Post on 29-Jun-2015
description
Amino acid catabolismAmino acid
degradationFates of carbon
skeleton
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MSB 206: REPRODUCTIVE AND URINARY SYSTEM
04/13/23
Amino acids and their R groups
• Twenty different amino acid are found in proteins• Most microorganisms and plants can
biosynthesize all 20• Animals (including humans) must obtain some of
the amino acids from the diet.• The amino acids that an organism cannot
synthesize on its own are referred to as essential amino acids.
• Humans require 8 essential amino acids
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Prop
ertie
s of
am
ino
acid
s
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Oxidative degradation of amino acids• Under three different metabolic circumstances
in animals:1. During the normal synthesis and degradation of
cellular protein; some amino acids that are released from protein breakdown
and are not needed for new protein synthesis undergo oxidative degradation.
2. When a diet is rich in protein and the ingested;amino acids exceed the body’s needs for protein synthesis, the
surplus is catabolized; amino acids cannot be stored.3. During starvation or in uncontrolled diabetes
mellitus;when carbohydrates are either unavailable or not properly
utilized, cellular proteins are used as fuel.
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Removal of the alpha amino group• Every amino acid contains an
amino group• The pathways for amino acid
degradation therefore include a key 1ST step in which the amino group is separated from the carbon skeleton
• It is then shunted into the pathways of amino group metabolism – different based on different organism.
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There are multiple transaminase enzymes which vary in substrate specificity.
Some show preference for particular amino acids or classes of amino acids as amino group donors, and/or for particular -keto acid acceptors.
H
R1 C COO- + R2 C COO-
NH3+ O
Transaminase
H
R1 C COO- + R2 C COO-
O NH3+
Role of Transaminases (aminotransferases)
Catalyze the reversible reaction
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aa
aaka
ka
04/13/23
Example of a Transaminase reaction: Aspartate donates its amino group, becoming
the -keto acid oxaloacetate. -Ketoglutarate accepts the amino group,
becoming the amino acid glutamate.
a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C O O
C H 2
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C O O
C H 2
C
C O O
O + +
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In another example, alanine becomes pyruvate as the amino group is transferred to -ketoglutarate.
a l a n i n e - k e t o g l u t a r a t e p y r u v a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C H 3
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C H 3
C
C O O
O + +
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The Ammonia that is generated following deamination rxn is Transported in the Bloodstream Safely as Glutamate or in combination with Glutamate to form Glutamine
• Un-needed glutamine is processed in intestines the, kidneys and liver
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Pyruvate – Alanine in SM
• In skeletal muscle, excess amino groups are generally transferred to pyruvate to form alanine
• Other than Gln and Glu, Alanine is therefore another important molecule in the transport of amino groups to the liver.
– See next slide
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Glutamate can Donate Ammonia to Pyruvate to
Make Alanine• Vigorously working muscles
operate nearly anaerobically and rely on glycolysis for energy
• Glycolysis yields pyruvate that muscles cannot metabolize aerobically; if not eliminated lactic acid will build up
• This pyruvate can be converted to alanine for transport of NH4 into liver
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Aminotransferases and pyridoxal phosphate• Aminotransferases are classic examples of enzymes
catalyzing bimolecular Ping-Pong reactions• In these rxns, the first substrate reacts and the product
must leave the active site before the second substrate can bind.
• The incoming amino acid thus binds to the active site, donates its amino group to pyridoxal phosphate, and departs in the form of an –keto acid.
• The incoming -keto acid then binds, accepts the amino group from pyridoxamine phosphate, and departs in the form of an amino acid.
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The prosthetic group of Transaminase is pyridoxal phosphate (PLP), a derivative of vitamin B6.
p y rid o x a l p h o sp h a te (P L P )
NH
CO
P
O O
O
O H
C H 3
CH O
H 2
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In the resting state, the aldehyde group of pyridoxal phosphate is in a Schiff base linkage to the -amino group of an enzyme lysine side-chain.
