Discovered by accident
1890s- Hans and Edward Buchner added sucrose( used as preservative) to yeast extract
Instead preserving yeast extract, sucrose was broken down and fermented and formed
Fermentation and other type of cellular metabolism could be studied in vitro
Glucose and pyruvate are phosphorylated
Each step in glycolysis is catalyzed by different enzyme
A closer look at glycolysis reactions
All 10 reactions of glycolysis
occur in cytosol
ATP production during
glycolysis occurs by substrate-
For each glucose molecule, the
How is glycolysis regulated?
High level of ATP inhibits a key
High level of ATP signals that
cell doesn’t need to produce
Feedback inhibition occurs
when an enzyme in a pathway is
inhibited by the product of the
cells that are able to stop the reaction when ATP is abundant can store glucose as a result natural selection favors individuals with phosphofructokinase
How high levels of substrate inhibit the enzyme?
Phosphofructokinase has 2 binding sites for ATP
ATP can bind to active site or at a site that changes the enzyme’s activity – regulatory
When the ATP concentrations are high, the molecule binds to regulatory site and changes
the conformation of enzyme in a way that decreases reaction rate (ATP acts as allosteric
9.4 pyruvate processing
In eukaryotes, Pyruvate is transported from cytosol to mitochondria
Mitochondria has 2 membranes and the
interior is filled with sac like structures
The region inside the inner membrane but
outside the cristae is called
Pyruvate moves across outer membrane
of mitochondria into intermembrane
space through small pores
Entry into matrix occurs through active
transport (though membrane protein
called pyruvate carrier located in inner
What happens when pyruvate enters
Coenzyme A (CoA) serves as a cofactor
Cofactors are often a metal ion or a
relatively small organic molecules called coenzymes.
Coenzymes bind to the active site and stabilize the reaction’s transition state
CoA act as coenzyme by accepting and then transferring an acetyl group to a substrate
In CoA, A stands for acetylation.
Acetyl CoA forms, when acetyl binds to sulfur atom on one end of the CoA.
Pyruvate reacts with CoA to form acetyl CoA.
This process occurs in an enzyme complex called pyruvate dehydrogenase.
Eukaryotes: pyruvate dehydrogenase is located in the matrix.
Prokaryotes: pyruvate dehydrogenase is located in cytosol.
The process 1. One of the C from pyruvate oxidized to
2. NAD reduced to NADH
3. Remaining 2 carbon acetyl molecule
transferred to CoA.
Acetyl CoA is the final product of the
pyruvate processing step.
When ATP is abundant, feedback inhibition
The pyruvate processing stops and the complex becomes phosphorylated
Phosphorylation changes the shape of the complex
And stops the reaction
High concentration of NAD , CoA or AMP speeds up the reaction catalyzed by the
pyruvate dehydrogenase complex.
9.5 Krebs cycle
Acetyl CoA is the key compound that feeds the Krebs cycle
Acetyl CoA reacts with oxaloacetate and forms citrate
Figure 9.19 (page 195)
Energy released by the oxidation of one molecule of acetyl CoA is used to produce 3
molecules of NADH one of FADH2 and one of the guanosine triphosphate (GTP) though
substrate level phosphorylation
Prokaryote: enzymes responsible for pyruvate processing and Krebs cycle are located in
Eukaryotes: enzymes responsible for for pyruvate processing and Krebs cycle are located
in mitochondrial matrix
How is Krebs cycle regulated?
Krebs cycle is regulated by feedback inhibition
Feedback inhibition occurs at 2 points in the cycle. (figure 9.20 page 196)
At first point, NADH binds to the enzyme’s active site (example of competitive
At second point, ATP binds to allosteric regulatory site.
Krebs cycle slows down when ATP and NADH are plentiful and ATP acts as an
allosteric and NADH acts as competitive inhibitor.
What happens to NADH and FADH ? 2 Each molecule of glucose that’s fully oxidized to 6