Top Banner
Amino Acid Metabolism
34

Amino Acid Metabolism - profkatz.comprofkatz.com/.../07/CH2210-Lecture-27-Amino-acid-metabolism-36-copy.pdf · Amino acids that can be converted into pyruvate, α-ketoglutarate, succinyl-CoA,

Feb 14, 2020

Download

Documents

dariahiddleston
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
  • Amino Acid Metabolism

  • Fate of Dietary ProteinDietary protein

    Denatured and partially hydrolyzed protein (large polypeptides)

    Stomach: HCl, pepsin

    Amino acids and dipeptides

    small intestine: proteases

    Amino acids in bloodstream

    intestinal lining: proteases

  • Overview of Amino Acid Catabolism

    H3N C C

    H

    RO

    OThe catabolism of amino acids takes

    place in three stages:

    1) Removal of the amino group, leaving the carbon

    skeleton of the amino acid.

    2) Breakdown of the carbon skeletons to a glycolytic

    intermediate, citric acid cycle intermediate, or acetyl-S-CoA.

    3) Oxidation of these intermediates to CO2 and H2O

    with the production of ATP.

    small pieces

    CO2 H2O

    ATP

  • aspartate oxaloacetateNH3

    C C

    H

    O

    O

    CH2C

    O

    OO

    C CO

    O

    CH2C

    O

    O

    NH4+

    glutamine

    α-ketoglutarate

    NH3

    C C

    H

    O

    OCH2C

    O

    H2NCH2

    O

    C CO

    OCH2C

    O

    OCH2

    2 NH4+

    alanine pyruvateNH3

    C C

    H

    H3CO

    O

    O

    C CH3CO

    ONH4+

    Representative Pathways of Amino Acid Catabolism

    phenylalanine

    fumarate

    acetoacetate

    NH3

    C C

    H

    CH2O

    OC

    CCCC C

    CH2 CC O

    OH3C

    O

    CH CCHO

    OC

    O

    O

    HH

    H

    H H

    NH4+CO2

    +

  • citrateoxaloacetate

    fumarate

    succinate

    succinyl-CoA

    malate isocitrate

    aconitate

    α-ketoglutarate

    oxalosuccinate

    pyruvate acetyl-CoA

    alanine

    aspartate

    glutamine

    phenylalanine

    phenylalanine

    acetoacetyl-CoA

  • citrateoxaloacetate

    fumarate

    succinate

    succinyl-CoA

    malate isocitrate

    aconitate

    α-ketoglutarate

    oxalosuccinate

    pyruvate acetyl-CoA acetoacetyl-CoA ketones

    gluconeogenesis

  • The α-keto acids derived from catabolism of many amino acids are intermediates in glycolysis and the citric acid cycle.

    The α-keto acids derived from catabolism of some amino acids are broken down to acetyl-CoA or acetoacetyl-CoA and are

    oxidized by the citric acid cycle or converted to ketone bodies.

    Amino acids that can be converted into pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate can be converted into glucose by gluconeogenesis and are said to be glucogenic.

    These α-keto acids may, therefore, replenish citric acid cycle intermediates.

    These amino acids are said to be ketogenic.

    Glucogenic and Ketogenic Amino Acids

  • citrate

    oxaloacetate

    fumarate

    succinate

    succinyl-CoA

    malate isocitrate

    aconitate

    α-ketoglutarate

    oxalosuccinate

    pyruvate acetyl-CoA acetoacetyl-CoA ketones

    gluconeogenesis

    alanine glycine threonine cysteine serine

    aspartate asparagine

    glutamate glutamine arginine histidine

    proline isoleucine methionine

    valine threonine

    aspartate phenylalanine

    tyrosine

    leucine lysine phenylalanine tyrosine

    tryptophan

    leucine isoleucine tryptophan threonine

    Glucogenic and Ketogenic Amino Acids

  • The Fate of the Amino Group of Amino Acids

    The extraction of the amino group from amino acids must be done in such a way so as not to increase

    blood ammonium ion levels above normal values.

