MSK TBL 1: Amino Acid & Skeletal Muscle Metabolism

nitrogen balance

amount of nitrogen in body

*most healthy people have 0 nitrogen balance

transaminases

shuttle nitrogen

[amino acid catabolism in peripheral tissues produces ammonia, esp in muscle]

glutamine synthetase

glutamate [1 N] + NH3 → glutamine [2 N]

fate of glutamine if energy is needed

converted into a ketoacid for catabolism

free ammonia converted to urea & intmd can make ATP

negative nitrogen balance

illness

• starvation

  • proteins converted to aa’s

• diabetes

positive nitrogen balance

growth

• childhood

• pregancy

• anabolic state

  • body needs more proteins & nitrogen, so less is removed & hunger functions activate

Hyperammonemia

occurs when excess nitrogen is NOT converted to urea

→ significant cognitive & movement issues

asterixus

flapping tremor when pt holds palms up

indicates hyperammonemia

Markers of skeletal muscle damage

AST [aspartate aminotransferase]

CK [creatine kinase]

Markers of liver disease

AST

ALT [alanine aminotransferase]

alkaline phosphatase

glutathione

Arginine catabolism

Arg enters TCA cycle & N is donated to urea cycle

Arginine anabolism

• Creatine & skeletal muscle metabolism

• Nitric oxide signaling [cGMP]

muscles use what as a storage fuel?

creatine phosphate

creatine’s starting material is

Arg & Gly

made in kidney

creatine kinase

catalyzes:

creatine reversibly donating its phosphate to ADP to make ATP

• allows muscles to contract for a longer period of time

creatinine

waste product

• amount of creatinine excreted each day is constant

• used as indicator of normal excretory function of kidneys

Sources of Energy/ATP for Muscle Contraction ranked shortest to longest

1. ATP concentration in muscle fiber [1-2 sec]

2. Phosphocreatine making ATP via CK enzyme

3. Glycolysis & glycogen hydrolysis [1 min]

4. Oxidative metabolism [hrs]

   1. requires O2

   2. uses fatty acids, glucose, and aa’s as substrates

nitric oxide signaling

• paracrine

• NO is a signaling molecule that uses cGMP signaling

clinically important cGMP signals

“BAD GraMPa”

BNP, ANP [cardiac hormones

EDRF [NO]

Endothelium-derived relaxing factor

• EDRF/NO

• smooth muscle relaxant

  • stops myosin activity

• vasodilation

  • decreases calcium to decrease BP

• NT

• other

  • decreases blood coagulation & phagocytosis

NO signaling mechanism: local secretion & NT

• NO synthase catalyzes conversion of Arg → NO

• NO is gas that readily crosses cell membranes

• activates guanylate cyclase, which makes cGMP

As a NT:

• NO released into NMJ as vesicles

• to stop signal, phosphodiesterase hydrolyzes cGMP

nitroglycerin

• NO variant

• used for symptom control in acute heart conditions

BCAA metabolism

• connected to developmental delays

• predominantly catabolized in skeletal muscle

• BCAA have a branch in incorrect location that doesn’t allow them to be a direct substrate for beta-oxidation

BCAA metabolism pathway: Ile or Val or alloisoleucine

1. amine removed by transamination & carried to urea cycle by glutamine

2. C is removed by oxidative decarboxylation [branched-chain ketoacid dehydrogenase]

   1. uses thiamine [B1] as cofactor

3. remaining aa skeleton shortened to make Propionyl-CoA

4. Propionyl-CoA converted to Methylmalonyl-CoA by propionyl-CoA carboxylase

   1. uses biotin [B7] as a cofactor

5. Methylmalonyl-CoA can enter TCA as Succinyl-CoA by enzyme methylmalonyl-CoA mutase

   1. uses cobalamin [B12] as a cofactor

BCAA metabolism pathway: Leu

*converted exclusively to ketogenic substrates

1. amine removed by transamination & carried to urea cycle by glutamine

2. C is removed by oxidative decarboxylation [branched-chain ketoacid dehydrogenase] → product Isovaleric-CoA

   1. uses thiamine [B1] as cofactor

3. remaining aa skeleton shortened

4. shorter skeleton converted into acetoacetate

   1. uses biotin [B7] as a cofactor

Maple Syrup Urine Disease [MSUD]

• caused by decreased activity of branched-chain ketoacid dehydrogenase

• manifests v. early [few days]

• Findings

  • poor feeding, vomiting, lethargy

• if untreated → irreversible neurological damage → cerebral edema at high levels, coma, & metabolic catastrophes

