metabolism of amino acid

7/17/2019 metabolism of amino acid

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Amino Acid Catabolism: N

Copyright © 1999-2008 by Joyce J. i!an.

All rights reser"ed.

#olec$lar %iochemistry &&



'here are m$ltiple transaminase en(ymes !hich vary in

substrate specificity.

)ome sho! pre*erence *or partic$lar amino acids or

classes o* amino acids as amino gro$p donors+ and,or *or

partic$lar -keto acid acceptors.

H

R1 C COO- + R2 C COO

-

NH3+ O

Transaminase

H

R1 C COO

-

+ R2 C COO

-

O NH3+

Transaminases aminotrans*erases

cataly(e the

re"ersible reaction

at right.



/ample o* a Transaminase reaction:

Aspartate donates its amino gro$p+ becoming the

α-eto acid oaloacetate.

α-etogl$tarate accepts the amino gro$p+

becoming the amino acid gl$tamate.

aspartate α-etogl$tarate oaloacetate gl$tamate

Aminotrans*erase 'ransaminase

COO−

CH2

CH2

C

COO−

O

COO−

CH2

HC

COO−

NH3+

COO−

CH2

CH2

HC

COO−

NH3+

COO−

CH2

C

COO−

O3 3



&n another eample+ alanine becomes pyr$"ate as

the amino gro$p is trans*erred to α-etogl$tarate.

alanine α-etogl$tarate pyr$"ate gl$tamate

Aminotrans*erase 'ransaminase

COO−

CH2

CH2

C

COO−

O

CH3

HC

COO−

NH3+

COO−

CH2

CH2

HC

COO−

NH3+

CH3

C

COO−

O3 3



'ransaminases equilibrate amino groups among

a"ailable α-eto acids.

'his permits synthesis o* non-essential amino acids+

$sing amino gro$ps *rom other amino acids 4 carbon

seletons synthesi(ed in a cell.

'h$s a balance o* di**erent amino acids is maintained+

as proteins o* "aried amino acid contents are

synthesi(ed.

Altho$gh the amino N o* one amino acid can be $sed

to synthesi(e another amino acid+ N must be

obtained in the diet as amino acids proteins.



Essential amino acids m$st be cons$med in the diet.

#ammalian cells lac en(ymes to synthesi(e theircarbon seletons α-eto acids. 'hese incl$de:

&sole$cine+ le$cine+ 4 "aline

5ysine

'hreonine

'ryptophan

6henylalanine 'yr can be made *rom 6he.

#ethionine Cys can be made *rom #et.

7istidine /ssential *or in*ants.



'he prosthetic gro$p o* 'ransaminase is

pyridoxal phosphate PLP+ a deri"ati"e o*

"itamin %8.

pyrido0al phosphate -656.

NH

CO

P

O−

O

O

OH

CH3

CH O−

+

H2



&n the resting state+ the aldehyde gro$p o* pyridoal

phosphate is in a Schiff base linage to the ε-amino

gro$p o* an en(yme lysine side-chain.

NH

C

O

P

O−

O

O

O−

CH3

HC−

+

H2

N

(CH2)4

Enz

H

3

RHC COO−

NH2

/n(yme 5ys-656 )chi** base

Amino acid



'he acti"e site lysine etracts 73+ promoting

ta$tomeri(ation+ *ollo!ed by reprotonation 4 hydrolysis.

NH

CO

P

O−

O

O

O−

CH3

HC−

+

H2

N

HC

H

3

R COO−

Enz−Lys−NH2

Amino acid-656 )hi** base aldimine

'he α-amino gro$p

o* a s$bstrate amino

acid displaces the

en(yme lysine+ to

*orm a )chi** base

linage to 656.



'he amino gro$p remains on !hat is no! pyridoxamine

phosphate PP.

A di**erent α-eto acid reacts !ith 6#6 and the process

reverses+ to complete the reaction.

NH

CO

P

O−

O

O

OH

CH3

CH2

NH2

H2

R C COO−

O

−

+

Enz−Lys−NH2

6yridoamine phosphate 6#6

α-eto acid

hat !as anamino acidlea"es as an

-keto acid.



