMEDIARY
METABOLISM
LECTURE 5
AIMS:
to review: gluconeogenesis;
the fates of galactose
and fructose.
[Lehninger
Instant Notes
Chapters 7, 14, 15
Section J]
75
GLUCONEOGENESIS
Definition:
Synthesis of Glc from a non-carbohydrate
source.
(So making Glc from, e.g., glycogen, doesn’t
count.)
The sources are:
1 certain amino-acids;
[see later How Organisms Handle Nitrogen
lectures]
2 glycerol
(a 3-C component of some lipids);
3 lactate
(but this has itself been made from
Glc/G-6-P, as we’ve seen in Lecture 4).
76
Purpose:
1 Metabolism of Glc/G-6-P to pyruvate
is the only way in which ATP can be
generated
(by substrate-level phosphorylation)
in cells without adequate O2,
(or lacking mitochondria);
all other fuels (e.g. fatty acids)
only yield ATP
by oxidative phosphorylation,
and so need O2 and mitochondria.
2 Some cells (e.g. in CNS),
although having access to O2 and with
mitochondria,
prefer to use Glc as a fuel.
77
Thus, the body always needs a supply of
Glc/G-6-P.
It can get Glc from the diet
(but one isn’t always eating).
It can use glycogen stores
(but these are limited).
So a mechanism of making Glc from other
molecules is essential.
The liver is a major gluconeogenic site.
78
Mechanism:
Gluconeogenesis operates by a reversal of
the process
glucose
pyruvate.
7 of the 10 reactions we saw in
glycolysis, catalysed by the same enzymes,
simply operate in the reverse direction.
However, 3 glycolysis reactions,
catalysed by
glucokinase/hexokinase,
phosphofructokinase,
pyruvate kinase,
(highlighted in our earlier scheme of glycolysis)
are
essentially
irreversible
conditions operating in cells.
under
They only work in the direction of
glycolysis.
79
These 3 reactions are by-passed,
by reactions exclusive to gluconeogenesis.
Glc
G-6-P
Fru-6-P
Fru-1,6-bisP
dihydroxyacetone-P
glyceraldehyde-P
phosphoenolpyruvate
pyruvate
80
Pi
glucose 6-phosphatase
Pi
fructose bisphosphatase
ADP
ATP
GTP
PEP carboxykinase
oxaloacetate (4C)
pyruvate carboxylase
ADP+ Pi
CO2
GDP
CO2
ATP
81
glycerol
some
amino-acids
lactate
82
WHY DRINKING AND
GLUCONEOGENESIS DON’T MIX
The fate of ingested ethanol:
In liver,
ethanol
ethanal
acetate
(ethanoate)
+
+
NAD NADH + H
+
NAD NADH + H
+
acetyl CoA
or
stored as fat
(see later Lipid Metabolism lectures)
citric acid
cycle
terminal respiratory
system
ATP synthesis
(by oxidative phosphorylation)
CO2+H2O
83
But
liver
NAD+
gluconeogenesis,
e.g.
lactate
is
needed
pyruvate
NAD+
NADH + H+
So drinking inhibits gluconeogenesis,
leading to:
lacticacidaemia (increased [blood lactate]),
hypoglycaemia (decreased [blood Glc]),
and, when untreated,
confusion, coma, death.
in
84
Drinking is particularly dangerous when
the body needs gluconeogenesis:
so,
avoid if you’re
athletic,
dieting,
or
a down-and-out derelict.
85
FATE OF ABSORBED GALACTOSE
AND FRUCTOSE
(Look back at Lecture 2.)
1 Galactose
Gal
ATP
galactokinase
ADP
Gal-1-P
UDPGlc
Glc-1-P
UDPGal
epimerase
glycolipids
glycoproteins
proteoglycans
lactose
86
The action of the epimerase means:
(i) absorbed Gal can be used to make
Glc, G-6-P, glycogen.
The flow is:
(ii) Gal-containing molecules can be made
from Glc,
so Gal is not essential in the diet.
(Good news for lactose-intolerant people!)
The flow is:
Glc-1-P
UDPGlc
UDPGal
glycolipids, etc.
