Class notes

Biochemistry - Glycogenesis

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Course
Biochemistry

Institution
University Of Ottawa (U Of O )

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Uploaded on June 22, 2022
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Biochemistry

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1. Class notes - Biochemistry notes
2. Class notes - Biochemistry - enzymes
3. Class notes - Biochemistry - glycolysis
4. Class notes - Biochemistry - glycogenesis
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Biochemistry Exam Aid

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1. Class notes - Biochemistry notes
2. Class notes - Biochemistry - enzymes
3. Class notes - Biochemistry - glycolysis
4. Class notes - Biochemistry - glycogenesis
5. Class notes - Biochemistry - proteins
6. Class notes - Biochemistry - tca cycle
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Hydrolysis :- catalyzed by α-amylase , consume H2O.

[salivary α-amylase hydrolyzes the internal (α1→4) glycosidic linkages of starch, producing short polysaccharide
fragments or oligosaccharides ]

Phosphorolysis :- Catalyzed by phosphorylase and Involve Pi

Fates of Pyruvate under Anaerobic Conditions : Fermentation

Under aerobic conditions, the pyruvate formed in the final step of glycolysis is oxidized to acetate (acetyl-CoA) , which
enters the citric acid cycle and is oxidized to CO2 and H2O .

The NADH formed by dehydrogenation of glyceraldehyde 3-phosphate is ultimately re-oxidized to NAD+ by passage of its
electrons to O2 in mitochondrial respiration.

Under hypoxic ( low-oxygen ) conditions , however—as in very active skeletal muscle, in submerged plant tissues, solid
tumors, or in lactic acid bacteria—NADH generated by glycolysis cannot be re-oxidized by O2 .

Failure to regenerate NAD+ would leave the cell with no electron acceptor for the oxidation of
glyceraldehyde 3-phosphate and the energy-yielding reactions of glycolysis would stop.

NAD+ must therefore be regenerated in some other way.

Most modern organisms have retained the ability to continually regenerate NAD+ during anaerobic glycolysis by
transferring electrons from NADH to form a reduced end product such as lactate or ethanol .

Pyruvate Is the Terminal Electron Acceptor in Lactic Acid Fermentation

When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH
produced in glycolysis , NAD+ is generated from NADH by the reduction of pyruvate to lactate.

The reduction of pyruvate in this pathway is catalyzed by lactate dehydrogenase .

some tissues and cell types (such as erythrocytes, which have no mitochondria and thus can not oxidize pyruvate to CO2)
produce lactate from glucose even under aerobic condition

In glycolysis, dehydrogenation of the two molecules of glyceraldehyde 3-phosphate derived from each molecule of
glucose converts 2 molecules of NAD+ to 2 of NADH.

Because the reduction of two molecules of pyruvate to two of lactate regenerates two molecules of NAD+ , there is no
net change in NAD+ or NADH .

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,The lactate formed by active skeletal muscles (or by erythrocytes) can be recycled .

It is carried in the blood to the liver, where it is converted to glucose during the recovery from strenuous muscular
activity.

When lactate is produced in large quantities during vigorous muscle contraction (during a sprint, for example), the
acidification that results from ionization of lactic acid in muscle and blood limits the period of vigorous activity.

The best-conditioned athletes can sprint at top speed for no more than a minute .

Although conversion of glucose to lactate includes two oxidation-reduction steps , there is no net change in the
oxidation state of carbon .

In glucose ( C6H12O6 ) and lactic acid ( C3H6O3 ) , the H:C ratio is the same.

Nevertheless , some of the energy of the glucose molecule has been extracted by its conversion to lactate — enough to
give a net yield of two molecules of ATP for every glucose molecule consumed.

Fermentation is the general term for such processes, which extract energy (as ATP) but do not consume oxygen or
change the concentrations of NAD+ or NADH .

Ethanol Is the Reduced Product in Ethanol Fermentation

Yeast and other microorganisms ferment glucose to ethanol and CO2 , rather than to lactate.

Glucose is converted to pyruvate by glycolysis , and the pyruvate is converted to ethanol and CO2 in a two-step process

In the first step, pyruvate is decarboxylated in an irreversible reaction catalyzed by pyruvate decarboxylase.

This reaction is a simple decarboxylation and does not involve the net oxidation of pyruvate.

Pyruvate decarboxylase requires Mg2+ and has a tightly bound coenzyme , thiamine pyrophosphate ( TPP ) .

In the second step, acetaldehyde is reduced to ethanol through the action of alcohol dehydrogenase , with the reducing
power furnished by NADH derived from the dehydrogenation of glyceraldehyde 3-phosphate.

This reaction is a well-studied case of hydride transfer from NADH .

Ethanol and CO2 are thus the end products of ethanol fermentation, and the overall equation is ;

As in lactic acid fermentation , there is no net change in the ratio of hydrogen to carbon atoms when glucose ( H:C ratio
= 12/6 = 2) is fermented to two ethanol and two CO2 (combined H:C ratio = 12/6 = 2).

In all fermentations, the H:C ratio of the reactants and products remains the same.

NO O2 is involved !

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, Ethanol fermentation

Pyruvate decarboxylase is present in brewer’s and baker’s yeast (Saccharomyces cerevisiae) and in all other organisms
that ferment glucose to ethanol, including some plants.

The CO2 produced by pyruvate decarboxylation in brewer’s yeast is responsible for the characteristic carbonation of
champagne .

In baking , CO2 released by pyruvate decarboxylase when yeast is mixed with a fermentable sugar causes dough to rise.

The enzyme is absent in vertebrate tissues and in other organisms that carry out lactic acid fermentation.

The production of ethanol as a renewable fuel

Ethanol can be produced from relatively inexpensive and renewable resources rich in sucrose, starch, or cellulose—
starch from corn or wheat, sucrose from beets or cane, and cellulose from straw, forest industry waste, or municipal
solid waste.

Alcohol dehydrogenase is present in many organisms that metabolize ethanol , including humans.

In the liver it catalyzes the oxidation of ethanol , either ingested or produced by intestinal microorganisms, with the
concomitant reduction of NAD+ to NADH.

In this case , the reaction proceeds in the direction opposite to that involved in the production of ethanol by
fermentation .

Alcohol dehydrogenase and acetaldehyde dehydrogenase determine how much you can drink!

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