Amino Acid Metabolism

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Amino Acid Metabolism
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Metabolism

• consists of both catabolic and anabolic processes
• Catabolism comprises all processes, in which
  complex molecules are broken down to simple ones
• Anabolism means any constructive metabolic
  process by which organisms convert substances
  into other components required for the organism's
  chemical architecture

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Introduction

• Amino acids (AAs) are the building blocks of
  proteins (precursors for proteins) (????)
• Energy metabolites (17.9KJ/g Pr)When degraded,
  amino acids produce glucose/carbohydrates and
  ketone bodies(????)
• Precursors for many other biological N-containing
  compounds ,Involved as direct neurotransmitters
  or as precursors to neurotransmitters, eg.
  (??????)
• - Tyrosine gives DOPA and dopamine
• - Precursors to peptide hormones and
  thyroid hormone
• - Precursors to histamine, NAD and other
  compounds of biological importance

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some major biological functions

• Detoxification of drugs, chemicals and metabolic
  by-products
• Excess dietary AAs are neither stored nor
  excreted. Rather, they are converted to common
  metabolic intermediates

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outline

• 1. The nutrition of protein
• 2. The digestion?absorption and putrefaction of
  protein
• 3. The general metabolism of AA
• 4. Metabolism of ammonia
• 5. Single AA metabolism

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Section 1 The nutrition of protein

• Nitrogen balance
• The requirements
• Classification of amino acids

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Nitrogen balance

• Zero or total nitrogen balance
• the intake the excretion
  (adult)
• Amount of nitrogen intake is equal to
  the amount of nitrogen excreted is zero or total
  nitrogen balance
• Positive nitrogen balance
• the intake gt the excretion
• during pregnancy, infancy, childhood and
  recovery from severe illness or surgery
• Negative nitrogen balance
• the intake lt the excretion
• following severe trauma, surgery or
  infections. Prolonged periods of negative balance
  are dangerous and fatal if the loss of body
  protein reaches about one-third of the total body
  protein

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The requirements

• The requirements of protein for the health the
  minimal requirement of protein is 3050 gram for
  the adult
• Advice 80 gram/day (??????) ? ? ?

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Classification of amino acids

• non-essential amino acids
• - can be synthesized by an organism
• - usually are prepared from precursors in
  1-2 steps
• Essential amino acids
• - cannot be made endogenously
• - must be supplied in diet
• eg. Leu, Phe..

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Nonessential Essential
Alanine Arginine
Asparagine Histidine
Aspartate Valine
Cysteine Lysine
Glutamate Isoleucine
Glutamine Leucine
Glycine Phenylalanine
Proline Methionine
Serine Threonine
Tyrosine Tyrptophan
????????
The amino acids Arg, His are considered
conditionally essential for reasons not
directly related to lack of synthesis and  they
are essential  for growth only
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nutritional value

• Legumes(??) poor in Trp, but rich in Lys
• Cereals (??) poor in Lys, but rich in
  Trp
• Mutual complementation of amino acids
• Protein deficiency-kwashiorkor, generalized edema
  and liver enlargement, abdomen bulged
• Suggestion the combined-action of protein in
  diet

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Section 2The digestion?absorption and
putrefaction of protein
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Digestive Tract of protein

• Proteins are generally too large to be absorbed
  by the intestine and therefore must be hydrolyzed
  to the amino acids
• The proteolytic enzymes responsible for
  hydrolysis are produced by three different
  organs the stomach?pancreas and small intestine
  (the major organ)

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Stomach

• HCl (parietal cells ) and Pepsinogen (chief cells
  )
• The pH of gastric juice is around 1.0. Food is
  retained in the stomach for 2-4 hrs
• HCl kills microorganisms, denatures proteins, and
  provides an acid environment for the action of
  pepsin
• Autocatalysis pepsinogen is converted to active
  pepsin(Pepsin A) by HCl
• Pepsin coagulates milk in presence of Ca2 ions

