Lecture 5 Flashcards
How are we thinking of oxidation levels in this class?
Suppose you are considering the oxidation state of the carbon in CH4 vs HOCH3. When the carbon is bound to an atom that is more electronegative than itself, the carbon is oxidized. Each bond to an atom more electronegative than the carbon can be thought of as removing 2 electrons from carbon.
In terms of their usage, how are NADH (NAD+) and NADPH (NADP+) different?
NADH is used for energy utilization
NADPH is used for biosynthesis and catabolism
How is a hydride represented? What does it symbolize?
H-
Hydride is a transfer of 2 electrons
Why is NADH kinetically stable? Why is NAD+ kinetically stable?
NADH is not aromatic but it is neutral in charge
NAD+ is charged but it is aromatic
Both NADH and NAD+ are kinetically stable. What does this mean as far as hydride transfer?
Enzymatic catalysis is needed to promote this hydride transfer.
We know the “business” end of NADH is used for hydride transfer. What does the rest of the molecule do and what does this result in as far as enzyme selectivity?
The rest of the NADH molecule interacts with the enzyme active site to hold the NADH in a specific orientation with respect to the enzyme and the substrate. This results in stereoselectivity for the hydride donation (or acceptance if NAD+).
What is alcohol dehydrogenase (ADH)?
A group of enzymes that interconvert between alcohols and aldehydes/ketones using cofactor NAD+/NADH.
What is unique about ADH?
It is highly stereoselective - holds the NAD+/NADH in such a manner that the hydride could only be added/removed to/from once face of the molecule.
What components (residues and otherwise) are in the ADH active site?
2 cysteine residues (deprotonated)
1 histidine residue (deprotonated)
Zn2+
Residues coordinate with Zn2+, which acts as lewis acid to draw electron density away from the oxygen of the alcohol (substrate), making the H of the alcohol group more acidic.
How do statins inhibit HMG coA reductase?
They mimic the transition state of HMG coA when acted upon by HMG coA reductase.
How are NAD+/NADH different from FAD/FADH2?
NAD+/NADH is stable in air (FAD/FADH2 is not)
NAD+/NADH can diffuse (FAD/FADH2 remains tightly bound to enzyme active site)
NAD+/NADH can only perform transfer of 2e- (FAD/FADH2 can transfer either 1 e- or 2 e-)
What are the 3 types of flavin dependent enzymes?
Reductases, oxygenases, monooxygenases
Reductases:
1) What is their function?
2) What do they require?
3) What is regenerated?
4) Example?
1) Required to keep other enzyme systems “charged” for redox
2) Require NADH or NADPH
3) FAD is regenerated through 2 e- reduction of other co-substrate
4) Glutathione reductase
Why do cells need glutathione (i.e. what roles does it play)? (2)
1) It is the most important cellular defense against reactive oxygen species, which can be harmful to DNA. Reduced glutathione (Glut-SH) is an antioxidant that combats ROS.
2) It reacts with xenobiotics, trapping reactive electrophiles in the cell (via SN2, nucleophilic acyl substitution, 1,4 addition and SNAr) before they cause harm to proteins and DNA. This is catalyzed by glutathione transferase (GST).
What types of reactions can glutathione engage in to scavenge xenobiotics?
1) SN2
2) Nucelophilic acyl substitution
3) 1,4 addition
4) nucleophilic aromatic substitution