87
2 Fructose
Fru
ATP
fructokinase
ADP
Fru-1-P
aldolase B
dihydroxyacetone-P
glyceraldehyde
both feed into glycolysis.
[Note: both Gal and Fru can also be substrates
for hexokinase (and glucokinase), as we saw in
Lecture 2, yielding the hexose 6-phosphates.]
88
WHAT POWERS SPERM?
ANSWER:
FRUCTOSE!
Seminal fluid contains Fru as sperm fuel.
Why?
Fru is not much used by seminal vesicles
or the female genital tract, as both are well
supplied with Glc.
This means that Fru of the seminal fluid
can be used ~exclusively by sperm.
They contain hexokinase, which converts
Fru to Fru-6-P, and then, through
glycolysis,
citric acid cycle,
terminal respiratory system,
lots of ATP is made
(by oxidative phosphorylation)
and away they go!
89
BI25M1
CARBOHYDRATES
AND
INTERMEDIARY
METABOLISM
LECTURE 6
AIM: to review the pentose phosphate
pathway.
[Lehninger
Instant Notes
Chapters 7, 14, 15
Section J]
90
Refer back to the start of Lecture 3
(pp.39-43):
[FATES OF
PHOSPHATE
LIVER
GLUCOSE
6-
It may be metabolised in the liver,
or converted back to Glc,
transported to other tissues,
and metabolised there.
Possible routes of metabolism,
either in liver or other tissues, are:
1
storage as glycogen;
2
catabolism through glycolysis to provide
ATP,
and further breakdown,
in the presence of O2,
to provide still more ATP;
3
metabolism
through
the
pentose
phosphate pathway,
to provide pentoses and NADPH].
91
THE
PENTOSE
PATHWAY
PHOSPHATE
Functions:
1
to produce NADPH.
This is used in
liver:
fatty acid synthesis
(See later Lipid Metabolism lectures);
steroid synthesis;
drug metabolism.
adipose tissue;
lactating mammary gland:
fatty acid synthesis.
adrenal cortex:
steroid synthesis.
red blood cells:
as an antioxidant.
92
[See Lehninger, Edition 4 p.513; Edition 5 p.517;
Instant Notes, p.89 for NADP structure.]
It is an example of a co-reactant that links
oxidative, catabolic pathways to
reductive, anabolic pathways.
(Energy Transformations Lecture 3)
P-P pathway (oxidative, catabolic)
G-6-P
products
NADP+
NADPH + H+
fatty acids
steroids
precursors
(reductive, anabolic)
93
2
to produce pentoses (5-C sugars),
e.g. ribose, 2-deoxyribose.
These are precursors of ATP, RNA,
DNA.
3
to metabolise the small amounts of
pentoses in the diet.
94
Mechanism:
It consists of 2 sections:
1 an oxidative, irreversible section,
that generates NADPH,
and converts G-6-P to a pentose
phosphate;
2 a reversible section,
that interconverts G-6-P and the
pentose phosphate
via Fru-6-P.
irreversible section
95
2NADP+
2NADPH + 2H+
G-6-P
pentose phosphate
CO2
96
Fru-6-P
reversible section
The flow pattern depends upon the need.
97
1 To metabolise dietary pentoses:
dietary pentoses
Fru-6-P
2 When pentoses are needed more than
NADPH:
e.g. in tissues in which much protein
synthesis is occurring,
so that lots of pentose-containing mRNA,
rRNA and tRNA is needed
(BI20M1 lectures).
Fru-6-P
98
3 When NADPH is needed more than
pentoses:
e.g. in tissues in which much fatty acid
synthesis is occurring.
NADPH is made by the irreversible
section,
but the pentoses also made are reconverted to G-6-P by the reversible
section.
NADPH
Fru-6-P
99
4 When both pentoses and NADPH are
needed:
flow occurs only through the irreversible
section.
NADPH
100
NADPH AS AN ANTIOXIDANT IN RED
BLOOD CELLS
O2 is carried in RBCs thus:
Hb–Fe2+O2
(BI20M1 Protein Structure lectures).
The electronegative O2 tends to remove an
electron from the Fe2+, producing:
Hb–Fe3+
(‘Met-Hb’)
and
O 2. –
(‘superoxide’).