    NH4+

    The normal concentration of NH4+ ion in the blood is 3.0 x 10-5 to 6.0 x 10-5 M.

    Nitrogen is present in the bloodstream in the form of ammonium ions:

    Above these concentrations (hyperammonemia) coma may result.

  • The pathway for the extraction of amino groups from amino acids consists of three phases:

    1) Conversion of amino groups from all amino acids into a single product, glutamate, by transamination.

    2) Conversion of glutamate into α-ketoglutarate by oxidative deamination, releasing NH4+ .

    3) Conversion of NH4+ into urea, which is extracted from the blood by the kidneys and excreted.

    The Fate of the Amino Group of Amino Acids

  • CHC

    O

    O

    CHC

    O

    O

    CHC

    O

    O

    CH2C

    O

    O NH3

    aspartate

    fumarate

    α-amino acid α-keto acid

    α-keto glutarate glutamate

    NAD+, (NADP)+,

    H2O

    NADH, (NADPH),

    NH4+ CO2

    Urea cycleH2N

    CNH2

    O

    Urea

    transamination

    oxidative deamination

  • CHC

    O

    O

    CHC

    O

    O

    CHC

    O

    O

    CH2C

    O

    O NH3

    aspartate

    fumarate

    α-amino acid α-keto acid

    α-keto glutarate glutamate

    CO2

    Urea cycleH2N

    CNH2

    O

    Urea

    transamination

    oxidative deamination

    NAD+, (NADP)+,

    H2O

    NADH, (NADPH),

    NH4+

  • Transamination

  • The first step in amino acid metabolism is the removal of the amino group by transamination followed by oxidative deamination.

    A transamination reaction can be represented as:

    Transamination

    Amino acid1 + α-ketoacid2 ⇔ α-ketoacid1 + amino acid 2

    HC

    CO O

    R1

    NH3 C

    CO O

    R2

    O C

    CO O

    R1

    O HC

    CO O

    R2

    NH3++

    Transamination reactions occur in all cells.

    The enzymes responsible for transaminations are called transaminases or amino transferases.

  • Most transaminases are specific for α-ketoglutarate but are less specific for the amino acid.

    This means that the amino groups of almost all amino acids end up on glutamic acid.

    L-amino acid + α-ketoglutarate ⇔ α-ketoacid + L-glutamate

    C

    CO O

    R

    H3N C

    CO O

    CH2

    O C

    CO O

    R

    O C

    CO O

    CH2

    H3N++H

    CH2

    CO O

    CH2

    CO O

    H

    Transamination

  • One exception to this rule is in skeletal muscle, where transaminases use pyruvate as the amino acceptor, producing alanine as the product.

    L-amino acid + pyruvate ⇔ α-ketoacid + L-alanine

    C

    CO O

    R

    H3N C

    CO O

    CH3

    O C

    CO O

    R

    O C

    CO O

    CH3

    H3N++H H

    Another example of a specific transaminase reaction is aspartate transaminase:

    L-aspartate + α-ketoglutarate ⇔ oxaloacetate + L-glutamate

    C

    CO O

    CH2

    H3N C

    CO O

    CH2

    O C

    CO O

    O C

    CO O

    H3N++H H

    CO O

    CH2

    CO O

    CH2

    CH2

    CO O

    CH2

    CO O

  • Transamination in Diagnostic Laboratory Medicine

    The presence of alanine transaminase (glutamate:pyruvate transaminase or GPT) and aspartate transaminase

    (glutamate:oxaloacetate transaminase, or GOT) in the bloodstream, above a certain base level, may indicate liver damage.

    Serum GPT and GOT (SGPT and SGOT) tests measure the

    severity and stage of liver damage.