NH
CO
P
OO
O
O
CH3
HC
H2
N
(CH2)4
Enz
H
+
RHC COO
NH2
Enzyme (Lys)-PLP Schiff base
Amino acid
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The active site lysine extracts H+, promoting tautomerization, followed by reprotonation & hydrolysis.
NH
CO
P
OO
O
O
CH3
HC
H2
N
HC
H
+
R COO Enz Lys NH2
Amino acid-PLP Shiff base (aldimine)
The -amino group of a substrate amino acid displaces the enzyme lysine, to form a Schiff base linkage to PLP.
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The amino group remains on what is now pyridoxamine phosphate (PMP). A different -keto acid reacts with PMP 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
Pyridoxamine phosphate (PM P)
-keto acid What was an amino acid leaves as an -keto acid.
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Several other enzymes that catalyze metabolism or synthesis of amino acids also utilize PLP as prosthetic group, and have mechanisms involving a Schiff base linkage of the amino group to PLP.
NH
CO
P
OO
O
O
CH3
HC
H2
N
HC
H
+
R COO Enz Lys NH2
Amino acid-PLP Shiff base (aldimine)
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Amino acid Carbon skeleton• Amino acids lose their amino groups to form -keto
acids - the “carbon skeletons” of amino acids.
• The -keto acids undergo oxidation to CO2 and H2O or can be converted by gluconeogenesis into glucose, the fuel for brain.
• As in carbohydrate and fatty acid catabolism, the processes of amino acid degradation converge on the central catabolic pathways.
• The carbon skeletons of most amino acids find their way to the citric acid cycle.
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central catabolic pathways• In cytoplasm (1)• In mitochondria (2, 3 & 4)
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Others e.g;GluconeogenesisGlycogenesis
Amino Acid Carbon Skeletons
Amino acids are grouped into 2 classes, based on whether or not their carbon skeletons can be converted to glucose: glucogenic ketogenic.
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Carbon skeletons of glucogenic amino acids are degraded to:
pyruvate, or
a 4-C or 5-C intermediate of Krebs Cycle. These are precursors for gluconeogenesis.
Glucogenic amino acids are the major carbon source for gluconeogenesis when glucose levels are low.
They can also be catabolized for energy, or converted to glycogen or fatty acids for energy storage.
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Summary of Amino Acid Catabolism
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Gluconeogenesis: synthesis
of Glucose from none carbohydrates
Gluconeogenesis is not just glycolysis in reverse--the enzymes in green print catalyze irreversible reactions
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Carbon skeletons of ketogenic amino acids are degraded to: acetyl-CoA, or acetoacetate.
Acetyl CoA, & its precursor acetoacetate, cannot yield net production of oxaloacetate, the gluconeogenesis precursor. For every 2-C acetyl residue entering Krebs Cycle, 2 C leave as CO2. Carbon skeletons of ketogenic amino acids can be catabolized for energy in Krebs Cycle, or converted to ketone bodies or fatty acids. They cannot be converted to glucose.
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The 3-C -keto acid pyruvate is produced from alanine, cysteine, glycine, serine, & threonine.Alanine deamination via Transaminase directly yields pyruvate.
a l a n i n e - k e t o g l u t a r a t e p y r u v a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C H 3
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C H 3
C
C O O
O + +
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Serine is deaminated to pyruvate via Serine Dehydratase.
Glycine, which is also product of threonine catabolism, is converted to serine by a reaction involving tetrahydrofolate (to be discussed later).