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Pancreas and small intestine

• Endopeptidase (pancreas)
• Trypsin carbonyl of arg and lys
• Chymotrypsin carbonyl of Trp, Tyr,
  Phe, Met, Leu
• Elastase carbonyl of Ala, Gly, Ser
• Exopeptidase (pancreas)
• Carboxypeptidase Aamine side of Ala,
  Ile, Leu, Val
• Carboxypeptidase B amine side of Arg,
  lys
• Aminopeptidase (small intestine)
• cleaves N-terminal residue of
  oligopeptidaes
• Dipeptidase (small intestine)

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carboxypepidase
endopeptidase

aminopeptidase
dipeptidase
Amino acids
1/3
Amino acids
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absorption

• There is little absorption from the stomach apart
  from short- and medium- chain fatty acids and
  ethanol
• Under normal circumstances, the dietary proteins
  are almost completely digested to their
  constituent amino acids, and these end products
  of protein digestion are rapidly absorbed from
  the intestine into the portal blood

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• Amino acids are transported through the brush
  border by the carrier protein and it is an active
  transport
• The classification of carrier protein
• aciditic basic neutral and
  gly-carrier
• 2. ?-glutamyl cycle (?-??????)
• 3. The bi-and tri- peptidase carrier system in
  the intestinal mucosa cell

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The mechanism of AAs absorption
K
Na
outer

Member
innner
ADPPi
K
Na
intestine
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• -gltamyl
• cyclotransferase

?-glutamyl cycle
5oxoprolinase
membrane
GCS synthetase
ADPPi
GSH synthetase
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Putrefaction

• Putrefaction the process of decay of
  un-digestive and un-absorbed protein and the
  products by bacterial, fungal in the intestine
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1.Amines(?)
False neurotransmitter are similar with
neurotransmitter
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2. Ammonia(?)-1
A. some amino acids are degraded by the in the
intestine bacteria
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2. Ammonia(?)-2
B. urea from the blood to the intestine with
resultant increased diffusion of NH3 into the
intestinal
Urea enzyme
Urea in blood
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• 3. The other toxic material
• phenol, indole, sulfureted hydrogen

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Section 4 The general metabolism of AA

• Protein and amino acid turnover
• Degradation of Amino Acids (Fate of amino group)
• The metabolism of a-ketoacid

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Protein and amino acid turnover
1-2
75-80
T1/2 ? (half time)
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introduction
1. Proteins constantly being synthesized and
degraded - need constant supply of amino acids
- need to degrade to protect from abnormal
proteins - regulate cellular processes
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2. Degraded by ubiquitin label -
Ubiquitin binds lysine side chain -
Targets for hydrolysis by proteosomes in cytosol
and nucleus - ATP required 3.
Degraded by the protease and the peptidase in the
Lysosome - non- ATP required -
the hydrolysis-selective are bad
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The ubiquitin degradation pathway
ATP AMPPPi
E3
E2-SH
(ubiquitin)
E1-SH
E2-SH
E1-SH
E1activiting enzyme E2carrier protein
E3ligase
ubiquitinational protein
ATP
19S regulate substrate
ATP
20S Proteasome
??-????
26S Proteasome
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Amino acid pool
Overview of the protein metabolism
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Degradation of Amino Acids -
Reactions in amino acid metabolism
Amino acid
Carboxylic group
Amino group
R group
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introduction

• Free amino acids are metabolized in identical
  ways, regardless of whether they are released
  from dietary or intracellular proteins
• The metabolism of the resulting amino group and
  nitrogen excretion are a central part of nitrogen
  metabolism

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FATE OF AMINO GROUP
DEAMINATION A. Transamination
B. Oxidative deamination
C. purine nucleotide cycle
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A. Transamination

• Transamination by Aminotransferase
  (transaminase)
• always involve PLP coenzyme (pyridoxal phosphate)
• reaction goes via a Schiffs base intermediate
• all transaminase reactions are reversible

