Amino Acid Metabolism

Amino acids undergo oxidative catabolism under three circumstances

• Leftover amino acids from normal protein turnover are degraded

• Dietary amino acids that exceed body’s protein synthesis needs are degraded

• Proteins in the body can be broken down to supply amino acids for energy when carbohydrates are scarce (starvation, diabetes mellitus)

Protein stimulates gastric mucosa to secrete

gastrin

HCL

release from parietal cells (pH 1.0 to 2.5; antiseptic; denatures proteins)

Pepsinogen

from chief cells of gastric gland

Pepsinogen is converted to _____ via autocatalytic cleavage

pepsin

Pepsin

cuts protein into peptides in the stomach

Product of gastric digestion

peptides

Small Intestine (SI) and Pancreas

• Causes secretin release to the blood

• Secretin stimulates bicarbonate release from pancreas into SI to neutralize acidic pH of gastric acid

Pancreatic secretions directly enter the SI from the

pancreatic duct

Presence of AA in SI (duodenum)

Causes Cholecystokinin (CCK) to be released into the blood – mostly aromatic AA

CCK

stimulates pancreatic enzyme release from pancreatic exocrine cells

Enteropeptidase released from SI cells converts trypsinogen to

trypsin

trypsinogen → trypsin

activates chymotripsinogen, procarboxypeptidases, proelastase, and more trypsin

Trypsin and chymotrypsin

Cleave proteins and larger peptides into smaller peptides in the SI

Aminopeptidase and carboxypeptidases A and B

Degrade smaller peptides into AA in the SI

Final product of protein hydrolysis

Free AA that are transported into SI epithelial cells, enter the

capillaries, then the liver.

zymogens

Trypsinogen, chymotripsinogen, procarboxypeptidases A & B

Pancreatic trypsin inhibitor

released by pancreas to prevent auto-digestion

Acute pancreatitis

• Caused by obstruction of the pathway by which pancreatic secretions enter the SI

• Zymogens are prematurely activated inside the pancreatic cells and damage the cells

protein turnover

• Proteins are continuously biosynthesized and degraded

• This allows replacement of damaged proteins and biological regulation

Passive diffusion

epithelial cells

Active transport

gills

Glu and gln

collecting point for NH2

AA transfer of NH2 to αKG to form

Glu

• Enters the mitochondria to release NH4+ (enzymatic transamination)

NH4+ is transferred to glu to form ___ in tissue

Gln

• Transported in the blood, and enters the liver mitocho. and releases NH4+

Skeletal muscle

NH2 is transferred to pyruvate to form alanine, which is transported to the liver and the amino group is transferred to αKG to form glutamine

Excess ammonia is transported to the liver for excretion as

urea

First step of degradation for all amino acids

removal of the amino group aka enzymatic

transamination

enzymatic transamination

Removal of amino group by aminotransferases (also known as transaminases) in the liver

All aminotransferases rely on the ________________ – requires vitamin B6

pyridoxal phosphate (PLP) cofactor

L-glutamine acts as temporary storage for nitrogen

Can donate the amino group when needed for AA biosynthesis or the amino group can be converted to urea

Assays of these enzymes in the blood provide diagnostic info

ALT, AST, Creatine Kinase

Aldehyde form (PLP) can react reversibly with amino groups (_______ amino groups)

accept

Aminated form (PMP) can react reversibly to _____ amino groups to α-keto acid

donate

Carbamoyl-phosphate synthetase I

• Ammonium is converted to carbamoyl-P

• two ATP required - one to activate bicarbonate, one to phosphorylate carbamate

Glutamate dehydrogenase

reductive amination of α-ketoglutarate to form glutamate

Glutamine synthetase

ATP-dependent amidation of γ-carboxyl of glutamate to glutamine

Ammonia collected in Glu is removed by

Glutamate dehydrogenase (liver)

transdeamination

transamination + oxidative deamination

NH4+ from intestines and kidneys can go

directly to the liver

Other tissues add the NH4+ to glutamate (via glutamine synthetase) to form

glutamine, which can help transport to blood then liver

Glutamine is deaminated in the

intestines, kidneys, and liver

Glutamine → glutamate + NH4+ via

glutaminase

Free NH4+ can be converted to

urea

pyruvate can be converted to ____ for transport to the liver

alanine

Urea Cycle

Converts toxic ammonia to non-toxic urea for excretion (liver)

