Biochemistry: Amino Acid Metabolism

T/F: There are storage forms of amino acids/proteins.

False

The carbons of excess amino acids from dietary protein are converted to what?

Fat, carbohydrate, or CO2

The carbons of excess amino acids are used; the ________ is waste.

Nitrogen

What is another name for amino acid metabolism?

Nitrogen metabolism

What is positive vs. negative nitrogen balance?

Positive: N intake > N excretion (gaining body protein); Negative: N excretion > N intake (losing body protein)

Examples of positive nitrogen balance

Growth, pregnancy, recovery

Examples of negative nitrogen balance

Starvation, low or incomplete dietary protein, illness

A patient recovering from surgery excretes less nitrogen than she takes in. What is her state?

Positive nitrogen balance

What percentage of protein is nitrogen?

About 16%

Where is the amino acid pool maintained?

Blood

What supplies amino acids to tissues?

The amino acid pool of the blood

Inputs to the amino acid pool

• Dietary protein (70-100 g/day)

• digestive enzymes (70-100 g/day)

• body protein breakdown

• synthesis of non-essential amino acids

Outputs from the amino acid pool

Protein synthesis, synthesis of N-containing compounds, degradation for energy or glucose, feces (~10 g/day)

About how much amino acid turnover occurs per day?

About 300 g/day, far more than you eat

The amount of urea in urine is proportional to what?

Dietary protein

Under constant weight (nitrogen balance), only the ____ level changes with changed protein intake.

Urea

Total N excreted on a 100 g vs. 50 g protein diet

• 100 g diet: 16.0 g N/day (urea 14.7); 50 g diet: 8.0 g N/day (urea 6.8)

• ammonia, uric acid, and creatinine stay about the same

Where are acid and pepsinogen secreted?

Glands of the stomach

What does stomach acid do to proteins?

Denatures them

Pepsin in the stomach is active at what pH?

1.5

Which enzyme functions best at low pH (around 1.5)?

Pepsin

What does pepsin produce?

Peptides

What does the pancreas secrete?

Bicarbonate and zymogens (secretion stimulated by CCK and secretin)

Activated enzymes in the first part of the small intestine digest ________.

Peptides

What are the enzymes in the duodenum?

Trypsin, chymotrypsin, elastase, carboxypeptidase

Where are trypsin, chymotrypsin, and elastase located?

Lumen of the small intestine

What enzymes act at the villi/brush border?

Aminopeptidase, dipeptidase

What do carboxypeptidase and aminopeptidases produce?

Amino acids

What is the final stage of dietary protein digestion?

Enzymes of the intestinal lumen and villi produce mostly free amino acids for absorption

Where does intracellular (body) protein degradation occur?

The 26S proteasome (the cell's mini stomach)

Intracellular proteins are tagged by _________ and degraded by what?

Ubiquitin; the proteasome

What enzymes add ubiquitin to target proteins?

E3 ubiquitin ligases

Poly-ubiquitinated proteins are ________.

Degraded (by the proteasome)

When is ubiquitin-proteasome degradation upregulated?

When the body needs energy

Aminopeptidases inside most cells degrade short peptides to what?

Amino acids

Essential amino acids are obtained from what?

The diet or degradation of body protein

Body protein is degraded with insufficient or incomplete _______ _______.

Dietary protein (causing negative nitrogen balance)

Essential amino acids mnemonic

PVT TIM HaLL: Phe, Val, Thr, Trp, Ile, Met, His, Leu, Lys (Arg conditionally essential)

A person on a leucine-deficient diet loses muscle mass because leucine is…

An essential amino acid

Average protein intake vs. requirement

About 100 g/day eaten vs. about 50 g/day required, so extra nitrogen must be disposed of

Nitrogen is removed in 3 steps:

1) Transfer to a common carrier (transamination); 2) Ammonia is regenerated in the liver (glutamate dehydrogenase); 3) Ammonia is incorporated into urea (urea cycle)

What is the essential first step in amino acid catabolism?

Amino acids transfer their nitrogen to a common carrier via transamination

Feed the excess nitrogen into which amino acid?

Glutamate

All ____________ use alpha-ketoglutarate as the amino group acceptor or glutamate as the donor.

Transaminases

What cofactor do transaminases require?

PLP (vitamin B6)

What does aspartate aminotransferase (AST) do?

Aspartate + alpha-ketoglutarate

What does alanine aminotransferase (ALT) do?

Alanine + alpha-ketoglutarate

What TCA cycle intermediate is transaminated to form glutamate?

Alpha-ketoglutarate

Removal of nitrogen occurs via ______.

Ammonia (ammonium ion, NH4+)

What catalyzes the release of ammonia from glutamate?

Glutamate dehydrogenase

Where does glutamate dehydrogenase work, and what does it regenerate?

Liver mitochondria; regenerates alpha-ketoglutarate

What does glutaminase do?

