Section 5: Oxygen transfer proteins

Question 1

Energy is a requirement of life, how is it produced?

Answer 1

respiration in which dioxygen is reduced to water
-enzymes carry out these reactions and increase the rate of reaction bu factors of 10^12 over the corresponding model reaction

Question 2

Oxygen is relatively unreactive, how is it made reactive?

Answer 2

Oxygen is mainly unreactive in its ground triple state howee most oxygen transfer proteins use coordination of O2 to a metal centre such as Fe or Cu to active the oxygen, significantly increasing the rate of reaction

Question 3

What is an oxygenase

Answer 3

Ans oxygenase is an enzyme that inserts oxygen into other molecules
a monooxygenase inserts one oxygen, and a dioxygenase inserts two

Question 4

What are the three oxygenases we’re studying

Answer 4

Cyctochrome P-450

Methane Monooxygenase

Question 5

What is cytochrome p-450?

Answer 5

A family of metalloenzymes which function as monooxygenases

found in mammals, birds, fish, plants, insects, yeasts, bacteria and other biological species

CO adducts have characteristic absorbtion bands at 450 nm

Question 6

What do cytochromes do?

Answer 6

catalyse aromatic and aaliphatic hydroxylation reactions and other oxidation reactions
these processees are essential for metabolism and the detoxification of harmful substance

One O atom is inserted and the second O is reduced to H2O (in cooperation with a reducing agent ed NADH, Fe-S proteins and also a P-450 reductase?)
PICTURE OF REACTIOn

Question 7

What is the structure of Cytochrome-450

Answer 7

Their structure is based on a haem ring, similarly to Hb and Mb
The iron is coordinated to the protein by a cysteine sulphur IMAGE

Question 8

What is the hardest hydrocarbon to hydroxylate? Why? Why is this a problem

Answer 8

Methane
-high CH bond energy
-no dipole moment
-no additional functional groups that assist binding to protein active site.
This is a problem as bacteria exist where methane is their sole carbon and energy source.

Question 9

What is MMO

Answer 9

Methane monooxygenase 
An Enzyme 
An oxygenase that catalyses the conversion of
methane to methanol (hydroxylates)
PICTURE eqn

Question 10

What is the mechanism for MMO? (General words)

Answer 10

-coenzyme=NAD
PICTURE
the active site of the enzyme contains two Fe(III) centres bridged by OH
Differes from the active site of Hr in that both of the irons are Fe(III) in the resting state of cf. Fe(II) centres in Hr resting state.

Question 11

Mechanism for MMO hydroxylation

Answer 11

PICTURE
1) Reduction to Fe(II) w/NADH
2) Addition of O2 to give peroxy intermediate
3) proton donation and expulsion of water
4) Fe accepts an H from methane to give ch3 radical
5) Fast genaration of methanol via donation of OH
PICTURE!!

Question 12

What is tyrosinase?

Answer 12

-contains a dinuclear active site similar to those fouund in Hc 2 x Cu
PICTURE
A monooxygenase which catalyses the hydroxylation of monophenols to diphenols
Can act as a two electron oxidase catalysing the oxidation of diphenols to quinines

Question 13

What is the mechanism of tyrosinase

Answer 13

1)addition of O2 to give Cu(II) oxy form
2)coordination of phenolic substrate in axial position on one Cu
3)Ortho-hydroxylation of the phenol
4) Intramolecular electron transfer to give quinone product and regenerates
PICTURE

Question 14

Mechanism: actual cycle

Answer 14

PICTURE

Question 15

Mechanism: words

Answer 15

1) RH binds and pushes out the water
2) reduction to high spin fe(II)
3) Binding of O2 and one electron transfer from Fe(II)
4) Acceptance of e to give peroxy state
5) protenation leading to to heterolytic bond cleavage producing Fe(IV)=O species with the porphyrin ring existing as a radical cation
6) Oxygen atom transfer to H substrate to give ROH and regeneration of resting site.