• intellectual disability

  • Leu prevents uptake of other aa’s → loss of NTs

• poor muscle tone

• characteristic odor: maple syrup smell in urine or ear wax

• Labs

  • ketosis: ketones in blood & urine

  • accumulation of 3 BCAAs & alloisoleucine

• in some cases, symptoms can be decreased by high levels of thiamine [vit. B1]

  • assuming some residual enzyme activity

Methionine catabolic pathway

1. Met makes S-adenosylmethionine [SAM], which transfers methyl groups to make products

2. SAM → homocysteine [has free SH group needed to make Cys]

3. cystathione beta synthase converts homocysteine → cystathione thru process called transsulfuration using Ser

   1. occurs in liver & kidney

   2. requires Vitamin B6

4. cystathione directly makes Cys

5. remaining C skeleton converted into Propionyl-CoA

SAM required to make

phospholipids, DNA, & NTs

normally, homocysteine levels are

v. low in serum b/c its either remethylated to methionine or catabolism

B12 cycle

• B12 carries methyl groups

• in Met Synth, methyl group comes from folate

• in catabolism its donated to folate

• in generation of succinyl-CoA, it moves the 1 carbon

source of B12

animal products

enzymes that require vitamin B12 [2]

1. methionine synthase

   1. remethylation of homocysteine

2. methylmalonyl-CoA mutase

   1. makes TCA cycle intmd, succinyl-CoA during catabolism

B12 cycle is connected to which other vitamin

B9

• both are catalytic cycles termed 1-carbon metabolism

B12 deficiency assessed by

• accumulation of methylmalonyl-CoA & urinary excretion of methylmalonic acid

• → anemia

• elevated homocysteine

  • Met makes SAM

B12 is methylated by

5-methyltetrahydrofolate [FH4-CH3]

• active folate

dihydrofolate reductase [FH2 reductase]

catalyzes synth of FH4 from folate

methylenetetrahydrofolate reductase [FH4 reductase]

converts FH4-methylene [also an active folate] to FH4-CH3

2nd pathway in liver to remethylate homocysteine to methionine

uses betaine as methyl donor and enzyme betaine methyltransferase

why is folate important in pregancy

allows RNA → DNA

which develops faster: B12 or B9 deficiency?

B9

• B12 takes a long time to develop because liver has ~1-2 yr supply of B12

B12 deficiency leads to what kind of anemia?

pernicious, macrocytic anemia

• low folate → poor DNA synth → immature red cells released from marrow [large bc have nucleus & organelles]

• neutrophils will also have >5 nuclear lobes

prolonged B12 deficiency →

• symmetric tingling

• proprioception loss

• weakened vasculature

*once neurological symptoms take place, its IRREVERSIBLE

methylamalonyl-CoA acidemia

• occur from either B12 deficiency or enzymatic mutation to methylmalonyl-CoA mutase

• methylmalonyl-CoA converts to methylmalonic acid in blood → blood pH imbalance

• appears in early infancy

• autosomal recessive

• findings

  • hypotonia

  • hepatomegaly

  • hyperammonemia

  • ketoacidosis

  • thromnopenia

  • heart problems can also occur

• high levels of methylmalonic acid & other ketones in urine

cystathionine beta synthase [CBS] deficiency

AKA homocystinuria

• CBS mutations → elevated homocysteine in blood & urine

• more common in people from Germany, Norway, Ireland, & Qatar

• usually appears at 3-5 yrs old

• findings

  • connective & fiber tissue problems

    • limited joint mobility

    • dislocation of ocular lens in eye

    • skeletal abnormalities

    • excessive blood clotting → BV obstruction → atherosclerosis, MI, or stroke

    • sight issues seeing far away [myopia = nearsighted]

*can be confused with Marfan Syndrome

• HIGH homocysteine & Met

• LOW Cys

isovaleric acidemia

• caused by dysfunction in leu catabolism

• less severe version of propionyl-CoA carboxylase deficiency

• urine odor that smells like feet

propionyl-CoA carboxylase deficiency

• caused by mutation to either gene that codes the 2 subunits of propionyl-CoA carboxylase

• imp in the following macronutrients: VOMIT

  • Val

  • Odd-chain fatty acid

  • Met

  • Ile

  • Thr

• some correlation w/ Saudia Arabia, Amish, & Inuit populations

• Findings

  • 1st, is severe vomiting in the 1st weeks after birth

    • encephalopathy & bone marrow suppression

  • manifests as failure to thrive, recurrent infections, cardiomyopathy, & pancreatic infections

• high levels of propionic acid & other ketones in urine