‘Met-Hb’ can’t bind O2,
and superoxide is a free radical,
which, through conversion to other
oxygen-containing free radicals and H2O2,
can chemically damage cell components,
particularly membranes.
101
How does the RBC cope?
Hb–Fe3+
reduced vit C
NADP+
Hb–Fe2+
oxidised vit C
NADPH + H+
102
O2.-
H2O2
reduced
glutathione
NADP+
H2O
oxidised
glutathione
NADPH + H+
103
BLACK-WATER FEVER
The first enzyme of the irreversible section is
glucose 6-phosphate dehydrogenase.
A genetic deficiency in it affects ~108 people,
mostly of African or Mediterranean origin.
Males affected most; females are carriers.
Symptom is haemolytic anaemia:
RBCs burst, and their contents
darken the urine (‘black-water fever’).
104
The condition often remains unnoticed,
until manifested in certain situations:
1 during particular infections;
2 on taking particular antibiotics,
antimalarials
antipyretics;
3 after eating Fava beans
(common in Mediterranean diets).
What happens is that:
G-6-P dehydrogenase deficiency
causes low [RBC NADPH]
causing accumulation of damaging oxygen
species in RBCs
which then have membranes that
‘suffer insult readily’.
105
In all the situations above,
the RBC cell membrane is put under
additional oxidative stress,
and this is enough to cause the bursting.
Treatment:
administer antioxidants.
Prevalence of the genetic condition suggests
that it has an evolutionary advantage.
Like people with sickle-cell anaemia,
female carriers are resistant to the RBCinfecting malarial parasite.
106
CARBOHYDRATE NOMENCLATURE AND STRUCTURES
A Guide for Reference
1.
Monosaccharides (ie, simple sugars)
Triose, tetrose, pentose, etc.
Biologically particularly important hexoses:
glucose (Glc), galactose (Gal), fructose (Fru) [Fig. 1-3]
2.
3.
Aldoses
(ie, having aldehyde group).
eg, Glc, Gal
Ketoses
(ie, having keto group)
eg, Fru [Fig. 3]
Asymmetric (chiral) carbons (ie, those having 4 different groups attached).
eg,
4.
[Fig. 1, 2]
Glc, Gal
have 4 each
(C2-5) [Fig. 1, 2]
Fru
has 3
(C3-5) [Fig. 3]
Epimers (ie, structural isomers differing in conformation about 1 carbon only).
eg, Glc, Gal are C4 epimers [Fig. 1, 2].
5.
Enantiomers (ie, structural isomers that are mirror images of each other).
eg, D-, L-Glc (so-labelled because of arrangement of groups around C5) [Fig. 1.4]
Most sugars in human biochemistry are D-sugars.
6.
Hemi-acetal/ -ketal rings (an -OH group reacts with an aldehyde / keto group).
Pyranose rings contain 6 atoms (5C, 1O);
Furanose rings contain 5 atoms (4C, 1O).
Predominant form of Glc, Gal [Fig. 5-7] and free Fru: pyranose.
Predominant form of Fru [Fig. 8] when part of a larger molecule: furanose.
7.
Anomers (ie, special kind of epimers produced when ring formation creates an additional asymmetric carbon).
eg, C1 in Glc, producing -D-Glc, -Glc. [Fig. 5, 6].
8.
Reducing/non-reducing sugars.
Interconversion of 2 anomers in solution occurs via the open-chain form. If this contains an aldehyde group, it
can be oxidised to –COOH, and so reduce something else.
9.
Glycosidic bonds.
The -OH of the anomeric C is reactive; this C may therefore link (formally, by water elimination,) to another
monosaccharide, or to a non-carbohydrate. - and -glycosides; O- and N-glycosides [Fig. 9, 10].
10. Derivatives.
Aldonic acids (-CHO oxidised to -COOH).
eg, on C1 of D-Glc [Fig. 11].
Uronic acids (CH2OH oxidised to -COOH).
eg, on C6 of D-Glc [Fig. 12].
Aminosugars (-OH replaced by -NH2).
eg, on C2 of D-Glc [Fig. 13].
107
108