Lecture 8 Flashcards
Which steps of glycolysis are heavily regulated and why?
Steps 1, 3 and 10
These steps have large negative delta G values, making them highly energetically favorable and thus the parts of the process that require the most regulation
How does pyruvate kinase act as a sensor for cellular energy needs?
Pyruvate kinase is regulated via negative feedback, whereby downstream products (ATP, Acetyl-coA and citrate) are able to allosterically inhibit the enzyme’s action. The presence of these molecules in abundance indicates that the cell is in a high energy state and does not require additional energy, so PK being regulated by these molecules allows it to sense the cells energy state.
How does pyruvate kinase act as a gatekeeper for further oxidative metabolism?
PK produces pyruvate and pyruvate is needed to continue oxidative metabolism via the kreb’s cycle and electron transport chain. Without the action of PK, oxidative metabolism cannot occur so when it is inhibited by downstream products, it prevents flux through the rest of oxidative metabolism.
What is an isozyme?
Different enzymes that catalyze the same reaction
What is allosteric?
Allosteric control indicates that an enzyme is affected by other compounds that bind to a site on the enzyme other than the active site
There are 4 isozymes of PK. One of these isozymes is less active than the others. Why would cancer cells preferentially express this isozyme?
PK must form a tetramer to be active. Isozyme PKM2 requires binding of F 1,6 BisP to form this tetramer and formation of the tetramer in PKM2 is prevented by the phosphorylation of tyrosine residue 105. The kinase that phosphorylates tyrosine 105 is quite active, so PKM2 is not able to form tetramers as easily as other isozymes of PK, making PKM2 less active overall than other isozymes. Cancer cells prefer PKM2 exactly because it is less active. This means that molecules produced in earlier steps of glycolysis will build up and this is preferred because these molecules can be used in other biosynthetic pathways that are important to the rapidly dividing tumor cell (i.e. nucleotide biosynthesis).
Why is decarboxylation of alpha keto acids thermodynamically favorable but kinetically inaccessible?
Decarboxylation is thermodynamically favorable because the overall reaction releases energy. However, in order for decarboxylation to occur, the bond that is being broken (sigma c-c) must be parallel to the pic=o. This is not the case for alpha keto-acids, where the sigma c-c bond is perpindicular to the pic=o so there is no favorable orbital alignment for the reaction to proceed making the reaction kinetically inaccessible.
How does TPP facilitate decarboxylation of alpha keto acids?
It acts as an electron sink via its conjugated pi system that allows the electrons from the broken bond (sigma c-c) to delocalize into the pi system as an electron sink.
Describe the structure and function of human PDC
PDC is a large protein complex that consists of 3 enzyme complexes working together. Complex 1 decarboxylates pyruvate and forms an enamine intermediate. Complex 2 facilitates the reaction of the enamine intermediate with oxidized lipoamide to form a thioester intermediate, which is then attacked by coenzyme A to form acetyl coA. Complex 3 uses FAD and NAD+ to oxidize the lipoamide once more.