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Aminotransferases

• Aminotransferases can have specificity for the
  alpha-keto acid or the amino acid
• Aminotransferases exist for all amino acids
  except proline and lysine
• The most common compounds involved as a
  donor/acceptor pair in transamination reactions
  are glutamate and a-ketoglutarate, which
  participate in reactions with many different
  aminotransferases
• to an alpha-keto acid ? alpha-amino acid

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Transamination
aminotransferases
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ALT and AST are components of a "liver
function test". Levels increase with damage to
liver (cirrhosis, hepatitis) or muscle (trauma)
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The mechanism of transamination
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Transamination

• Interconversion of amino acids
• Collection of N as glu
• Provision of C-skeletons for catabolism

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B. Oxidative Deamination

• L-glutamate dehydrogenase (in mitochondria)
• Glu NAD (or NADP) H2O ? NH4
  a-ketoglutarate NAD(P)H H
• Requires NAD or NADP as a cofactor
• Plays a central role in AA
  metabolism ?

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urea cycle
?
It is inhibited by GTP and ATP, and
activated by GDP and ADP
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Combined Deamination
?
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Combined deamination
Transamination Oxidative Deamination
The major pathway !!!
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NH3
AA
Asp
IMP
?-Keto glutarate
H2O
AST
aminotransferases
C. purine nucleotide cycle
AMP
?-Keto acid
Oxaloacetate
fumarate
malate
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The metabolism of a-ketoacid

• Biosynthesis of nonessential amino acids
• TCA cycle member amino acid ?a-keto
  acid nonessential amino acid
• A source of energy (10) ( CO2H2O )
• Glucogenesis and ketogenesis

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Classification of amino acids

• glucogenic amino acid are converted into
  either pyruvate or one of the citric acid cycle
  intermediates
• (a-ketoglutarate, succinyl CoA, fumarate or
  malate)
• ketogenic amino acid will be deaminated via
  Acetylc-CoA and thus can be made into a ketone
  body. such as Leucine and lysine
• glucogenic and ketogenic amino acid
  isoleucine, phenylalanine, tryptophan and
  tyrosine, threonine

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Ammonia is toxic, so cells need to get rid of
it..
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Sources
2. glutamine (glutaminase, kidney) 3. catabolism
from bacteria in intestine (two) 4. purine and
pyrimidine catabolism
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Metabolism of ammonia

• Fix ammonia onto glutamate to form glutamine and
  use as a transport mechanism
• Transport ammonia by alanine-glucose cycle and
  Gln regeneration
• Excrete nitrogenous waste through urea cycle
  

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Transport of ammonia

• alaninie - glucose cycle
• regenerate Gln

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Alanine-Glucose cycle

• In the liver alanine transaminase tranfers the
  ammonia to a-KG and regenerates pyruvate. The
  pyruvate can then be diverted into
  gluconeogenesis. This process is refered to as
  the glucose-alanine cycle

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Gln regeneration
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Urea synthesis

• Synthesis in liver (Mitochondria and cytosol)
• Excretion via kidney
• To convert ammonia to urea for final excretion

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The urea cycle1932 by Hans Krebs and Kurt
Henseleit as the first metabolic cycle elucidated
arginase
Ornithine cycle
Krebs-henseleit cycle
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1
OCT
Mitochondria
2
(???)
(???)
5
3
cytosol
4
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UREA CYCLE (liver)
1. Overall Reaction NH3 HCO3
aspartate 3 ATP H2O ? urea fumarate 2 ADP
2 Pi AMP ppi 2. Requires 5 enzymes
2 from mitochondria and 3 from cytosol
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Regulation of urea cycle
1.Mitochondrial carbamoyl phosphate synthetase I
(CPS I)   CPS I catalyzes the first
committed step of the urea cycle    CPS I
is also an allosteric enzyme sensitive to
activation by N-acetylglutamate(AGA) which is
derived from glutamate and acetyl-CoA
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Increased rate of AA degradation requires higher
rate of urea synthesis    ? AA degradation ?
?glutamate concentration ? ?synthesis of
N-acetylglutamate ? ?CPS I activity ? ?urea
cycle efficiency
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• 2. All other urea cycle enzymes are controlled by
  the concentrations of their substrates
•     Deficiency in an E ? ?(substrate) ??rate
  of the deficient E
• 3. The intake of the protein in food
• the intake ? ??urea synthesis

