The Molybdenum Cofactor: Moco

What early experiments indicated:

… but it wasn’t entirely correct.

Nit-1 Nit-1 Nit-1 Nit-1

active nitrate reductase

nitrate reductase

sulfite oxidase

xanthine dehydrogenase

aldehyde oxidase

apo NRapo SO apo XDH apo AO

dissociatedMoco

O

Mo OS

SX

O

Mo SS

SX

O

Mo OHS

SX

O

MoS

S

X

S

S

(a) (b) (c) (d) (e)

O

MoS

S

O

S

S

ASP

Now we know that there is not just one Moco, but a family of related Moco structures in molybdenum enzymes:

Questions asked of molybdoenzymes and their model compounds:

-What is the redox potential ( energy of) Mo redox reactions?

- What are the structural details? What is bond order? (angles, bond distances)

-How well do models mimic reactions of Mo in enzymes? in structure? in reactivity?

A “second generation” Moco model:the Holm-Berg model

N

SSMo

O O

Mo(6+)O2(L-NS2)

N

SSMo

O

Mo(4+)O(L-NS2)

sol

Mimicking the Catalytic Reactions of Moco

Mo(6+)O2(L-NS2) Mo(4+)O(L-NS2)

PPh3O=PPh3

Me-S(=O)-MeMe-S-Me

DMSO = dimethylsulfoxideDMS = dimethylsulfide

Berg-Holm Model

Mo(6+)O2(L) + 2 H+ Mo(4+)O(L) + H2O

SO32- SO4

2-

2 e- acceptor (like FAD+ )

2 e- reduced (like FADH2

Sulfite Oxidase

What would the Berg-Holm model system suggest?

• a 2 e- process between Mo(6+) and Mo(4+), and only the Mo(6+) and Mo(4+) ox. states are required.

• BUT, it was known that Mo(5+) plays a role (by EPR)

O

Mo+6

OS

S

O

MoOS

S

O

Mo+4

S

S

O

Mo+4

OS

S

SO

OO

OS

O

OO

O

Mo+4

S

SH

H

O

Mo+5

OHS

S

a

- H+, - e -

- H+, - e -

+ SO32-

- SO42-

+ H2O

oxygen

atom

transfer

coupled

proton

electron

transfer

A proposed mechanism for Moco Catalysis of Sulfite Oxidation: OAT and CEPT

Why such a BIG Ligand on Mo?

Mo

S

S O

S

O

S

Mo

S

S

S

O

SX X=O

1st equivalent

Mo

S

S O

S

O

S

Mo

S

S

S

OS

MoSS

S

O

S

Mo SS

O

S

O

S

conproportionation2nd equivalent

•Mo(5+)- Mo(5+) dimer•CN = 6•Terminal vs bridging Mo-O

Step 1:

Step 2:

BIG Ligand is intended to prevent dimerization

Except… it didn’t!!!Later researchers showed formation of

(L-NS2)Mo

O

O Mo(L-NS2)

O

Technique: Infrared Spectroscopy

Application to Berg-Holm model and Mo-dimer:• detection of Mo=O groups

• number of absorption related to number Mo=O • frequency related to Mo oxidation state• frequency also reveals Mo-O-Mo

Technique: Column Chromatography

Review:• chromatographic separations are based on:

• dipolar interaction of molecules with solid support (SiO2)• partitioning of molecule between support and solvent

In practice:• Silica gel column chromatography elutes most non-polar first, most polar last.• Different species may be selectively eluted with increasing the polarity of solvents, e.g., CH2Cl2, then acetone, then methanol

This week’s reactions:

1. MoO2(detc)2 + excess PPh3 --> RED

2. RED + propylene sulfide --> BLUE

3. Chromatography to separate:

O

Mo OS

SX

O

Mo SS

SX

O

Mo OHS

SX

O

MoS

S

X

S

S

(a) (b) (c) (d) (e)

O

MoS

S

O

S

S

ASP

What is the sulfur donor ligand for the family of related Moco structures in molybdenum enzymes?

S

S

O

HN NH

HNNO

O

NH2

MoO NN

NH2

O

HO

P OP O

O

O

OO

OH

Mo

The Ligand is calledMolybdopterin

pterin

dithiolene

nucleotide