)e"eral other en!ymes that cataly(e metabolism or

synthesis o* amino acids also $tili(e PLP as prosthetic

gro$p+ and ha"e mechanisms in"ol"ing a )chi** base

linage o* the amino gro$p to 656.

NH

CO

P

O−

O

O

O−

CH3

HC−

+

H2

N

HC

H

3

R COO−

Enz−Lys−NH2

Amino acid-656 )hi** base aldimine



Chime /ercise

'!o neighboring st$dents or st$dent gro$ps sho$ld

team $p+ each displaying one o* the *ollo!ing:

'ransaminase !ith PLP in )chi** base linage to

the acti"e site lysine resid$e.

'ransaminase in the PP *orm+ !ith glutarate+ an

analog o* α-etogl$tarate+ at the acti"e site.

)t$dents sho$ld then sho! and eplain the str$ct$re

displayed by them to the neighboring st$dent or

st$dent gro$p.



&n addition to e$ilibrating amino gro$ps among

a"ailable α-eto acids+ transaminases funnel amino

groups *rom ecess dietary amino acids to those amino

acids e.g.+ gl$tamate that can be deaminated.

"arbon skeletons o* deaminated amino acids can be

cataboli(ed *or energy+ or $sed to synthesi(e gl$cose or

*atty acids *or energy storage.

;nly a *e! amino acids are deaminated directly.



&t is one o* the *e! en(ymes that can $se N#$% or N#$P% as e− acceptor.

;idation at the α-carbon is *ollo!ed by hydrolysis+

releasing N7<3.

−OOC

H2

C

H2

C C COO−

O

+ NH4+

NAD(P)+

NAD(P)H

−OOC

H2

CH2

C C COO−

NH3+

Hgl$tamate

α-etogl$tarate

=l$tamate ehydrogenase

H2O&lutamate$ehydrogenase

cataly(es a ma>or

reaction that e**ects

net removal of N *rom the amino

acid pool.



)$mmari(ed abo"e:'he role o* transaminases in *$nneling amino N to

gl$tamate+ !hich is deaminated "ia =l$tamate

ehydrogenase+ prod$cing N7<3.

Amino acid -ketoglutarate NADH + NH4

+

-keto acid glutamate NAD+ + H2O

Transaminase Glutamate Dehydrogenase



)ome other path!ays *or deamination o* amino acids:

1. Serine $ehydratase cataly(es:

serine pyruvate % N'(%

2. Peroxisomal 5- and -amino acid oidases cataly(e:

amino acid % )#$ % '*+

-keto acid % N'(% % )#$'*

)#$'* % +* )#$ % '*+*

"atalase cataly(es: 2 '*+* * '*+ % +*

HO CH2

HC COO

−

NH3

+

C COO−

OH2O NH4+

C COO−

NH3

+

H2C H3C

H2O

serine aminoacrylate pyr$"ate

)erine ehydratase



#ost terrestrial land animals con"ert ecess nitrogen to

urea+ prior to ecreting it.

?rea is less toic than ammonia.

'he ,rea "ycle occ$rs mainly in liver.

'he 2 nitrogen atoms o* $rea enter the ?rea Cycle as N'-

prod$ced mainly "ia =l$tamate ehydrogenase and asthe amino N of aspartate.

'he N7@ and 7C;@− carbonyl C that !ill be part o* $rea

are incorporated *irst into carbamoyl phosphate.

H2N C

O

NH2

$rea



"arbamoyl Phosphate

Synthase 'ype & cataly(es

a @-step reaction+ !ith

carbonyl phosphate and

carbamate intermediates.#mmonia is the N inp$t.

'he reaction+ !hich

in"ol"es clea"age o* * .P bonds o* A'6+ is essentially

irreversible.H2N C OPO3

2−

O

H2N C O−

O

HO C

O

OPO32−

HCO3−

ATP

NH3

ADP

ATP

Pi

ADP

carbonyl phosphate

carbamate

carbamoyl phosphate



Alternate *orms o*

"arbamoyl Phosphate

Synthase Types // 0 ///

initially generate ammonia

by hydrolysis o* glutamine.'he type && en(yme incl$des

a long internal tunnel

thro$gh !hich ammonia 4

reaction intermediates s$ch

as carbamate pass *rom one

acti"e site to another. H2N C OPO32−

O

H2N C O−

O

HO C

O

OPO32−

HCO3−

ATP

NH3

ADP

ATP

Pi

ADP

carbonyl phosphate

carbamate

carbamoyl phosphate



"arbamoyl Phosphate Synthase is the committed step o*

the ?rea Cycle+ and is s$b>ect to reg$lation.

