M1. Nitrogen Fixation and Assimilation

M2. Amino acid metabolism

M3. The urea cycle

M4. Hemes and chlorophylls

Section M Nitrogen Metabolism

1. The nitrogen cycle

2. Nitrogen fixation

3. Nitrogen assimilation 同化作用

Nitrogen fixation and assimilation

The nitrogen cycle

The nitrogen cycle is the movement

of nitrogen through the food chain

from simple inorganic compounds ,

mainly ammonia, to complex organic

compounds .

脱氨作用

分解代谢

Nitrogen fixation

Nitrogen fixation is the conversion of N2 g

as into ammonia, a process carried out b

y some soil bacteria, cyanobacteria 蓝 细菌 , and the symbiotic 共生的 bacteria Rh

izobium 根瘤菌 that invade the root nodul

es of leguminous 豆类 plants.

Nitrogen fixation

This process is carried out by the nitrogenase固氮酶 complex, which consists of a reductase and an iron-molybdenum 钼 -containing nitrogenase. At least 16ATP molecules are hydrolyzed to form two molecules of ammonia. Leghemoglobin 豆血红蛋白 is used to protect the nitrogenase in the Rhizobium from inactivation by O2 .

nitrogenase

铁氧还蛋白

Nitrogen assimilation

Ammonia is assimilated by all organisms i

nto organic nitrogen-containing compoun

ds(amino acids,nucleotides,etc.) by the ac

tion of glutamate dehydrogenase (to form

glutamate) and glutamine synthetase (to fo

rm glutamine).

Amino acid family

Amino acid degradation

• Amino acids are degraded by the

removal of the α-amino group and

the conversion of the resulting

carbon skeleton into one or more

metabolic intermediates.

Amino acid degradation

• Amino acids are termed glucogenic if their carbo

n skeletons can give rise to the net synthesis of

glucose ,and ketogenic of they can give rise to k

etone bodies. Some amino acids give rise to mor

e than one intermediate and these lead to the sy

nthesis of glucose as will as ketone bodies. Thu

s these amino acids are both glucogenic and ket

ogenic.

pathway

Transamination

• The α-amino group of most amino acids is transferred to α-ketoglutarate to form glutamate and the corresponding α-keto acid

• α-amino acid + α-ketoglutarate α-keto acid + glutamate

• Enzyme: transaminases

Aminotansferase

Vitamine B6

PLP

Oxidative deamination of glutamate

glutaminase

Amino acid oxidases

NH3

Metabolism of phenylalanine

四氢生物蝶呤

Inborn errors of metabolism

• Inherited metabolic disorders

遗传代谢紊乱• Alkaptonuria 尿黑酸症• Homogentisate oxidase 尿黑酸氧化

酶• Phenylketonuria 苯丙酮尿• Phenylalanine hydroxylase

苯丙氨酸羟化酶

The urea cycle

1. Ammonia excretion

2.The urea cycle

3. Link to the citric acid cycle

4. Hyperammonemia

5. Formation of creatine phosphate

6. The activated methyl cycle

7. Uric acid

Ammonia excretion

• Ammonotelic animal 排氨 ammonia

• Uricotelic animal 排尿酸 uric acid

• Ureotelic animal 排尿素 urea

• terrestrial reptile 陆生爬行动物• Aquatic animal 水生动物

Ammonia excretion

• Amminia------ammonotelic organisma—a

quatic animals

• Uric acid ------uricotelic organisms birds

and terrestrial reptiles

• Urea-----ureotelic organisms—terrestrial

vertebrates

The urea cycle

• In the urea cycle ammonia is first combined wi

th CO2 to form carbamoyl phosphate. This then

combines with ornithine to form citrulline. Citrulline then condenses with aspartate, the source of the second nitrogen atom in urea,to form argininosuccinate. This compound is in turn split to arginine and fumarate, and the arginine then splits to form urea and regenerate ornithine.

NH4+ + HCO3

- + H2O + 3ATP +aspartate

urea + 2ADP + AMP + 2Pi +PPi +

fumarate

Urea cycle

Urea cycle

• The reaction place: mitochondria, cytosol

• The enzymes are involved in reaction:

1.Carbarroyl phosphate synthetase 氨甲酰磷酸合酶2.ornithine transcarbamoylae 鸟氨酸转氨甲酰酶3.argininosuccinate synthetase 精氨琥珀酸合成酶4.argininosuccinase 精氨琥珀酸酶5.arginase 精氨酸酶

Urea cycle 2

Urea cycle 3

Urea cycle 4

Urea cycle 5

Link to the citric acid cycle

Oxaloacetate has several possible fates

• Transamination to aspartate which can then feed back into the urea cycle;

• Condensation with acetyl CoA to form citrate which then continues on round the citric acid cycle ;

• Conversion into glucose via gluconeogenesis ;

• Conversion into pyruvate

Hyperammonemia

• A block in any of the urea cycle en

zymes leads to an increase in the

amount of ammonia in the blood,

so-called hyperammonemia

ammonia

The reason of brain damage in hyperammonemia.

1. Excess ammonia leads to the formation

of glutamate and glutamine.