H O C H 2
HC C O O
N H 3+
C C O O
OH 2 O N H 4+
C C O O
N H 3+
H 2 C H 3 C
H 2 O
s e r i n e a m i n o a c r y l a t e p y r u v a t e S e r i n e D e h y d r a t a s e
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The 4-C Krebs Cycle intermediate oxaloacetate is produced from aspartate & asparagine. Aspartate transamination yields oxaloacetate. Aspartate is also converted to fumarate in Urea Cycle. Fumarate is converted to oxaloacetate in Krebs cycle.
a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C O O
C H 2
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C O O
C H 2
C
C O O
O + +
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Asparagine loses the amino group from its R-group by hydrolysis catalyzed by Asparaginase.
This yields aspartate, which can be converted to oxaloacetate, e.g., by transamination.
C
C H 2
H C
C O O
N H 3+
OH 2 N
C O O
C H 2
H C
C O O
N H 3+
H 2 O N H 4+
a s p a r a g in e a s p a r t a t e A s p a r a g in a s e
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The 4-C Krebs Cycle intermediate succinyl-CoA is produced from isoleucine, threonine, valine, & methionine.
Propionyl-CoA, an intermediate on these pathways, is also a product of -oxidation of fatty acids with an odd number of C atoms.
NB: Isoleucine and Valine are branched-chain amino acids
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The branched chain amino acids initially share a common pathway.
Branched Chain -Keto Acid Dehydrogenase (BCKDH) is a multi-subunit complex.
Genetic deficiency of BCKDH is called Maple Syrup Urine Disease (MSUD).
High concentrations of branched chain keto acids in urine give it a characteristic odor.
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MSUD• People with MSUD have a mutation that
results in a deficiency for one of the 6 proteins that make up this complex.
• Therefore, they can't break down leucine, isoleucine, and valine.
• They end up with dangerously high levels of these amino acids in their blood, causing the rapid degeneration of brain cells and death if left untreated.
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Propionyl-CoA is carboxylated to methylmalonyl-CoA. A racemase yields the L-isomer essential to the subsequent reaction.Methylmalonyl-CoA Mutase catalyzes a molecular rearrangement: the branched C chain of methylmalonyl-CoA is converted to the linear C chain of succinyl-CoA.
C C H 3
C S-C o A
O
C C H 3
C S-C oA
O
C O O
C
C S-C oA
O
C O O
C C
C O O
C
C
O
H
H
CoA-S H
HH HH
H
H
H
H
H CO 3
A T P A D P
+ P i
p r o p i o n y l - C o A D - m e t h y l m a l o n y l - C o A L - m e t h y l m a l o n y l - C o A s u c c i n y l - C o A
P r o p i o n y l - C o A M e t h y l m a l o n y l - C o A M e t h y l m a l o n y l - C o A C a r b o x y l a s e ( B i o t i n ) R a c e m a s e M u t a s e ( B 1 2 )
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bk
The 5-C Krebs Cycle intermediate -ketoglutarate is produced from arginine, glutamate, glutamine, histidine, & proline. Glutamate deamination via Transaminase directly yields -ketoglutarate.
a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C O O
C H 2
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C O O
C H 2
C
C O O
O + +
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Glutamate deamination by Glutamate Dehydrogenase also directly yields -ketoglutarate.
O O CH 2C
H 2C C C O O
O
+ N H 4+
N A D (P )+
N AD(P)H
O O CH 2C
H 2C C C O O
N H 3+
Hglu tam ate
-ke toglu tara te
G lu tam ate D ehydrogenase
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Histidine is first converted to glutamate. The last step in this pathway involves the cofactor tetrahydrofolate. Tetrahydrofolate (THF), which has a pteridine ring, is a reduced form of the B vitamin folate.
NH
HNN
HN
H2N H
H
H
CH2
HNO C
O
NH
CH
COO
CH2
CH2
COO
Tetrahydrofolate (THF)
pteridine -aminobenzoate glutamate
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In the pathway of histidine degradation, N-formiminoglutamate is converted to glutamate by transfer of the formimino group to THF, yielding N5-formimino-THF.
