Amino acid metabolism

proteins

• Only foodstuff that can form structures (tissues and enzymes)

• Made up of amino acids• Protein synthesis,

enzyme formation• Can serve as fuel during

long-term work• 0.8 g/kg recommended

for adults; probably too low for athletes

Protein structure: Amino acids

• Essential vs non-essential– Essential:

NOT made by body

– Non-essential: made by the body

Protein structure

• Carboxyl and amino termini come together to from protein structures (peptides)

Amino acid

• Digested in stomach and small intestine– Hydrocholoric acid

(stomach)– Trypsin,

chymotrypsin, carboxypeptidase (from pancreas)

– Polypeptidases and dipeptidases in intestinal cells finish digestion

Proteins in the diet

• The amino acid pool– Free amino acids in the liver, skeletal muscle, plasma, interstitial

fluid and intracellular water– All interconnected in that metabolism in one affects the others– Continuous excretion of nitrogenous end-products– Necessitates constant input of new amino acids– So, CONSTANT Protein turnover

Nitrogen balance

• Nitrogen is a component of AAs– Thus, used as a

marker of protein metabolism

• Protein intake necessary to balance nitrogen turnover (input vs excretion)– 0.8-1.0 g/kg is

sufficient for most– 1.2-1.6 g/kg is the

highest recommendation for athletes

Removal of nitrogen

• Before amino acids can be used as fuel, nitrogen group must be removed

• Two ways– Deamination– Transamination

• Glutamate is a key player in both

Removal of Nitrogen

• Deamination– Occurs in liver

1) Requires NAD+ as oxidizing agent

2) Produces ammonium ion

3) α-ketoglutarate can be used in Kreb’s cycle

• Called anaplerotic (to fill) addition to Kreb’s cycle

12

3

Removal of Nitrogen• Transamination

– Much more common

– Transfers amine group from amino acid to keto acid

• SGPT and SGOT transaminases in liver

AA

AA

Keto acid

Keto acid

Excretion of nitrogenous wastes

• Ammonia (small amt)• Most is excreted as urea• Urea cycle

1) Formation of carbamoyl phosphate from ammonia and Co2

2) Addition of aspartate3) Production of fumarate

(Kreb’s cycle intermediate)4) Produces Urea

12

4

3

Gluconeogenic amino acids

• Some amino acids used for gluconeogenesis1)Pyruvate to OOA2)OOA to PEP3)PEP begins “reverse

glycolysis” or gluconeogenesis

• So, amino acids that give rise to pyruvate and oxaloacetate– Can form

phosphoenolpyruvate– Can be converted to

glucose

1

2

Anaplerotic and cataplerotic reactions• Anaplerotic (adding to)• Cataplerotic (emptying)

– These Rx add to or deplete the Kreb’s cycle

• Glutamate-glutamine• Key intraorgan

nitrogen transport vehicle, fuel source for GI tract and immune system and gluconeogenic precursor

Branched chain amino acids

• Leucine, Isoleucine and valine (LIV)

• Catabolized mostly in skeletal muscle

• Leucine:– Forms acetyl-CoA,

acetoacetate and glutamate

• Leucine is thus called ketogenic

Transamination

AA metabolism• AA can be used in the following ways

– Structural (proteins)– Anaplerotic additions to Kreb’s cycle

• This keeps the Kreb’s cycle working

– Oxidized directly• Branched chain AA

– Other contributions to energetics• Ketogenic

– Produce ketone bodies when broken down

• Glucogenic– Contributes to gluconeogenesis

Glucose-alanine cycle• Used during fasting• Alanine can come

from glycolysis or AA metabolism– Glycolysis

• Kreb’s cycle backs up during starvation

– Pyruvate transaminated to alanine

– Alanine converted to glucose in liver

Glucose-alanine II

• Other amino acids can also form alanine (glucogenic AA, anything that gives rise to pyruvate or OOA)

• So when Pyruvate builds up, converted to Alanine (1)

• Alanine shuttled to liver– Converted to

glucose

1

Effects of endurance training on AA metab

• Greater rates of AA metabolism in trained subjects – Greater oxidation

in human subjects during exercise

AA metabolism• Note that leucine

oxidation increases during exercise– This increases is linear

with respect to exercise intensity

– Particularly true in fasted state

• Note that alanine appearance increases during exercise (1) and this can come from AA leucine (2)

• Also, glucose infusion reduces AA oxidation (3)

AA metabolism1

2

3

AA metabolism• However, exercise training does not appear to increase

AA metabolism in human subjects

– If anything, it is reduced

Ammonia scavenging during high intensity exercise

• During high intensity exercise, AMP is formed– Adenylate kinase Rx

• ADP + ADP ATP + AMP

• AMP then inhibits AK Rx if it builds up

• AMP deaminated to IMP• Muscle releases ammonia

(NH4+) during contraction

– Contains nitrogen

• Purine nucleotide cycle

Ammonia scavenging

• Formation of glutamine (1) helps to transport ammonia in blood– Ammonia is toxic– Transamination

• Glutamine goes to kidney (2)