2. It may compromise energy production.

3. It also leads to increase [H+ ]

Formation of creatine phosphate

• The urea cycle is also the starting point f

or the synthesis of another important m

etabolite creatine phosphate. This phosp

hate provides a reservoir of high-energy

phosphate in muscle cells.

ATP

• As the energy released upon is hydrolysis is greater than that released upon the hydrolysis of ATP (ΔG for creatine phosphate hydrolysis=-10.3kal

mol-1 compared with –7.3 kcal mol-1 for ATP hydrolysis ).

Creatine phosphate

• The first step in the formation of creatine phosphate is the condensation of arginine and glycine to form guanidinoacetate 胍基乙酸 . Ornithine is released in this reaction and can then be re-utilized by the urea cycle. The guanidinoacetate is then methylated by the methyl group donor S-adenosyl methionine to form creatine, which is in turn phosphory-lated, by creatine kinase to form creatine phosphate.

Creatine phosphate

The activated methyl cycle

• S-Adenosyl methionine serves as d

onor of methyl groups in numerous

biological reactions [e.g.in the for-m

ation of creatine phosphate and in th

e synthesis of nucleic acids].It is for

med through the action of the activat

ed methyl cycle .

The activated methyl cycle

• During donation of its Methylgroup to anoth

er compound, S-adenosyl methionine is con

verted into S-adenosyl homocysteine. To re

generate S-adenosyl methionine, the adeno

syl group is removed from the S-adenosyl h

omocysteine to form homo-cysteine 高半胱氨酸 .

The activated methyl cycle

• This is then methylated by the enzyme homo

cysteine methyltransferase, one of only two v

itamin B12 containing enzymes found in euka

ryotes, to form methionine. The resulting met

hionine is then activated to S-adenosyl methi

onine. With the release of all three of the pho

sphate from ATP.

Methyl cycle

Uric acid

• Uric acid is the main nitrogenous waste pr

oduct of uricotelic organisms (reptiles, bir

ds and insects), but is also formed in ureo

telic organisms from the breakdown of the

purine bases from DNA and RNA.

• Gout 痛风

HEMES AND CHLOROPHYLLS

1. Tetrapyrrol

2. Biosynthesis of hemes and chorophylls

3. Heme degradation

Tetrapyrroles

• The tetrapyrroles are a family of pigments base

d on a common chemical structure that include

s the hemes and chlorophylls. Hemes are cyclic

tetrapyrroles that contain iron and are commonl

y found as the prosthetic group of hemoglobin,

myoglobin and he cytochromes.

Heme is as the prosthetic roup

 

Globin 珠蛋白

Protein

Heme Cytochomes 细胞色素 Catalases 过氧化氢酶 Enzyme

Peroxidase 过氧化物酶

chlorophylls

• The chlorophylls are modified tetrapy

rroles containing magnesium that occ

ur as light-harvesting and reaction ce

nter pigments of photosynthesis in pl

ants, algae and photosynthetic bacter

ia.

structure

Biosynthesis of hemes and chlorophylls

The staring point for heme and

chlorophyll synthesis is aminolaevulinic

acid 氨基乙酰丙酸 (ALA) which is made in

animals from glycine and succinyl CoA by

the enzyme ALA synthase.

Biosynthesis of hemes and chlorophylls

• This pyridoxal 吡哆醛 pyosphate-requi

ring enzyme is feedback regulated by h

eme. Two molecules of ALA then conde

nse to form porphobilinogen 胆色素原 i

n a reaction catalyzed by ALA dehydrat

ase 脱水酶 .

Biosynthesis of hemes and chlorophylls

• Porphobilinogen deaminase catalyzed the

condensation of four porphobilinogen to f

orm a linear tetrapyrrole. This compound

then cyclizes to form uroporphyhrinogen

尿卟啉原 III, the precursou of hemes, chlo

rophylls and vitamin B12.

Biosynthesis of hemes and chlorophylls

• Further modifications take place to form pr

otoporphyrin 原卟啉 IX. The biosynthetic pa

thway then branches, and either iron is ins

erted to form heme, or magnesium is insert

ed to begin a series of conversions to form

chlorophyII.

pathway

protoporphyrin

Heme degradation

• Heme is broken down by heme oxygenase to the linear tetrapyrrole biliverdin 胆绿素 . This green pigment is them converted to the red-orange bilirubin 胆红素 by biliverdin reductase. The lipophilic bilirubin is carried in the blood bound to serum albumin, and is then converted into a more water-soluble compound in the liver by conjugation to glucuronic acid 葡萄糖醛酸 .

heme

Heme

• The resulting bilirubin

• Diglucuronide 胆红素二葡糖苷酸 is secret

ed into the bile, and finally excreted in the

feces. Jaundice is due to a build up of ins

oluble bilirubin in the skin and whites of t

he eyes.

Heme

• In higher plants heme is broken down to the

phycobiliprotein 藻胆蛋白 phytochrome whic

h is involved in coordinating light responses,

while in algae it is metabolized to the light-ha

rvesting pigments phycocyanin and phycoer

ythrin.