HC C CH2
HC COO
NH3+N NH
CH
OOCHC CH2 CH2 COO
HN NHCH
OOCHC CH2 CH2 COO
NH3+
THF
N 5-formimino-THF
NH4+
H2O
H2O
histidine
N-formimino-glutamate
glutamate GKM/MUSOM/MSB206:.REP.URI.SYS.201404/13/23
Aromatic Amino Acids
Aromatic amino acids phenylalanine & tyrosine are catabolized to fumarate and acetoacetate.
Hydroxylation of phenylalanine to form tyrosine involves the reductant tetrahydrobiopterin.
Biopterin, like folate, has a pteridine ring.
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Overall the reaction is considered a mixed function oxidation, because one O atom of O2 is reduced to water while the other is incorporated into the amino acid product.
CH2 CH COO
NH3+
CH2 CH COO
NH3+
HO
phenylalanine
tyrosine
O2 + tetrahydrobiopterin
H2O + dihydrobiopterin
Phenylalanine Hydroxylase
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deamination via transaminase) accumulate in blood & urine.Mental retardation results unless treatment begins immediately after birth. Treatment consists of limiting phenylalanine intake to levels barely adequate to support growth. Tyrosine, an essential nutrient for individuals with phenylketonuria, must be supplied in the diet.
Transaminase Phenylalanine Phenylpyruvate (Phenylketone) Phenylalanine Deficient in Hydroxylase Phenylketonuria
Tyrosine Melanins
Multiple Reactions
Fumarate + Acetoacetate
Genetic deficiency of Phenylalanine Hydroxylase leads to the disease phenylketonuria.Phenylalanine & phenylpyruvate (the product of phenylalanine
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High [phenylalanine] inhibits Tyrosine Hydroxylase, on the pathway for synthesis of the pigment melanin from tyrosine.
Transaminase Phenylalanine Phenylpyruvate (Phenylketone) Phenylalanine Deficient in Hydroxylase Phenylketonuria
Tyrosine Melanins
Multiple Reactions
Fumarate + Acetoacetate
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Tyrosine is a precursor for synthesis of melanins and of epinephrine and norepinephrine.
Individuals with phenylketonuria have light skin & hair color.
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H3C SH2C
H2C
HC COO
NH3+CH2
+
O
OHOH
HH
HH
AdenineH3C S
H2C
H2C
HC COO
NH3+
HSH2C
H2C
HC COO
NH3+
SH2C
H2C
HC COO
NH3+CH2
O
OHOH
HH
HH
Adenine
methionine
homocysteine
S-adenosyl-methionine(SAM)
S-adenosyl-homocysteine
ATP PPi + Pi
adenosine H2O
acceptor
methylated acceptorTHF
N5-methyl-THF
Methionine S-Adenosylmethionine by ATP-dependent reaction.
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H3C S CH2
CH2
HC COO
NH3+CH2
+
O
OHOH
HH
HH
Adenine
HS CH2
CH2
HC COO
NH3+
S CH2
CH2
HC COO
NH3+CH2
O
OHOH
HH
HH
Adenine
homocysteine
S-adenosyl-methionine (SAM)
S-adenosyl-homocysteine
adenosine H2O
Acceptor (THF)
methylated acceptor)
SAM is a methyl group donor in synthetic reactions.
The resulting S-adenosylhomocysteine is hydrolyzed to homocysteine.
Homocysteine may be catabolized via a complex pathway to cysteine & succinyl-CoA.
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Or methionine may be regenerated from homocysteine by methyl transfer from N5-methyl-tetrahydrofolate, via a methyltransferase enzyme that uses B12 as prosthetic group.
The methyl group is transferred from THF to B12 to homocysteine.
Another pathway converts homocysteine to glutathione.
H3C S CH2
CH2
HC COO
NH3+
HS CH2
CH2
HC COO
NH3+
methionine
homocysteine
THF
N5-methyl-THF
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The Catabolism of Lysine and Tryptophan are quite complex and will not be discussed in detail
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The End
Thanks for your attention
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