Biochemistry, glycolysis, metabolism, pyruvate and its emzyme complex

1. CLASS-2
10.12.2022
CARBOHYDRATE​
METABOLISM​
Neeraja S Raj​

2. CONTENTS​
• Fate of pyruvate
• Fermentation
• Pyruvate dehydrogenase complex
• Other monosaccharides

3. Fate of pyruvate...

4. • In the absence of oxygen,fermentation will occur.The key fermentation enzyme in mammalian cells is lactate
dehydrogenase,which oxidizes NADH to NAD + , replenishing the oxidized coenzyme for glyceraldehyde-3-
phosphate dehydrogenase.
• Without mitochondria and oxygen,glycolysis would stop when all the available NAD+ had been reduced to NADH.
By reducing pyruvate to lactate and oxidizing NADH to NAD+ , lactate dehydrogenase prevents this potential
problem from developing.
• There is no net loss of carbon in this process:pyruvate and lactate are both three carbon molecules.In aerobic
tissues,lactate does not normally form in significant amounts. However,when oxygenation is poor (during
strenuous exercise in skeletal muscle,a heart attack, or a stroke),most cellular ATP is generated by anaerobic
glycolysis,and lactate production increases.
• In yeast cells,fermentation is the conversion of pyruvate (three carbons) to ethanol (two carbons) and carbon
dioxide (one carbon).While the end products are different,the result of both mammalian and yeast fermentation is
the same:replenishing NAD+ .
FERMENTATION

5. (1) A glucose molecule is broken down via
glycolysis, yielding two pyruvate
molecules. The energy releasedby this
exothermicreactions is used to
phosphorylatetwo ADP molecules,
yielding two ATP molecules,and to
reduce two moleculesof NAD+to
NADH.
(2) The two pyruvate moleculesare
broken down by pyruvate decarboxylase,
yielding two acetaldehydemolecule and
giving off two molecules of carbon
dioxide.
(3) The two moleculesof NADH reduce
the two acetaldehyde moleculesto two
moleculesof ethanol;this converts
NADHback into NAD+ which is
catalyzedby alcoholdehydrogenase
(ADH1 in baker's yeast)
Ethanol / alcoholic fermentation

6. Lactic Acid Fermentation Pyruvate produced by glycolysis is converted
to lactate.This reaction is catalysed by the enzyme
lactate dehydrogenase (LDH).
The bacteria that make yogurt carry out lactic acid
fermentation,as do the red blood cells in your body,which
don’t have mitochondria and thus can’t perform cellular
respiration.
Muscle cells also carry out lactic acid fermentation,though
only when they have too little oxygen for aerobic
respiration to continue—for instance,when you’ve been
exercising very hard.It was once thoughtthat the
accumulation of lactate in muscles was responsible for
soreness caused by exercise,but recent research suggests
this is probably not the case.
Lactic acid produced in muscle cells is transported through
the bloodstream to the liver, where it’s converted back to
pyruvate and processed normally in the remaining reactions
of cellular respiration.
Pyruvate + NADH + H+ → Lactate + NAD+

7. PYRUVATE DEHYDROGENASE
COMPLEX
• Pyruvatefrom aerobic glycolysis enters mitochondria,
where it may be converted to acetyl-CoAfor entry into the
citric acid cycle if ATP is needed, or for fatty acid synthesis
if sufficient ATP is present.
• The pyruvate dehydrogenase complex(PDH) reactionis
irreversible and cannot be used to convert acetyl-CoAto
pyruvate or to glucose.
• Pyruvatedehydrogenasein the liver is activatedby insulin,
whereasin the nervous system, the enzyme is not
responsive to hormones.
• This makes sense because high insulin levels signal to the
liver that the individual is in a well-fed state; thus, the liver
should not only burn glucose for energy, but shift the fatty
acid equilibrium towardproduction and storage,rather
than oxidation.
• Pyruvatedehydrogenaseis actually a complex of enzymes
carrying out multiple reactions in succession.
Pyruvate dehydrogenase complex

8. •Insufficient amounts of any of the cofactors
or coenzymes can result in metabolic
derangements.
•Pyruvate dehydrogenase is inhibited by its
product acetyl-CoA.
•This control is important in several contexts
and should be considered along with pyruvate
carboxylase, the other mitochondrial enzyme
that uses pyruvate (in gluconeogenesis).
•Essentially,the buildup of acetyl-CoA (which
happens during β-oxidation) causes a shift in
metabolism:pyruvate is no longer converted
into acetyl-CoA (to enter the citric
acid cycle), but rather into oxaloacetate (to
enter gluconeogenesis).
Enzymes Abbrev. Cofactors
Pyruvate
Dehydrogenase (PDC)
E1
TPP (thiamine pyrophosphate),
Magnesium
Dihydrolipoyl
Transacetylase (DLAT)
E2
Lipoate,
Coenzyme A
Dihydrolipoyl
Dehydrogenase (DLD)
E3
FAD,
NAD+

9. Other monosaccharides... While glucose represents the primary monosaccharide used by cells, other
monosaccharides such as galactose and fructose can also contribute to ATP production
by feeding into glycolysis or other metabolic processes.
GALACTOSE
METABOLISM

10. FRUCTOSE
METABOLISM

11. THANKYOU