H2N C OPO32−

O

HCO3− + NH3 + 2 ATP

+ 2 ADP + Pi

Carbamoyl 6hosphate)ynthase

carbamoyl phosphate



"arbamoyl Phosphate Synthase has an absol$te

re$irement *or an allosteric acti"ator N -acetylglutamate.

'his deri"ati"e o* gl$tamate is synthesi(ed *rom

acetyl-CoA 4 gl$tamate !hen cell$lar gl$tamateB is high+signaling an excess of free amino acids d$e to protein

breado!n or dietary intae.

O CO NH



H2N C OPO32−

O

CH2

CH2

CH2

HC

COO−

NH3+

NH3+

CH2

CH2

CH2

HC

COO−

NH3+

NH

CO NH2

COO−

CH2

HC

COO−

NH2

CH2

CH2

CH2

HC

COO−

NH3+

NH

C NH2+

COO−

CH2

HC

COO−

HN

AMP + PPi

ATP

CH2

CH2

CH2

HC

COO−

NH3+

NH

C

NH2+

H2N

COO−

HC

CH

COO−

C NH2H2N

O H2O

Pi

ornithine

$rea

citr$lline

aspartate

arginino-s$ccinate

*$marate

arginine

carbamoyl phosphate

?rea Cycle

1

2

@

<

,rea "ycle

/n(ymes inmitochondria:

1. ;rnithine'rans-

carbamylase

/n(ymes incytosol:

2. Arginino- )$ccinate

)ynthase@. Arginino- s$ccinase

<. Arginase.



or each cycle+ citrulline m$st lea"e the mitochondria+ andornithine m$st enter the mitochondrial matri.

An ornithine1citrulline transporter in the inner mitochondrialmembrane *acilitates transmembrane *l$es o* citr$lline 4

ornithine.

cytosol

mitochondrial matrix

carbamoyl phosphate

6i

ornithine citr$lline


$rea aspartate

arginine argininos$ccinate

*$marate



A complete 2rebs "ycle *$nctions only !ithin

mitochondria.

%$t cytosolic iso(ymes o* some rebs Cycle en(ymes are

in"ol"ed in regenerating aspartate *rom fumarate.

cytosol


carbamoyl phosphate6i


ornithine citr$lline$rea aspartate


*$marate



)umarate is con"erted to oxaloacetate "ia rebs Cycle

en(ymes $marase 4 #alate ehydrogenase.+xaloacetate is con"erted to aspartate "ia

transamination e.g.+ *rom gl$tamate.

Aspartate then reenters ?rea Cycle+ carrying an amino

gro$p deri"ed *rom another amino acid.

aspartate α-etogl$tarate oaloacetate gl$tamateAminotrans*erase 'ransaminase

COO−

CH2

CH2

C

COO−

O

COO−

CH2

HC

COO−

NH3+

COO−

CH2

CH2

HC

COO−

NH3+

COO−

CH2

C

COO−

O3 3



'ereditary deficiency o* any o* the ?rea Cycle

en(ymes leads to hyperammonemia - ele"ated

ammoniaB in blood.'otal lac o* any ?rea Cycle en(yme is lethal.

/le"ated ammonia is toic+ especially to the brain.

&* not treated immediately a*ter birth+ se"ere mental

retardation res$lts.



6ost$lated mechanisms *or toicity o* high ammoniaB:

1. 7igh N7@B !o$ld dri"e &lutamine Synthase:

glutamate % #TP % N'- glutamine % #$P % Pi

'his !o$ld deplete gl$tamate D a ne$rotransmitter 4

prec$rsor *or synthesis o* the ne$rotransmitter =A%A.

2. epletion o* gl$tamate 4 high ammonia le"el !o$ld

dri"e &lutamate $ehydrogenase reaction to reverse:

glutamate % N#$3P4% -ketoglutarate %

N#$3P4' % N'(%

'he res$lting depletion o* α-etogl$tarate+ an essential

rebs Cycle intermediate+ co$ld impair energy

metabolism in the brain.



