Protein Catabolism

Overview of Protein Catabolism

Proteases are zymogens

Excess amino acids are not stored

Normal protein turnover 1-2% of body protein per day

~75-80% amino acids reused; lose 30-40 g/day

Not excreted as amino acids or ammonia

Protein(diet)

stomach/intestine

Aamino acids Protein

(body)

B

CD urea

cycle

Nitrogensecretion

.

.

Methods of Nitrogen Excretion

• Uricotelic (birds, reptiles)

• Ammonotelic (fish)

• Ureotelic (mammals)

Two Amino Acids are Used to Transport Nitrogen

• Transaminase• Nonspecific for one

pair, specific for the other pair– Pyr-Ala– KG-Glu

are the two most common

-amino acid

-amino acid'

-keto acid

-keto acid'

transaminase

PLP

H3N C

CO2

CH2

H

CH2

CO2

H3N C

CO2

CH2

H

CH2

CO NH2

NH4

Glutamine Synthetase

H2OMg2+

ATP ADP+Pi

.

.

• Gln synthetase particularly important to the brain

Transamination Mechanism

N

PO43-

CH3

HO

CHN

H

Enz-Lys

H

C CO2R

NH3

H

N

PO43-

CH3

HO

CH

NH

C CO2R

H

H

Enz-Lys NH3

N

PO43-

CH3

HO

CH2

NH3

C CO2R

O

N

PO43-

CH3

HO

CH

NH

C CO2R

H

N

PO43-

CH3

HO

CH

NH

C CO2R

HH

N

PO43-

CH3

HO

CH2

NH

C CO2R

HH2O

H

Urea Cycle

Importancemeans of excreting nitrogen in nontoxic form

LocationMitochondrial matrix and cytoplasm of liver and kidney

ReactionsEnergeticsRegulation

Intracellular Ammonium Generation

• Glutaminase mitochondria only• Liver quantitatively more important

H3N C

CO2

CH2

H

CH2

CO2

H3N C

CO2

CH2

H

CH2

CO NH2

NH4

Glutaminase

H2O

• Oxidative deamination of Glu Only• Glu DH can use NAD or NADP

H3N C

CO2

CH2

H

CH2

CO2

C

CO2

CH2

CH2

CO2

O

ATP, GTP, NADHADP

NH4

Glutamate dehydrogenase

H2O

NAD(P)H

+ H+NAD(P)+

.

.

Carbamoyl Phosphate Synthetase I

• Liver Mitochondrial enzyme• RDS of urea cycle• Activated by (requires) NAcGlu

.

.

carbamoylphosphate

C

O

NH2 OPO32-

NH4 2 ATP 2 ADP+ 1 Pi

carbamoyl phosphatesynthetase I

HCO3Mg2+

Ornithine Transcarbamoylase

• Mitochondrial enzyme• Transported to Cp after synthesis

.

.

Pi

ornithinetranscarbamoylase

carbamoylphosphateC

O

NH2 OPO32-

H3N C

H

CH2

CO2

CH2

CH2

NH3

L-ornithine

H3N C

H

CH2

CO2

CH2

CH2

NH C NH2

O

L-citrulline

Arginosuccinate Synthetase

• Cytoplasmic enzyme

.

H3N C

H

CH2

CO2

CH2

CH2

NH C NH2

O

citrulline

.

ATP AMP+ PPi H2O

arginosuccinatesynthetase

Asp

CNH3 H

CO2

CH2

CO2

H3N C

H

CH2

CO2

CH2

CH2

NH

CHN NH C H

CO2

CH2

CO2arginosuccinate

Mg2+

Arginosuccinase

• Enzyme restricted to liver and kidney• Fumarate converted back to Asp

.

.

H3N C

H

CH2

CO2

CH2

CH2

NH

CHN NH C H

CO2

CH2

CO2arginosuccinate

H3N C

H

CH2

CO2

CH2

CH2

NH

CH2N NH2

arginosuccinase

fumarate

C H

CO2

C

CO2

H

Arginase

• Primarily liver enzyme• Ornithine transported back into mitochondrion by

ornithine-citrulline antiporter

.

.

H3N C

H

CH2

CO2

CH2

CH2

NH

CH2N NH2

H3N C

H

CH2

CO2

CH2

CH2

NH3

arginase

H2O

C

O

NH2H2Nurea

Regulation

• GluNAc as described earlier occurs by changing ATP affinity

• 10-20–fold change in enzyme levels based on “nitrogen balance” i.e. how much protein is consumed relative to needs

EnergeticsFalse Claim #27 of Vegetarians

• “consuming protein uses energy”

• 4 ATP equivalents consumed in the urea cycle

• However, the carbon backbone is now available for energy use

• Carnivores obtain large amount of energy from amino acid carbon, herbivores ~10-15%

Carbon Backbone Catabolism

• Glucogenic amino acids vs. ketogenic amino acids

• Glucogenic are converted to metabolite of glycolysis (e.g., pyruvate) and can be converted into glucose

• Ketogenic form molecules such as acetoacetate, which can be converted to fat

• Most amino acids are glucogenic and ketogenic• Leu is the sole amino acid which is ketogenic

Ala

-keto acid' -amino acid'

PLP

Transaminase

to glycolysis

CO2

C O

CH3

pyruvate

CO2

C H

CH3

H3N

Ala

Gln, Glu

H3N C

CO2

CH2

H

CH2

CO2

H3N C

CO2

CH2

H

CH2

CO NH2

NH4

Glutaminase

H2O -keto acid'-amino acid'

PLP

Transaminase C

CO2

CH2

CH2

CO2

Oto Kreb's cycle

Asn, Asp

-keto acid'-amino acid'