  • Vitamin B-6 as a Coenzyme

    All transaminases require the coenzyme pyridoxal phosphate (derived from pyridoxine, vitamin B-6):

    N

    C

    HO

    H3C

    CH2

    O H

    H

    O P

    O

    O

    O

    pyridoxal phosphate

    Vitamin preparations may contain the precursor to pyridoxal phosphate in different forms:

    N

    C

    HO

    H3C

    CH2OH

    O H

    HN

    CH2NH2

    HO

    H3C

    CH2OH

    H

    N

    CH2OH

    HO

    H3C

    CH2OH

    H

    pyridoxalpyridoxamine

    pyridoxine

  • The Transaminase Mechanism

    N

    CHO

    H3C

    CH2

    N H

    H

    O PO32-

    CHCO

    O

    R

    CC

    O O

    RH3N H

    H2O

    CC

    O O

    RO

    H2O

    pyridoxamine phosphate

    N

    CHO

    H3C

    CH2

    N H

    H

    O PO32-

    CCOO

    R

    H

    N

    CHO

    H3C

    CH2

    O H

    H

    O PO32-

    N

    CHO

    H3C

    CH2

    H3N

    H

    O PO32-

    H

    H

    pyridoxal phosphate

  • CC

    O O

    CH2

    O

    CH2C

    O O

    CC

    O O

    H3N HCH2CH2C

    O O

    NH4++

    NADH (NADPH)NAD+ (NADP+) H2O

    glutamate dehydrogenase

    Oxidative Deamination

    This reaction is reversible and provides a mechanism for

    1) generating ammonium ion for excretion as urea

    2) generating a-ketoglutarate

    3) assimilating ammonium ion for use in other metabolic pathways in the liver and kidneys

    glutamate α-ketoglutarate

    DOES NOT ENTER THE

    BLOODSTREAM

  • Amino Group and Ammonia Transport

    Amino groups collected in extrahepatic tissues in the form of glutamate must be packaged in a non-toxic form for

    transport through the blood to the liver.

    Glutamate, itself, cannot pass through the cell membranes.

    Two different transport forms are used:

    1) Production of glutamine in most cell types

    2) Production of alanine in muscle cells.

  • Glutamine Production

    + NH4+ + ATP + H+ + ADP +Pi

    CC

    O O

    H3N HCH2CH2C

    O O

    CC

    O O

    H3N HCH2CH2C

    O NH2

    glutamine synthetase

    In almost all cell types, glutamine synthetase catalyzes the formation of glutamine from glutamate:

    The glutamine, thus formed, is electrically neutral, nontoxic, and can pass through the cell membranes into the blood. The concentration of glutamine in the blood is higher than any other amino acid.

    L-glutamate L-glutamine

  • Amino Group and Ammonia Transport

    Once in the liver, the reverse reaction takes place and glutamine is deaminated to ammonium ion and glutamate.

    CC

    O O

    H3N HCH2CH2C

    O NH2

    CC

    O O

    H3N HCH2CH2C

    O O

    + H2O + NH4+

    Glutamate can then be oxidatively deaminated to ammonium ion and α-ketoglutarate.

    L-glutamate L-glutamine

    CC

    O O

    H3N HCH2CH2C

    O O

    L-glutamate

    CC

    O O

    CH2

    O

    CH2C

    O O

    α-ketoglutarate

    + NAD+ + H2O + NADH + NH4+

    glutamate dehydrogenase

  • L-glutamate

    CC

    O O

    H3N HCH2CH2C

    O O

    CC

    O O

    CH2

    O

    CH2C

    O O

    α-ketoglutarate

    + NADP+ + H2O+ NADPH + NH4+

    glutamate dehydrogenase

    In active muscle cells, large quantities of ammonium ion are produced. After two reactions, alanine is formed. Like glutamine, alanine is

    electrically neutral:

    CC

    O O

    CH2

    OCC

    O O

    H3N H

    CH2C

    O O

    CH2CH2C

    O O

    CC

    O O

    CH3

    O CC

    O O

    CH3

    H3N++ H

    α-ketoglutarateL-glutamate

    pyruvate alanine

  • CC

    O O

    CH3

    H3N HCC

    O O

    H3N HCH2CH2C

    O NH2

    Like glutamine, alanine is electrically neutral and readily traverses membranes and enters the blood stream.

    alanine

    L-glutamine

    In the liver, the combination of transamination and oxidative deamination reaction releases ammonium ions.

