Proteins Part 2

Protein digestion: Trypsinogen

Trypsinogen gets converted to trypsin in the pancreas

Trypsin activates all other enzymes (Zymogens):

• Chymotrypsinogen → chymotrypsin

• Protelastase → Elastase

• Procarboxypeptidase → Carboxypeptidase

AA absorption

Facilitated diffusion

Active transport **

• NA dependent transporter (needs ATP)

Absorbed through the PEPT1

Assessing Protein Quality: Protein Efficiency Ratio (PER)

Values compared to 2.0 g of whole egg

Assessing Protein Quality: Chemical score

Test protein is chemically digestion into free AA

Nitrogen Balance

Intake of N and N loss

N should = 0

Not enough protein → NB < 0 (-ve)

• Poor protein quality

Atkins diet

High protein

Low CHO

Higher Fat intake

South Beach diet

Protein consistent but macronutrient content varies

Zone diet

Balanced diet

Protein supplements

Balance of AA are best

Supplements usually high in BCAAs

Rapidly absorbed and delivered to muscle

Bypass the liver

Marasmus

Low protein and calorie malnutrition

Body switches to starvation mode

Use energy reserves

Kwashiorkor

Protein deficiency

High CHO foods → Cassava

Weaned from breast milk to cassava

Osmotic imbalance in gut = swelling

Liver enlarged due to inability to export fat from liver (no VLDL made)

Reactions of Protein Catabolism

Excess AA

Amino group → Ammonium

• Gets excreted into urea cycle

• Gets excreted into urine

Carbon skeleton → Glucose or Acetyl CoA

• Glucogenic or ketogenic AA

Differences in fed and fasted state

Fasted state involved formation of glutamine and alanine

Fed state mostly glutamine, involves liver and kidney

Fed state excretes NH₄⁺ as urea

Fasted state mainly excretes as NH₄⁺ directly

Alkalosis

Fed state with high dietary protein intake

High AA catabolism increase Bicarbonate

Increase pH to more basic

Acidosis

Fasting/starvation

AA broken down and release glucogenic AA

pH drops

Glutamate

Important in AA catabolism

End product of transamination rxns

α-ketoacid → α-ketoglutarate

Aspartate

Donates Amino group in urea cycle (turns into asparagine)

α-ketoacid → oxaloacetate

Alanine

Inter-organ N carrier

Muscle to liver

α-ketoacid → pyruvate

Glutamine

Most abundant

Inter-organ carrier

Donate amino group to other rxn (turns into glutamate)

4 rnx more N → organs → excretion: Transamination

amino acid ↔ α-keto acid → Aminotransferases

• Aminotransferases → GPT and GOT

Glutamate almost always present in transamination reactions

α-ketoglutarate is almost always one of the reactants

• α-ketoglutarate = amino group acceptor

Glutamate is almost always one of the products

4 rnx more N → organs → excretion: Oxidative deamination

Glutamate main AA that undergoes oxidative deamination

• Because its main product of transamination

Amino group released from glutamate backbone

Glutamate → α-ketoglutarate + NH₄⁺

• Done by Glutamate dehydrogenase

4 rnx more N → organs → excretion: Glutamine production

Glutamate → Glutamine

• Done by Glutamine synthetase

Fed: NH₄⁺ travels to liver in

Fasted: NH₄⁺ travels to kidney

4 rnx more N → organs → excretion: Glutamate regeneration

Opposite to glutamine production

Release Amino group from glutamine side chain

Fed: active in liver → amino group for urea synthesis

Fasted: active in kidney

4 rnx more N → organs → excretion: Urea Cycle

Toxic NH₄⁺ converted to urea in liver

Urea transported to kidney

Aspartate donates Amino group (to form urea)

Uses Bicarbonate nearby (prevent alkalosis)

Requires ATP

Cahill cycle (Glucose Alanine Cycle)

Pyruvate transaminated to Alanine

α-ketoglutarate + alanine → glutamate + pyruvate

Glutamate → Urea

Pyruvate → Glucose

Fed state N metabolsim

AA catabolism → Glutamine formation (from glutamate)

Glutamine transported to liver

Deliver Amino group for urea production

Urea transported to kidney

Excreted in urin

Fasted state of N metabolsim

AA catabolism → Glutamine formation (from glutamate) + Alanine (from pyruvate)

Glutamine transported to kidney

• Converted to glutamate and remove amino group

• Amino group excreted as urine

Alanine sent to liver

Ketogenic

Degraded AA can be converted into Acetyl CoA

Leucine and Lysine

Glucogenic

Degraded AA can be converted into Glucose

Alanine, Glutamate, Aspartate