• Urea (3) and glucose formed (4)

1

2

3

4

Neuro-endocrine control of blood glucose

Hormones

• Chemical messengers– Produced and stored in a gland– Secreted into the blood– General and specific effects

• Two basic types– Steroid

• Produced from cholesterol by adrenal cortex and gonads

– Polypeptides• Amino acids

Hormones

• Powerful effects• Precisely regulated

– Feedback control (negative feedback)

• Mechanisms of action– Affect cell permeability

(insulin)– Activate an enzyme

(epinephrine)– Protein synthesis (GH)

Blood glucose homeostasis• When fed

– Liver glycogenolysis

• When fasted– Gluconeogenesis

• SNS helps in this– Epi stimulates liver

glycogenolysis and gluconeogenesis

• Hormones– Released into blood– Epinephrine and nor-

epinephrine

• Maintenance of blood glucose levels is paramount– Fuel source– Anaplerotic additions to

Kreb’s– Allows fat metabolism– Needed by brain and

CNS• Hormones that help

maintain blood glucose– glucoregulatory

Hepatic glucose production during exercise

Glucose homeostasis• How difficult is this?• Normal adult

– Blood volume = 5L– Blood glucose = 100 mg/dl (1 g/L)– 5g or 20 kcals (4kcal/g) worth of energy– Only enough to support 1 min of maximal activity!

• This means– We must get plenty of CHO prior to and even during

activity– Liver supplements this

Glucose homeostasis• Glucose

production increased in 2 ways– Increased

absorption from gut and liver output

• Liver glycogenlosis

• Liver gluconeogenesis

Glucose homeostasis

• Note how addition of arm exercise increases catecholamine levels– glucoregulatory

hormone (raises blood glucose)

• Insulin falls– Decreases blood

glucose• Thus hormonal

changes help maintain blood glucose levels

Why increased?

Catecholamines and blood glucose• Epinephrine and nor-

epinephrine• Epi binds to β-receptor

– Activates adenylate cyclase– Muscle contraction increases

intracellular Ca2+ and Pi• Stimulates glycogenolysis

– Muscle and liver– Supports liver glucose

production – Also increases lipolytic rate

Cyclic AMP• Made from ATP (1)• Intracellular messenger• Activates many processes

in metabolism• Example

– Glycogenolysis• Epinephrine binds to

receptor (2)• Adenyl-cyclase creates

cAMP (3)• cAMP activates

phosphorylase

1

2

EPI

3

Insulin and glucagon• Insulin

– β cells of the islets of langerhans of pancreas

• Glucagon– α cells

• Along with epinephrine and nor-epinephrine, main hormones of glucose homeostasis

Insulin response to exercise

• Falls in response to exercise– Epinephrine

suppresses insulin secretion

• Thus– Glucose

production is increased

Why insulin?• Insulin

– Helps facilitate glucose transport across sarcolemma during rest

– Uses glucose transporters (GLUT)

– GLUT-4 • Insulin mobilizes

transporters from intracellular pool

• Transporters move to sarcolemma

Glucose transport: exercise• Muscular contraction

– “insulin-like” effect– GLUT-4 can

translocate due to insulin or Ca2+

• So, muscular contractions– Cause release of

Ca2+

– This causes translocation of Glut-4 receptors

– Important as epinephrine (released during exercise) inhibits insulin

insulin

Neuro-endocrine control of

hepatic glucose production • Gluconeogenesis

– Liver and kidneys• 3 different enzymes

than glycolysis– Pyruvate carboxylase– PEP carboxylase– Fructose 1,6

biphosphatase• Glucose 6-

phosphatase– Liver only

• So, muscle resynthesizes glycogen, liver and kidneys, glucose

Pyruvate kinase

Gluconeogenesis

• Those 4 enzymes are either nonexistent or in small supply in skeletal muscle

• Found in large amts in liver and kidneys• Pyruvate kinase (last step of glycolysis):

– virtually irreversible in skeletal muscle– In liver, can be inhibited by cAMP and

phosphorylation (Ca2+-dependent protein kinase)

– Reduces glycogenloysis and promotes gluconeogenesis

Gluconeogenesis• Pyruvate coverted to

oxaloacetate (A)– High acetyl-CoA, low ADP

• Oxaloacetate converted to Phosphoenolpyruvate (B)– Low ADP

• Phosphoenolpyruvate converted to Fructose 1, 6 bisphosphate (C)

• F 1,6 bisphosphate converted to F6P (D)– High citrate, low AMP

• Converted to glucose (E)

A

B

C

D

E

Hepatic glucose productionThe following hormones increase gluconeogenesis• Inhibit pyruvate kinase

– Glucagon– Epinephrine– Nor-epinephrine

• Insulin– Inhibits gluconeogenesis

Can muscle make glucose?

• Glycolytic muscle can produce glycogen from lactate– Glyconeogenesis

• Likely occurs early in recovery

• Muscle lacks G6 phosphatase– So can’t release glucose from

cell• However, it is possible that

debranching enzyme can release glucose from glycogen– May help explain very rapid inc

in blood glucose (fig 9-13)