Treatment o* de*iciency o* ?rea Cycle en(ymes

depends on !hich en(yme is de*icient:

limiting protein intake to the amo$nt barely

ade$ate to s$pply amino acids *or gro!th+ !hile

adding to the diet the α-eto acid analogs o*

essential amino acids.

Liver transplantation has also been $sed+ sinceli"er is the organ that carries o$t ?rea Cycle.

t l



tiss$es !here they generate arginine 4 ornithine+ !hich are

prec$rsors *or other important molec$les.

/.g.+ #rgininosuccinate Synthase+ !hich cataly(es

synthesis o* the prec$rsor to arginine+ is in most tiss$es.

#itochondrial #rginase //+ distinct *rom the cytosolic ?rea

Cycle Arginase+ clea"es arginine to yield ornithine.

cytosol


carbamoyl phosphate

6i ornithine citr$lline

ornithine citr$lline$rea aspartate


*$marate

'he complete,rea "ycle is

signi*icantly onlyin liver.

7o!e"er some en!ymes o* the

path!ay are inother cells and



'he amino acid arginine+ in addition to being a constit$ento* proteins and an intermediate o* the ,rea "ycle+ is

prec$rsor *or synthesis o* creatine 4 the signal molec$le

nitric oxide.

H2N C N

NH2+

CH2

CH3

C

O

O−

creatine

NH2 NH2 NH2



)ynthesis o* the radical species nitric oide 5N+ *rom

arginine is cataly(ed Nitric +xide Synthase+ a distant

relati"e o* cytochrome 6<E0.

$ifferent isoforms o* Nitric ;ide )ynthase e.g.+ eN+S

epressed in endothelial cells and nN+S in ne$ronal cells

are s$b>ect to di**ering reg$lation.

+H3N CH COO

−

CH2

CH2

CH2

NH

C

NH2

NH2+

NADPH NADP+

O2 H2O O2 H2O

+H3N CH COO−

CH2

CH2

CH2

NH

C

NH2

N OH

+H3N CH COO

−

CH2

CH2

CH2

NH

C

NH2

O

1/2 NADPH 1/2 NADP+

+ NO

Nitric ;ide )ynthase

arginine hydroyarginine citr$lline



5N+ is a short-li"ed signal molecule !ith di"erse roles

in di**erent cell types+ incl$ding reg$lation o* smoothm$scle contraction+ gene transcription+ metabolism+ and

ne$rotransmission.

#any o* the regulatory effects of 5N+ arise *rom its

acti"ation o* a sol$ble cytosolic &uanylate "yclase

en(yme that cataly(es synthesis o* cyclic-&P

analogo$s in str$ct$re to cyclic-A#6.

"ytotoxic effects of 5N+ obser"ed $nder someconditions are attrib$ted to its non-en(ymatic reaction

!ith s$peroide ;2F− to *orm the strong oidant

peroxynitrite ;N;;−.



Polyamines incl$de putrescine+spermidine+ spermine.

+rnithine is a ma>or precursor *or

synthesis o* polyamines.

Con"ersion o* ornithine to p$trescine is

cataly(ed by +rnithine $ecarboxylase.



'he cationic polyamines ha"e di"erse roles in cell

gro!th 4 proli*eration.

isr$ption o* polyamine synthesis or metabolism leads

to disease in animals 4 h$mans.



7o!e"er+ Ca33-acti"ated Peptidylarginine $eiminases con"ert arginine resid$es !ithin proteins to citrulline as

a post-translational modi*ication.

'here is no tGNA *or

citrulline 4 this amino acid

is not incorporated

translationally into proteins.



is essential to terminal differentiation of skin cells.

/cessi"e protein citr$llination+ !ith prod$ction o*

antibodies against citrullinated proteins+ is *o$nd to be

a *actor in the autoimmune diseases s$ch as rhe$matoid

th iti d lti l l i

)$bstit$tion o* citrulline+

!hich lacks arginineHs

positive charge+ may alter

str$ct$re 4 properties s$ch as

binding a**inities o* a protein.

/.g.+ citrullination o* certain proteins+ incl$ding keratin

intermediate *ilament proteins+

metabolism of amino acid

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