# 1 # 1 Amino Acid MetabolismAmino Acid Metabolism

1. General FeaturesNitrogen Balance & Metabolic Pools

2. DegradationTransamination & Glutamate Dehydrogenases

3. Urea Cycle4. Sulfur-containing amino acids5. Creatine & Creatinine

ObjectivesObjectives

• Outline the general features of the amino acid (AA) pool & how these relate to positive & negative N balance.

• Essential vs non-essential AAs

• Interorgan transport and utilization of glutamine and alanine

N balance = NN balance = Ninin - N - Noutout

1 Major dietary source of N is Protein (>95%), since the diet has very few free amino acids

2 AA are used for Protein Synthesis & N containing compounds

3 AA in excess are degraded (used for energy)

N is disposed of in urea (80%), ammonia, uric acid or creatinine in urine with small amounts in fecal matter (undigested)

Positive Nitrogen BalancePositive Nitrogen Balance

Negative Nitrogen BalanceNegative Nitrogen Balance

1. Stress

2. Decreased Intake

3. Lack of an essential AA

Specificity of Some Proteolytic EnzymesSpecificity of Some Proteolytic EnzymesEnzyme Occurrence pH optimum Major site of action

Trypsin Intestine 7.5 to 8.5 Arginyl, lysyl bonds

Chymotrypsin Intestine 7.5 to 8.5 Aromatic amino acyl bonds (Phe, Trp, Tyr)

Pepsin Stomach 1.5 to 2.5 Wide range of specificity

Carboxypeptidas e Intestine 7.5 to 8.5 C-terminal amino acid

Aminopeptidase Intestinal mucosa

N-terminal amino acid

Very few AA are present in the free form in the diet. Most are absorbed following digestion as AA + peptides

(important in transport deficiencies).Various enzymes cleave different bonds.

Sterospecific Transport Systems Sterospecific Transport Systems for Amino Acidsfor Amino Acids

Amino acid specificity Amino acids transported Human disease

1. Small neutral amino acids

Alanine, serine, throenine

2. Large neutral and aromatic amino acids

Isoleucine, leucine, valine, tyrosine, tryptophan, phenylalanine

Hartnup disease

3. Basic amino acids Arginine, lysine, omithine, cystine

Cystinuria

4. Proline, glycine Glycinuria

5. Acidic amino acids Glutamic and aspartic acids

Uptake (transport) systems exist especially in intestine & kidney.

Lack of specific transporter results in a disease state.This can be partially overcome through uptake of peptides.

Metabolic Pool of Amino Acids • Metabolic pool AA has no storage form in mammals (as with other life

forms) as free AA or as specialized storage form (such as glycogen for glucose, TG for FA) but a certain percentage of muscle & structural proteins are “expendable”.

• AA are used for proteins, N compounds, energy (also via glucose) but increased protein breakdown will eventually compromise normal protein function.

• Therefore need a small mobile pool of free AA in cells and blood– Pool size is regulated (no more than 50% changes) – Pool size is small relative to flux

»16g in = 16g out : Nequilibrium

[AA = 30g] [300g cell protein]

Metabolic Pool of Amino AcidsMetabolic Pool of Amino Acids

The three major draws on the amino acid pool areThe three major draws on the amino acid pool are:

1 AA → Proteins (not covered here)

2 AA → Catabolism (NH4+ discarded,

glutamine used)

3 AA → N containing compoundsas - whole/part AA integrated

or - only the amino (N) group is used• AA catabolized, then NH4

+ incorporated into glutamine

This synthesis is not compromised even when dietary P is decreased because they are essential for cellular function.

Major Functions of Amino Acids Major Functions of Amino Acids Derived from Dietary ProteinDerived from Dietary Protein

OxidationGlycogenic amino acids: --Blood glucose--EnergyKetogenic amino acids: -Acetyl CoA-Stored fat-Energy

Biosynthesis of nitrogen-containing metabolites

Heme Blood cell

Choline PL

Glycosamine Sugar

Nucleotides DNA

Protein synthesis Protein

Biogenic amines Neurotransmitters

Carnitine Heart

Creatine phosphate « Energy »

Maintenance of Pool SizeMaintenance of Pool Size• Surplus AA to biosynthetic requirements

are degraded

• AA oxidation is the major mechanism of degradation Rate

• Oxidation Pool Size

and relates to enzyme induction.• Balanced composition of AA pool is

important to meet all cell requirements for all AA

Summary of Protein MetabolismSummary of Protein Metabolism

a) α keto acids are funneled into the Krebs cycle (glucogenic/ketogenic)

b) NH4+ is cleared via urea, NH4

+, with uric acid however major product is urea (80%)

c) Creatine/creatinine important for energy consideration

Essential & Non-essential AAEssential & Non-essential AA

1. EAA : Humans (mammals) cannot synthesize their carbon skeletons de novo. Some EAA are considered essential because we cannot synthesize enough, especially for growth (children).

2. NEAA: Synthesized from intermediates of glucose/TCA cycle except TYR (cannot make aromatic ring).

3. The grouping of E vs NE was determined experimentally by feeding diets lacking in a single AA and measuring Nin/Nout. . If balance negative then = EAA

Essential & Non-essential AAEssential & Non-essential AA

1. The difference between EAA & vitamins that is AA are needed in substrate amounts to make proteins and nitrogen compounds. Vitamins are needed in catalytic amounts (co-factors for enzymes) and therefore can be reused.