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Hyper-ammonemia and the toxic of the ammonia

• Hyperammononemia ammonia intoxication -
  tremors, slurring of speech, and blurring of
  vision, coma/death
• Cause by cirrhosis of the liver or genetic
  deficiencies

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Section 5 Single AA metabolism

• Decarboxylation
• some neurotransmitters precursors for the
  decarboxylation of AAs production bioactive
  amines

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?-aminobutyric acid (GABA) Glutamine can be
decarboxylated in a similar PLP-dependent
fashion, outputting ?-aminobutyric acid
(neurotransmitter, GABA)
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Taurine
L-cysteine can be decarboxylated and converted
into outputted the taurine
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Histamine Histidine can also be
decarboxylated in a similar PLP-dependent
fashion, outputting the Histamine
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5-hydroxytryptophan (5-HT)
5-HT
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• Polyaminies (putrescine,spermidine,spermine)
• CO2 is then cleaved off in a PLP-dependent
  decarboxylation, resulting in the polyaminies
  (such as SAM, spermidine, spermine)

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One carbon unit metabolism

• One carbon unit some AA may turn into the group
  including one carbon in the AA metabolism
• Such as
• methl -CH3
  ??
• methylene -CH2
  ???/???
• methenyl -CH
  ???/???
• formyl -CHO
  ???
• formimino -CHNH
  ????
• But
  without CO2

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One carbon unit metabolism

• Folic acid / folate is an essential vitamin, and
  as such it cannot be synthesized within the human
  body
• Folate itself is not an active cofactor its
  doubly-reduced form, is tetrahydrofolate (THF)
• Tetrahydrofolate (THF) the carrier of one
  carbon
• accepts one carbon groups from amino acids

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Folate is reduced first by dihydrofolate
reductase (DHFR) into dihydrofolate (DHF),
oxidizing an NADPH in the process. DHFR, again
oxidizing an NADPHH/NADP, can also reduce DHF
into THF
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(Folate)
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THF biosynthetic pathways

• There are three important biosynthetic precursors
  synthesized from THF
• N5,N10-methylene-THF
• N5-methyl-THF
• N10-formyl-THF
• N5,N10-methylene-THF acts in a central processing
  role in the synthesis of all of these enzymes in
  order to synthesize either of the other two, one
  must first produce N5,N10-methylene-THF

N5,N10??????
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biosynthesis methionine
N5-methyl-THF
Histidine
THF
biosynthesis thymidylate
N5,N10-methylene-THF
N5,N10??????
NAD
THF
N5,N10-methenyl-THF
N5,N10??????
Glycine cataboase
H2O
biosynthesis purines
NADH
N10-formyl-THF
Tryptophan
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• Sulfanilamide (??) is a compound that the human
  body can create. It is a close analog to PABA,
  and it has the effect of stopping folate
  synthesis in bacteria. There are a wide variety
  of sulfa drugs (????) based on PABA analogs
  such as sulfanilamide
• Trimethoprim (TMP) and pyrimethamin(???), on the
  other hand, are DHFR inhibitors. Because
  bacterial DHFR is structurally simpler than human
  DHFR, these two drugs have a more drastic effect
  on bacteria than they do on us

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Metabolism of sulfur-containing amino acids
cysteine
cystine
Met
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Methionine Catabolism

