Lecture 7: Amino Acid Metabolism, Nitrogen Metabolism & Urea Cycle

amino acid metabolism involves removing the _______ to use in other ______, then excrete the ______

carbon skeleton; pathways; nitrogen

we can’t store _____ in the body, so we must _____ or _____ it

nitrogen; excrete; recycle

removing the carbon skeleton means replacing the ______ group with a ______ group

amino; ketone

the carbon skeletons produced from amino acid metabolism can then be used as intermediates in ______ and ______

glycolysis; TCA cycle

nitrogen is removed from amino acids as _______, which is excreted from the body in the ______ cycle

ammonia; urea

what are the 4 different ways that carbon skeletons can be used for?

energy production; gluconeogenesis; FA synthesis; Ketone body synthesis

when the amino acid carbon skeleton is used for energy production; it is oxidized down into _________

CO2 and H2O

when the amino acid carbon skeleton is used for fatty acid synthesis; it is turned into ______

acetyl-CoA

the urea cycle converts ______ into _____

ammonia; urea

which amino acids play a central role in amino acid metabolism? (4)

alanine; aspartate; glutamate; glutamine

removing the amino group from aspartate makes ________

oxaloacetate

removing the amino group from alanine makes ________

pyruvate

removing the amino group from glutamine makes ________

glutamate

removing the amino group from glutamate makes ________

alpha-ketoglutarate

removing 2 amino groups from glutamine makes ______

a-ketoglutarate

why are Ala, Asp, Glu, and Gln important in amino acid metabolism?

they are easily converted to glycolysis and TCA intermediates

______ and ______ are used to transport amino groups/NH3 BETWEEN TISSUES

glutamine; alanine

______ is used to carry amino groups/NH3 INSIDE CELLS

glutamate

what do all reactions that use free NH3 have in common?

incorporate free ammonia into a molecule

what enzyme is used to convert glutamate into glutamine?

glutamine synthetase

what enzyme is used to make glycine?

glycine synthase

what cofactor is used by glycine synthase to make glycine?

N5, N10-methyltetrahydrofolate

how is glycine made?

CO2 + NH3 + methyl-THF

what do reactions that release free NH3 have in common?

they separate the amino group from carbon skeleton of amino acid

what enzyme is used to REMOVE the amino group from glutamate?

glutamate dehydrogenase

what enzyme is used to REMOVE the amino group from glutamine?

glutaminase

glutaminase is used in the _____ to generate free ammonia for excretion (organ)

liver

what cofactor is used throughout amino acid metabolism?

PLP

what does PLP stand for?

pyridoxal phosphate

what is transamination?

movement of amino groups between molecules

is transamination reversible or irreversible?

reversible

aspartate transaminase reactions are used to transfer the amino group from ________ to _______

aspartate; glutamate

aspartate transaminase transfers an amino group from aspartate to _______, which generates ______ and _______

a-ketoglutarate; oxaloacetate; glutamate

transamination occurs in a ______ mechanism

ping-pong

how many stages are in transamination?

2

what is the general reaction for stage 1 of aspartate transamination?

aspartate + PLP → oxaloacetate + PMP

what is the general reaction for stage 2 of aspartate transamination?

a-ketoglutarate + PMP → glutamate + PLP

what is the form of PLP when it has an amino group attached?

PMP

transaminases are used for both ______ and ______

synthesis; degradation

what drives the direction of transamination?

concentration of substrate and products

how does PLP help catalyze transamination?

PLP acts as an electron sink

what is the substrate for stage 1 of aspartate transamination?

aspartate

what is the substrate for the second stage of aspartate transamination?

a-ketoglutarate

what is the product of the 1st stage of aspartate transamination?

oxaloacetate

what is the product of the second stage of aspartate transamination?

glutamate

what is the general reaction for alanine transaminase?

alanine + a-ketoglutarate → pyruvate + glutamate

what is the specific name of the enzyme that catalyzes aspartate transamination?

glutamate-oxaloacetate transaminase

how can transaminase levels be used to indicate a heart attack of liver damage?

high expressed/active in the tissue; damage leaks it into blood serum

in the absence of a substrate PLP is ________ to the enzyme active site via a ______ residue side chain

covalently attached; lysine

upon binding a substrate, PLP forms a ______ complex with an ______ substrate, displacing the _______

covalent; amino acid; lysine

when PLP binds the substrate is _____ from the enzyme, but it does not ______

detaches; leave

what forms when PLP binds to the amino acids?