PLP

TransaminaseH3N C

CO2

CH2

H

CO2

H3N C

CO2

CH2

H

CO NH2

NH4

Asparaginase

H2O

C

CO2

CH2

CO2

O to Kreb's cycle

Thr, Ser, GlyCO2

C H

CH

H3N

CH3

OH

Ser hydroxymethyltransferase

PLP

CO2

C H

H

H3NCO2 and NH4

+

MeTHF

THF

H2OSer hydroxymethyl

transferase

CO2

C H

CH2OH

H3N

Ser dehydratasePLP

THF MeTHF

NAD+ NADH+ H+

major p/w: Gly synthase

pyruvate to glycolysisCO2 C

O

CH3

NH4+

CH3CHO

Pro

N

CO2H

H

CH2

CH2

C

C

O

CO2

HH3N

H

Glu--semialdehyde

1. Pro dehydrogenase2. spontaneous

NAD+

NADH+ H+

Glu--semialdehyde DH

NADH+ H+

NAD+

CH2

CH2

CO2

C

CO2

HH3N

.

.

Met

H3N C

CH2

CH2

S

CH3

CO2

H

Met adenosyltransferase

ATP PPi + Pi

H3N C

CH2

CH2

S

CH3 O

CH2

OHHO

NN

N N

NH2

CO2

H

S-Adenosylmethinonine (SAM)

Acceptor methylated acceptor

SAM SAH

SAH

H2O Adenosine

H3N C

CH2

CH2

CO2

H

SH

homocysteine

3 steps

CoASH

Ser CO2 + NH4+

2 NAD 2 NADH

proprionyl CoA

Arg

CH2

CH2

C

C

O

CO2

HH3N

H

Glu--semialdehyde

-keto acid' -amino acid'

PLP

TransaminaseH3N C

H

CH2

CO2

CH2

CH2

NH

CH2N NH2

H3N C

H

CH2

CO2

CH2

CH2

NH3

arginase

H2O

C

O

NH2H2Nurea

.

.

Phe

CO2

C H

CH2

H3N

CO2

C H

CH2

H3N

OH

Phe Hydroxylase

O2 H2O

NADNADH+ H

.

.

Glu TAase

PLP

KG Glu

CO2

C O

CH2

OH

Cu2+

ascorbateO2 CO2

Dioxygenase

HO

OH

CH2

CO2

homogentisate

1,2-Dioxygenase

O2HO

OH

CH2

CO2

homogentisate

C O

CH2

CH

C

CH

CH2

O

CO2

CO2

maleylacetoacetate

C O

CH2

CH

C

C

CH2

O

CO2

CO2

H

fumerylacetoacetate

cis-trans isomerase hydrolase

H2O

C

CO2

C

H

CO2

H

acetoacetateCH3

C

CH2

O

CO2

thiolase

CoASH

acetate andacetyl CoA

Phenylketonuria (PKU) is a Congenital Absence of Phe

Hydroxylase CO2

C H

CH2

H3N

Phe Hydroxylaseabsent

O2 H2O

NADNADH+ H

.

CO2

C O

CH2

LeuCO2

C H

CH2

H3N

CH

CH3

CH3

Leu

CO2

C O

CH2

CH

CH3

CH3

-ketoisocaproate isovaleryl CoA

C O

CH2

CH

CH3

CH3

S-CoAGlu TAase

PLP

-KG Glu

ß-methylcrotonyl CoA

C O

CH

CH

CH3

CH3

S-CoAbranched-chainamino acid DH

PLP

NAD NADH+ H

ß-methylglutaconyl CoA

C O

CH

CH

CH2

CH3

S-CoA

CO2

ß-hydroxy-ß-methylglutaryl CoA(HMG-CoA)

CH3

CH2

CO2

C O

S-CoA

C

HO C

-keto aciddehydrogenase

CoASH CO2

crotonase

Mg2+

ATP ADP+ Pi

CO2

biotinhydrating enzyme

H2O

HMG-CoA lyase

acetoacetate

C

CH2

CO2

CH3

O

C O

CH3

S-CoA

Acetyl-CoA

Maple Syrup Urine Disease

• The result of branched chain amino acid dehydrogenase deficiency

• Oxidation products of keto compounds accumulate

CO2

C H

CH2

H3N

CH

CH3

CH3

Leu

CO2

C O

CH2

CH

CH3

CH3

-ketoisocaproate isovaleryl CoA

C O

CH2

CH

CH3

CH3

S-CoAGlu TAase

PLP

-KG Glu

branched-chainamino acid DH

PLP

NAD NADH+ H

-keto aciddehydrogenase

CoASH CO2

HistidineCO2

C H

CH2

H3N

N NH

CO2

C H

C

N NH

H

urocanate

Histidase

NH4

CO2

CH2

CH2

N NH

O

4-Imidazolone-5-propionate

urocanase

H2O

Imidazolonepropionatehydrolase

H2O

CO2CH2CH2CHCO2

HN NH2

N-Formiminoglutamate (Figlu)

H3N C

CO2

CH2

H

CH2

CO2

THFN5-formiminoTHF

glutamateformiminotransferase

Cysteine Has Two Catabolic Pathways

H3N C

CO2

CH2SH

H C

CO2

CH2SH

O

mercaptopyruvate

sulfurtransferase

2H H2S

C

CO2

CH3

O

pyruvate

cysteinedioxygenase

[O]

H3N C

CO2

CH2SH

H

transaminase

-ketoacid

-aminoacid

PLP

transaminase

-ketoacid

-aminoacid

PLPH3N C

CO2

CH2SO2

H

cysteinesulfinate

ß-sulfinylpyruvate

C

CO2

CH2SO2

Odesulfinase

SO32-

C

CO2

CH3

O

pyruvate