• The principal fates of methionine are
  incorporation into polypeptide chains(protein
  synthesis), and use in the production of
  a-ketbutyrate and cysteine via S-adenosyl
  methionine (SAM)

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S-adenosyl methionine (SAM)

• is a powerful methylating agent (in the
  methylating gene regulation of DNA and RNA, It is
  constantly regenerated in a cyclical) with uses
  in many biochemical pathways
• formed from ATP and Met

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S-adenosylmethionine (SAM)

• methyl group is donated to form several products
• norepinephrine --gt epinephrine
• gamma-butyric acid --gt
  carnitine
• guanidinoacetate --gt
  creatine
• (???)

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Met cycle
FH4 is produced!!!
SAHH
S-????????
??????
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N5,N10 -methylene- FH4
NADH
Carbon donors (serine, glycine) combine with THF
NAD
FH4
N5-CH3-FH4
Only one!
??????
homocysteine
methionine
vitamin B12
ATP
adenosine
FH4 and Met cycle
all 3 phosphate groups are lost!
PPi Pi
H2O
s-adenosylmethionine
s-adenosylhomocysteine
S-????????
methyl group donated to biological substrate,
e.g. norepinephrine
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Regulation of the Met metabolism

• If methionine and cysteine are present in
  adequate quantities, SAM accumulates and is a
  positive effector on cystathionine
  synthase(??????), encouraging the production of
  cysteine and a-ketobutyrate
• If methionine is scarce, SAM will form only in
  small quantities, thus limiting cystathionine
  synthase activity. Under these conditions
  accumulated homocysteine is remethylated to
  methionine, using N5-methyl THF and other
  compounds as methyl donors

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(???)
Creatine metabolism
H2O
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Metabolism of Cystine and Cysteine
2H
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Cysteine Catabolism

• The pathway is catalyzed by a liver desulfurase
  and produces pyruvate and hydrogen sulfide (H2S)
• The enzyme sulfite oxidase uses O2 and H2O to
  convert HSO3- to sulfate (SO4-) , and H2O2. The
  resultant sulfate is used as a precursor for the
  formation of 3'-phosphoadenosine-5'-phosphosulfate
  , PAPS

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Metabolism of aromatic amino acid

Phenylalanine
Tyrosine
tryptophan
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Phenylalanine metabolism
Phenylalanine hydroxylase
THF
DHF
NADP
NADPHH
tyrosine
phenylalanine
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Tyr metabolism
1. Catecholamine/melanin
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Tyr Catabolism
???
??????
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???
????
????
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Phenylalanine Hydroxylase PKU

• Phenylketonuria (PKU) lack of phenylalanine
  hydroxylase
• - cant hydroxylate phenylalanine to
  tyrosine

Phe 0.1 mM normally ? 1.2 mM in PKU

1 in 20,000 homozygous 1 in 150 heterozygous
IQ study 53 ? 93
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• People with phenylketonuria must avoid excess
  phenylalanine, but both tyrosine and
  phenylalanine are essential amino acids, so they
  shouldnt exclude it completely or brain
  disorders with result

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The Metabolism of Branched Chain Amino Acids
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• Branched-chain amino acids (BCAAs)
• isoleucine, leucine and valine
• The catabolism of all three BCAAs initiates in
  muscle and yields NADH and FADH2 which can be
  utilized for ATP generation

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Isoleucine / Leucine / Valine
?-ketoglutarate
transamination
A metabolic block here causes maple syrup urine
disease
glutamate
?-keto acid
NAD, CoASH
?-keto acid dehydrogenase
NADH, CO2
gluconeogenesis
?-keto-S-CoA
(if Leu or Lys, only this path can be used)
ketogenesis
Proprionyl-CoA
Succinyl-CoA
??CoA
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• Most nitrogen metabolism pathways are very
  complex
• require many steps
• require input of ATP and NADPH
• are regulated by feedback inhibition
  mechanisms (allosteric)