the ____ is the only tissue that can produce urea

liver

Excess glutamate is metabolized in

mitochondrial matrix

free NH4+ can react with HCO3- to form

carbamoyl phosphate

Glutamate dehydrogenase can use either ______ or ______ as electron acceptor

NAD+ or NADP+

Ornithine + carbamoyl P =

citrulline

Citrulline + aspartate

Argininosuccinate

Argininosuccinate forms

fumerate (goes to CAC) and arginine (still in cytosol)

Arginine forms

urea + ornithine

Carbamoyl phosphate synthase I

• 1st step of urea cycle

• Regulated; carbamoyl P synthase I uses 2 ATP

• captures free NH4+ in the mitochondrial matrix (adds HCO3-)

2nd nitrogen requiring reaction

Entry of Aspartate into the Urea Cycle

first amino group entering urea cycle

from carbamoyl phosphate

the second group to enter the urea cycle

from aspartate

Total of ____ATP required to produce one urea molecule

4

the net ATP required for the Urea Cycle is

1.5

urea contains

• Carbon and a nitrogen (orange) derived from carbamoyl phosphate

• Nitrogen (purple) derived from aspartate

N-acetylglutamate, synthesized from

glutamate and acetyl-CoA (Allosterically activates carbamoyl phosphate synthetase)

Carbamoyl phosphate synthase I is activated by

N-acetylglutamate (short-term regulation)

Expression of urea cycle enzymes increases when needed

• long term regulation

• high protein diet

• starvation

transfer N onto Glutamate

• Transamination: AA + α-ketoglutarate ↔ α-keto acid + glu

• Glutamate dehydrogenase: NH4+ + NADH + α-ketoglutarate ↔ glu + NAD+

Glu →Gln (plasma transport form)

Glutamine synthetase: glutamate + NH4+ →glutamine + H2O

Convert Gln to excretion form in liver & kidney

• Glutaminase (in liver & kidney): glutamine + H2O glutamate + NH4+

• Asparaginase (in liver & muscle): asparagine + H2O  aspartate + NH4+

• Urea cycle

purely ketogenic

leucine and lysine

5 entry points into CAC

• Acetyl-CoA

• αKG

• Succinyl-CoA

• Fumerate

• OAA

glucogenic AA

alanine, glycine, cysteine, theonine, tryptophan, asparagine, aspartate, valine, methionine, glutamate, glutamine, histidine, proline, arginine

ketogenic AND glucogenic AA

tyrosine, phenylalanine, threonine, isoleucine, tryptophan,

The first reaction in phenylalanine degradation is the

hydroxylation reaction of tyrosine biosynthesis

Transamination of Tyr yields

p-hydroxyphenylpyruvate

Ring opening and isomerization gives

4-fumaryl-acetoacetate, which is hydrolyzed to acetoacetate and fumarate

Alkaptonuria and phenylketonuria

Two human genetic diseases arising from specific enzyme defects in Phe degradation

Leucine, Isoleucine, and Valine are oxidized for fuel in

muscle, adipose tissue, kidney, and brain

n amino acid biosynthesis, ________ is the primary source of N, Via transamination (aminotransferase) reactions

glutamate

THF

carries activated one-carbon units for the formation of new C-S, C- C and C-N bonds

5,10-methylene-THF

is carrying a methylene unit attached to the 5 and 10 positions that can be used to create new C-C bond

5-methyl-THF

is used to transfer the methyl group to homocysteine, to form a C-S bond

10-formyl-THF

is used to create new C-N bonds

S-adenosylmethionine (SAM)

• preferred cofactor for methyl transfer in biological reactions

• Methyl is 1000 times more reactive than THF methyl group

• Synthesized from ATP and methionine

Methylcobalamin, or methyl- B12,

is used in the methionine synthase reaction

5′-Deoxyadenosylcobalamin

is used by a number of enzymes, including methylmalonyl-CoA mutase

Pernicious anemia

is caused by deficiency of a glycoprotein needed for intestinal absorption of vitamin B12, leading to

intracellular deficiencies of B12 coenzymes