Coenzymes and vitamins 2

Question 1

Nucleotide substrate (cosubstrates)

Answer 1

Nucleosides acting as cosubstrates:

• ATP,GTP
• S adenosylmethionine
• Nucleotide sugars e.g UDP-glucose

Core structure:

• Nitrogenous base (adenine, uracil)
• Sugar (ribose)
• business end - reactive group of cosubstrates e.g phosphoryl group or glycosyl group.

Question 2

ATP transfer reagent (nucleoside substrate)

Answer 2

Nitrogenous base
Sugar group
reactive group (business end) = whole molecule

Its a versatile group, donation of:

• Phosphoryl (Pi) , 1 phosphate group is used
• Prophosphoryl (PPi), 2 phosphate group is used
• Adenylyl (AMP), the pyrophosphate is hydrolysed and the rest is used
• Adenosyl

Question 3

S adenosylmethionine (ATP related coenzyme)

Answer 3

SAM synthesis from ATP and methionine
Met + ATP > SAM + Pi + PPi

Core structure:

• Nitrogenous base
• Sugar
• business end, +ve sulfonium on methionyl group.

The +ve sulfonium (sulfur) makes it a nucleophilic acceptors

SAM is a donor of virtually ALL METHYL GROUPS

e.g Phospholipids, proteins, DNA, RNA

Question 4

Nucleotide-sugar coenzymes (activated sugars)

Answer 4

Structure:

• Nitrogenous base
• Sugar
• Pyrophosphoryl (for linkage)

business end: glycosyl group (all sorts of sugars)

GROUP TRANSFER OF SUGARS in carb metabolism

cosubstrate formation:
G-1-P + UTP > UDP-Glu + PPi
PPi > Pi + Pi

usage:
UDP-Glu + acceptor > Glu-acceptor + UDP

regeneration: (atp dependant)
UDP + ATP > UTP + ADP

Question 5

Dinucleotide cosubstrates - NAD,NADP,NADPH

Answer 5

2 Types:

• Nicotinamide coenzymes: NAD, NADH, electron transfer
• Riboflavin-derived coenzymes FAD (not FMN)

Question 6

Nicotinamide coenzyme

Answer 6

NAD : Nicotinamide Adenine Dinucleotide
NADP : Nicotinamide Adenine Dinucletide Phosphate

Bother from nicotinamide, which is derived from vitamin niacin (B3)

Niacin: aromatic 6 ring structure with nitrogen (pyridine)
It has an acid and amide group (C00H > NH2)

NADPH: coenzymes important in many redox reactions
group transfer of electrons

NAD,NADP are electron deficient, NADPH carry pair of electrons

business end: Nitrogen

NADPH are fairly stable in solution containing oxygen

Reducing power: biological reducing agents

Stability of the reduced di-nucleotide allows for them to carry reducing power from one enzyme to another i.e mobile form of reducing power

NB, Flavin coenzyme cannot carry reducing power

Miotochondia onxidation of NADH is coupled to ATP synthesis

NADPH is used a reducing agent in biosynthetic reactions

NADH sits at the top of metabolites active in most reactions

Question 7

NAD(PH)- binding in most dehydrogenases

Answer 7

Rossmann fold- consists a paor of Ba,Ba,B motifs

B strnads form one extended parallel B-sheet

Each of the rossman fold motifs bind to one half of the dinucleotide

Rossman fold motif is able to discriminate NADH and NADPH via critical Gly in a tight B-turn

Protein engineering of this aa results in change of preference for coenzyme

Question 8

NAD and NADH

Answer 8

The absorb wavelength of 340 nm due to dihydropyridine ring

As NAD and NADP do NOT have a dihydropyridine ring, absorbance at 340 does not occur.

Question 9

Lactate dehydrogenase (LDH)

Answer 9

Lactate + NAD > NADH + Pyruvate (-ve)+ H(+ve)

Question 10

Mechanism of Lactate dehydrogenase (LDH)

Answer 10

contain di-amine group help position of substrate

NAD binds to the rossman fold

Arg171 forms an ion pair with the carboxylate group of the substrate

His195, a base catalyst in the active site, abstracts a proton from the C-2hydroxyl group of lactate, facilitating transfer of the hydride ion H- from C2 of substrate to C4 of the bound NAD

2nd hydrogen (on His195) released as proton H+ to regenerate base catalyst

In the reverse reaction, H is transferred from the reduced coenzyme NADH to C2 of the oxidized substrate, pyruvate.