Converts glutamine to glutamate, releasing ammonia

What is the sum of the reactions catalyzed by transaminases and glutamate dehydrogenase?

Amino acid + NAD(P)+ + H2O -> alpha-keto acid + NH4+ + NAD(P)H

Where is NH4+ made?

Mitochondria of liver cells

The ammonia is excreted in the form of ____ through the ____ cycle.

Urea; urea

Where is urea synthesized?

Liver

Where does the urea cycle take place within the cell?

Starts in mitochondria, finishes in cytosol

Two atoms of ________ are needed to make one molecule of urea.

Nitrogen

Where does nitrogen enter the urea cycle?

At two points: carbamoyl phosphate (from free ammonia) and the amino group of aspartate

The two nitrogens of urea come directly from:

Carbamoyl phosphate and aspartate

Aspartate for the urea cycle is regenerated by transamination of which TCA intermediate?

Oxaloacetate

How much energy does the urea cycle cost?

4 high-energy phosphate bonds per urea

Urea cycle mnemonic

Ordinarily Careless Crappers Are Also Frivolous About Urination: Ornithine, Carbamoyl phosphate, Citrulline, Aspartate, Argininosuccinate, Fumarate, Arginine, Urea

Urea cycle step 1 (location, reaction, enzyme)

Mitochondria; NH4+ + CO2 -> carbamoyl phosphate; CPS I (rate-limiting)

Which precursors does mitochondrial carbamoyl phosphate synthetase use?

NH4+, ATP, and CO2

Where is NH4+ incorporated?

Into carbamoyl phosphate (and eventually urea)

Urea cycle step 2 (location, reaction, enzyme)

Mitochondria; ornithine + carbamoyl phosphate -> citrulline; ornithine transcarbamoylase (OTC)

What combines with carbamoyl phosphate in the mitochondria?

Ornithine

What catalyzes ornithine + carbamoyl phosphate?

Ornithine transcarbamoylase (OTC)

Ornithine + carbamoyl phosphate exits the mitochondria as?

Citrulline

An OTC-deficient patient will have low urea and low levels of what other compound?

Citrulline

Urea cycle step 3 (location, reaction, enzyme)

Cytosol; citrulline + aspartate -> argininosuccinate (2nd N enters); argininosuccinate synthetase

Urea cycle step 4 (location, reaction, enzyme)

Cytosol; argininosuccinate -> arginine + fumarate (fumarate goes to TCA); argininosuccinate lyase

Urea cycle step 5 (location, reaction, enzyme)

Cytosol; arginine -> urea + ornithine; arginase

What does arginine produce?

Urea + ornithine

What catalyzes arginine to urea?

Arginase

What is the key regulatory step of the urea cycle?

The first irreversible reaction, catalyzed by CPS I

What primarily controls CPS I (the first regulatory step)?

The allosteric activator N-acetylglutamate (NAG)

Production of carbamoyl phosphate is controlled by ___.

NAG

Where is NAG formed?

Only in mitochondria

What is NAG synthesized from?

Acetyl-CoA and glutamate

What stimulates NAG synthesis?

Arginine

Regulatory logic of the urea cycle

More amino acids -> more glutamate and arginine -> more NAG -> faster urea cycle

What are the two fates of amino acid carbon skeletons?

Glucogenic or ketogenic

What does glucogenic mean?

The skeleton becomes pyruvate or a TCA intermediate that can reach oxaloacetate -> PEP -> glucose

What does ketogenic mean?

The skeleton becomes acetyl-CoA or acetoacetyl-CoA, which cannot make net glucose

When amino acids are broken down, their carbons form either ________ or ___ _____ intermediates.

Pyruvate or TCA cycle intermediates

If these intermediates can form OAA, you can make _______.

Glucose

T/F: In mitochondria, almost all amino acids are degraded into TCA cycle compounds.

True

T/F: You can get blood glucose from lysine and leucine.

False; they are purely ketogenic

Which amino acid pair can never raise blood glucose?

Leucine and lysine

Amino acids that are both glucogenic and ketogenic

Phe, Tyr, Trp, Ile, Thr

Amino acids entering at pyruvate

Alanine (also Ser, Gly, Cys, Thr, Trp)

Oxaloacetate is the TCA entry point for which amino acids?

Asparagine and aspartate

Amino acids entering at alpha-ketoglutarate

Glutamate, glutamine (also His, Pro, Arg)

Amino acids entering at succinyl-CoA

Val, Ile, Met, Thr

Amino acids entering at fumarate

Phe, Tyr

Amino acids entering at acetyl-CoA/acetoacetyl-CoA

Leucine and lysine (purely ketogenic); also Phe, Tyr, Trp, Ile, Thr

What are the branched-chain amino acids?

Leucine, isoleucine, valine

What do branched-chain amino acids form?

Branched-chain alpha-keto acids (by transamination)