2. All AA must be provided simultaneously (not hours after).3. Normally: no “disease” can be attributed to deficiency in a

single AA except Pellagra which is due to a lack of niacin nicotinate a component of NAD

TRP nicotinate NADCorn P is deficient in TRP therefore corn diet pellagra. Preparation is important: Because TRP → necessary to make niacinCorn + Alkali absorption of nicotinate present in corn → niacin

Essential & Non-essential AA

Conditionally essential(i) ARG:can be made, but not enough(ii) HIS: controversial (essential for growth in children)(iii) PHE essential, TYR can be made from PHE but when enzyme is missing (phenyl- ketonuria) then PHE > TYR; Therefore TYR is essential(iv) MET CYS; Similarly, if MET > CYS then CYS essential

Even with excess, important in excretion NH4

+ therefore continue to be made

Glycogenic and KetogenicAmino Acids

Glycogenic Alanine, Arginine Asparagine, Aspartate Cysteine, Glutamate Glutamine, Glycine

Glycogenic and Ketogenic Isoleucome Phenytolanine Tryptophen Tyrosine

Ketogenic

Leucine Lysine

Histidine, Methionine Proline, Serine

Threonine, Valine

1.Glucogenic: converted to glucose via pyruvate2.Ketogenic: converted to ketone bodies 3.Some are both

4.During fasting when FA are the major fuel FA cannot be converted to

glucose therefore AA → glucose & ketone bodies (especially for brain)

-AA → pyruvate → liver → glucose

-keto AA + FA → ketone bodies (acetoacetate & 3 hydroxybutyrate)

Specialized Amino Acid Roles1. Certain NEAA continue being synthesized even when

adequate levels are supplied in diet because of a specialized role

2. ARG → urea synthesis

ASP → urea synthesis

GLU → conduit for disposal of N

3. ALA & GLN → key role in exchange between tissues (liver & skeletal muscle)

4. Liver: major site gluconeogenesis (AA → Glucose)

major site urea synthesis (kidneys to a lesser extent)

5. Skeletal Muscle: 60% total body protein, 50% total body AA pool and is the major source to provide AA carbons → hepatic gluconeogenesis

AA are released from muscle during the post- absorptive state (O/N fast). Of the AA released by muscle ALA= 30% & GLN= 25% (total> 50%)

But output (ALA+GLN) > abundance in muscle proteins which contain 7-10% ALA & 6% GLN

Where does this ALA & GLN come from?

Sources of Alanine (from Muscle)Sources of Alanine (from Muscle)

(i)Muscle: Protein → ALA + AA

AA → NH4+ + α keto acids

α keto acids → ALA (“simplest” AA).

Therefore total ALA released > ALA derived from proteins

(ii) Liver: ALA → NH4+ + α keto acids

NH4+ → urea

(iii) As well Glucose → Pyruvate (no N) → ALA (with N)

Therefore ALA serves as a vehicle for transport of NH4+ from

muscle to liver (NH4+ is generated through breakdown of AA

→ energy).

(iv) Because free NH4+ is very toxic even at low levels therefore

Pyruvate + NH4+ → ALA (non-toxic)

(v) In liver: NH4+ → urea for excretion

Sources of Glutamine (from Muscle)Sources of Glutamine (from Muscle)

(i) Extra GLN released is also made from other AA & serves as a non-toxic transport of NH4

+ from muscle → kidneys & gut (previous fig)

(ii) Kidneys: GLN → ALA (to the Liver )

& GLN → glucose (blood) +NH4+ (Urine)

(iii) Gut: GLN → ALA (to the liver)

Response to Food DeprivationResponse to Food Deprivation

(i) For the first 7 days, maintain blood glucose (brain use 65% of glucose 400 - 600 Cal)

(ii) > 7 days: Protein proteolysis decreases (protect essential proteins) therefore use over a prolonged period compromises organism.

(iii) → Switch to Ketone bodies

Case # 1 Amino Acid Metabolism: Case # 1 Amino Acid Metabolism: General Features Case DiscussionGeneral Features Case Discussion

A worker reported to a physician with many of the symptoms of pellagra: swollen tongue, dermatitis, and nervous disturbances. The man's diet consisted principally of sweet corn with a small amount of other sources of protein. The identical twin of the man had no similar complaints, and although the twin's diet was high in sweet corn, it was mixed with significant amounts of beans.

3. Pellagra is caused by a deficiency in

A. Pyridoxal phosphate B. Ascorbic Acid C. Niacin D. Vitamin B12

E. Riboflavin

1. Sweet corn protein is deficient in

1. Tryptophan 2. Glutamic acid 3. Lysine 4. Arginine

2. Is tryptophan an EAA?

YESYES

3. Match the numbers with the following letters as appropriate:A. Sweet corn protein 1. High Biologic ValueB. Bean proteins 2 Deficient in tryptophanC. Both 3. Calorie valueD. Neither 4. Essential fatty acids

4. A vitamin is defined as a compound that is1. An essential component of the body2. Mainly synthesized in plants3. Not synthesized in adequate amounts in the body4. Only synthesized in animals

5. The identical twin, who had no complaints1. Had sufficient tryptophan to biosynthesize niacin2. Derived enough niacin from beans3. Did not cook his food in water4. Added vitamin C to his diet