Nitrogen metabolism: fixation and amino acids

Question 1

What is the most common form of nitrogen?

Answer 1

N2 (triple bond - gas)

Question 2

What converts N2 to a useful form? (4)

Answer 2

Diazotrophs: azo, Brady, photo, Sino - rhizobium

Question 3

What enzyme do diazotrophs use to fix N2?

Answer 3

Nitrogenase

Question 4

What is the structure of nitrogenase?

Answer 4

Fe-protein: (homodimer with 2 ATP binding sites)

MoFe-protein: contains Fe and molybdenum

Question 5

What is nitrogenase inhibited by and how do diazotrophs stop this occurring?

Answer 5

Oxygen

Thick walled cells and no photosystem 2 in root nodules - absence of O2 (must be oxygen freer to work)

Question 6

What does nitrogen fixation require?

Answer 6

An electron source (oxidatively or photosynthetically)

Question 7

Why does nitrogen fixation require electrons? (3)

Answer 7

Electrons are transferred to ferrodoxin
Electrons require ATP-dependent protein changes
2 ATP bind to Fe-protein and hydrolyse as an e- passes to MoFe-protein

Question 8

Where does N2 reduction occur and what is the cost?

Answer 8

N2 reduction occurs on the MoFe protein

e- transfer occurs 6 times per N2 (12 ATP for 1 molecule)

Question 9

What is the cost of nitrogenase reducing H20 to H2?

Answer 9

8 e- and 16 ATP

Question 10

What are 3 ways we can obtain fixed N2 without plants?

Answer 10

Lightning discharges
Decaying organic matter
Applied fertiliser

Question 11

What is the haber-bosch process and what are the conditions?

Answer 11

Industrial N2 creator 
High temp (200*C)
Metal catalyst (iron)
High pressure (200atm)

Question 12

How is nitrogen used once its fixed? (2)

Answer 12

Assimilated into cells biomolecules

Forms amino acids then transaminated (aminotransferase)

Question 13

Which 2 amino acids are vital for nitrogen use and why?

Answer 13

Glutamate ands glutamine

Act as N donors

Question 14

What does glutamine synthetase do?

Answer 14

Incorporates an ammonium ion glutamate ti from glutamine

Question 15

Which amino acids are essential? (10)

Answer 15

Arginine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine

Question 16

What amino acids are non-essential? (10)

Answer 16

Alanine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
serine
Tyrosine

Question 17

What is the difference between an essential and non-essential amino acid?

Answer 17

Essential: Can’t be synthesised from scratch (diet)

Non - essential: Can be synthesised from intermediates

Question 18

Where from are essential amino acids synthesised? (3)

Answer 18

Lysine, methionine & threonine: aspartate
Leucine, isoleucine & valine: pyruvate
Phenylalanine, tyrosine & tryptophan: glucose derivatives

Question 19

How are non-essential amino acids synthesised?

Answer 19

Common metabolic intermediates: pyruvate, oxaloacetate

Question 20

What are amino acids precursors for? (5)

Answer 20

Nucleotides
Nucleotide coenzymes
Hormones
Heme (globin)
Neurotransmitters

Question 21

How are nucleotides synthesised?

Answer 21

De novo (from scratch)
Salvaged

Question 22

Where are purines and pyrimidines synthesised?

Answer 22

Purines: on the ribose
Pyrimidines: Cytoplasm

Question 23

What is the one carbon pool?

Answer 23

Single carbons in a variety of oxidation states

Question 24

What is methotrexate?

Answer 24

Inhibits FH2 to FH4

Used for cancer treatment