  • Glucose

    Pyruvate Lactate

    2 ATPGlucose

    Pyruvate

    Alanine

    Urea

    6 ATP

    4 ATP

    Alanine

    𝛂-amino acid

    𝛂-keto acid

    Liver Muscle tissue

    N

    Glucose/Alanine Cycle

  • Overall Reaction:

    NH4+ + HCO3- + 3 ATP + aspartate (-NH3+)

    urea + 2 ADP + AMP + 4 PO43- + fumarate

    Urea Cycle

  • CNH2H2N

    O

    C

    C

    O O

    HH3N

    CH2CH2CH2NH

    COH2N

    C

    C

    O O

    HH3N

    CH2CH2CH2NH3

    C

    C

    O O

    HH3N

    CH2CH2CH2NH

    CNH2H2N

    C

    C

    O O

    HH3N

    CH2CH2CH2NH

    CNH2N C

    CO O

    H

    CH2C

    O O

    arginine

    ornithine

    citrulline

    arginosuccinate

    urea

    Urea Cycle

  • ornithine

    arginine

    argininosuccinate

    citrulline

    citrulline

    ornithine

    urea

    glutamate

    glutamate a-ketoglutarate

    NH4+

    Pi

    mitochondria

    cytosolaspartate

    fumarate

    ATP

    AMP, 2Pi

    Urea Cycle

    carbamoyl phosphate

    HCO3-2 ATP

    2 ADP,Pi

    Urea Cycle

  • CNH2H2N

    O

    C

    C

    O O

    HH3N

    CH2CH2CH2NH

    COH2N

    C

    C

    O O

    HH3N

    CH2CH2CH2NH3

    C

    C

    O O

    HH3N

    CH2CH2CH2NH

    CNH2H2N

    C

    C

    O O

    HH3N

    CH2CH2CH2NH

    CNH2N C

    CO O

    H

    CH2C

    O O

    arginine

    ornithine

    citrulline

    arginosuccinate

    urea

    C

    CO O

    H

    CH2

    CO O

    H3N

    C

    C

    H

    H

    C

    O

    O

    C O

    O

    aspartate

    fumarate

    COH2N

    P O

    O

    O

    O

    P O

    O

    O

    NH4+ , HCO3-, 2ATP

    2ADP,Pi

    carbamoyl phosphate

    Urea Cycle

  • Overall Reaction:

    NH4+ + HCO3- + 3 ATP + aspartate

    urea + 2 ADP + AMP + 4 PO43- + fumarate

    Urea Cycle

  • ATP, via TCA

    cycle

    Glucose

    Fatty acids

    Ketones

    Urea

    NH4+

    Amino acid pool

    Dietary protein pyruvate, acetyl-CoA,

    acetoacetate, TCA cycle

    intermediates

    Liver proteins Plasma proteins

    Other nitrogen-containing

    compounds

  • aquatic invertebrates,

    bony fishes, crocodiles

    mammals, sharks, some bony fishes,

    turtles

    birds, insects, reptiles, land gastropods

    scorpions, spiders

    NH3H2N

    C O

    H2N

    CHN

    CNH

    C

    C

    NH

    C

    HN

    O

    O

    O

    Nitrogen excretion products for various organisms

    Water solubility

    Energy needed to produce

    -NH2 groups

    CHN

    CN

    C

    C

    NH

    CH

    N

    H2N

    O

    ammonia urea uric acid guanine

  • H2N

    C O

    H2N

    NH3

    H2N

    C O

    H2N

    NH3