cofactor-substrate adduct

PLP cofactor is electron _______, which stabilizes the ______

withdrawing; carbanion

PLP-catalyzed reactions go through multiple _______

intermediates

the transamination reaction maintains the ______ of the amino acids in the _____ form

chirality; L

in the 1st half of the transamination reaction; the amino group is transferred to the ______ forming a ______ intermediate, and releasing an ___________

cofactor; pyridoxine; a-keto acid carbon skeleton

in the 2nd half of the transamination reaction; an ______ reacts with ______ to yield an ________

a-keto acid; PMP; amino acid

what is the goal of the urea cycle?

remove free ammonia from the body

what are 3 situations that we use amino acids/free NH3?

protein rich diet; gluconeogenesis; starvation

excess amino acids will store the carbons as ___

fat

why does starvation generate lots of NH3

start breaking muscle proteins down to generate energy

in order to use the amino acid carbon skeletons; we need to:

remove and dispose of the amino group

why do we need to get rid of ammonia from out body?

excess NH3 is toxic; changes blood pH

where does the urea cycle occur?

liver

every tissue breaks down _______ BUT, the urea cycle only occurs in the ______, which means free ammonia needs to be _____

amino acids; liver; transported

how does nitrogen get from the muscles to the liver?

glucose-alanine cycle

what is the path of ammonia from MUSCLE to liver? (the molecules that carry it)

glutamate in muscle → alanine in blood → glutamate in liver

transporting nitrogen to the liver beings in the muscle cells; the NH3 taken out of the amino acid and placed on _______ (catalyzed by ________). To export OUT of the cell into circulation, the NH3 is transferred to ______ by __________

glutamate; transaminase; alanine; alanine transaminase

once ______ bring the NH3 into the liver cells, it is transferred to ________, via _________ for transport inside the cell (MUSCLE to liver)

alanine; glutamate; alanine transaminase

once alanine transferred the NH3 to glutamate in the liver cells, it is converted into __________, which can them be plugged into the ______ cycle.

pyruvate; Cori

how is alanine moved between the muscle and liver cells during the glucose-alanine cycle?

the cori cycle

what is the path of ammonia from TISSUE OTHER THAN MUSCLE to liver? (the molecules that carry it)

glutamate in the tissue → glutamine in the blood → glutamate in the liver

NH3 is transported to the liver from tissue other than muscle on _______

glutamine

why is glutamine used to transport NH3 in the bloodstream?

its neutral and non-toxic

NH3 is added to ______ to form glutamine, the enzyme that catalyzes this is ______

glutamate; glutamine synthase

when NH3 needs to be extracted from glutamine (once in liver), the nitrogen is removed by the enzyme __________, reforming ________

glutaminase; glutamate

amino groups from amino acids in other tissues accumulate in the liver as ___________

glutamate

where is glutaminase located?

mitochondria

in the liver, most NH3 for excretion is generated from:

oxidative deamination of glutamate

where is glutamate dehydrogenase located (inside cell)?

mitochondria

the deamination of glutamate produced _____ and _______

a-ketoglutarate; free NH3

the urea cycle begins in the _______ and is then moved to the ______

mitochondrial matrix; cytosol

the urea cycle begins with the formation of _________

carbamoyl-phosphate

what is carbamoyl-phosphate made from?

ammonia and bicarbonate

what is the enzyme that catalyzes the 1st step of the urea cycle?

carbamoyl-phsopahte synthetase-1 (CPS-1)

carbamoyl-phosphate is an ______ intermediate

activated

what does carbamoyl-phosphate react with? what does it make?

ornithine; citrulline

what happens to the citrulline made in the mitochondria?

transported to cytosol

what happens to citrulline after it is transported to the cytosol?

reacts with aspartate to make argininosuccinate

what happens after argininosuccinate if made?

breaks N-C bond to form fumarate and arginine

in the urea cycle, arginine is attacked by ______, which releases ____ and regenerates ____

H2O; urea; ornithine

what enzyme catalyzes the release of urea from arginine?

arginase

how many nitrogens does urea have?

2

where does the aspartate come from in the urea cycle?

glutamate transfers nitrogen to make aspartate

where in the cell is aspartate made?

mitochondrial matrix

how does aspartate leave the matrix to react in the urea cycle?

aspartate/glutamate antiporter

after reacting in the urea cycle, aspartate is converted into _______, which can be transported back into the ________ via the ________

malate; matrix; malate/a-ketoglutarate antiporter

the malate/aspartate shuttle maintains the _________, and links the urea cycle to the ______ cycle

carbon balance; TCA

the _____ produced helps to offset the cost of the urea cycle

NADH

the liver bears the ________ burden of transporting nitrogen

energetic