Exam 1- Unit 4: Urea Cycle & Ketogenic & Glucogenic Amino Acids

What is the urea cycle?

• First proposed in 1932 by Hans Krebs & Kurt Henseleit (5 years prior to discovery of Krebs/CAC)

• First metabolic pathway discovered

• In cycle, 2 Ns are excreted, atoms come from NH+ & Asp

What does the carbon atom come from in urea cycle?

HCO3-, however enzyme carbonic anhydride can combine CO2 & H20 to form H2CO3 which can be ionized to HCO3- & oxygen atom comes from water

What is step 1 of the urea cycle?

• Occurs in mitochondrial matrix & catalyzed by Carbamoyl phosphate synthetase I

• Transporters exist to move both citrus line & ornithine out of & into matrix, respectively

What is step 2 of urea cycle?

• Catalyzed by enzyme Orinithine trasncarbomoylase

• Carbamoyl phosphate- good leaving group, acid anhydride, high transfer potential (high energy bond)

• 1 carbon less than Lys

What is step 3 of urea cycle?

• Catalyzed by enzyme Argininosuccinate synthetase

• PPi- pyrophosphate

• Hydrolysis releases energy & drives reaction forward- 2 high energy bonds are used

What happens in step 4 of urea cycle?

• Catalyzed by enzyme Arginosuccinase

• Produces arginine & fumarate

• Can be recycled back to CAC to regenerate oxaloacetate

• Contains the C/H/O skeleton of Asp

What happens in step 5 of urea cycle?

• Catalyzed by enzyme ArginasE

• Total cost to make urea= 4 ATP

• Produces Omithine & urea

• Omithine recycled back to step 2

What are the 2 high energy bonds produced in urea cycle?

• 2 ADP + 2 Pi

• AMP + PPi

What are the N sources for urea cycle?

NH4+ + Aspartate

What does nitrogen come from in urea cycle?

AAs & aminotrasnferase enzymes

How is urea cycle linked w/ CAC?

• Aspartate-arginine-succinct shunt of CAC purpose: connects both cycles

• Fumerate is converted to malware to enter matrix

• Asp comes out of matrix

• Aminotrasnferase is required

What collects nitrogen from AAs?

Glutamate

How is the urea cycle long-term regulated?

During prolonged starvation/dietary intake is primarily protein, carbon skeletons will be used as fuel & urea production will increase

• Synthesis of 5 enzymes involved in urea cycle will increase

How is the urea cycle short-term regulated?

Allosteric activation occurs w/ increasing concentrations of:

• Acetyl CoA & Glutamate (Substrates for N-acetyl glutamine synthase)

• Arginine (urea cycle intermediate)

  • Higher concentration Arg signified high NH4+ that needs removed

  • Activates N-Acetyl glutamate synthase & stimulates the 1st step in urea cycle

What are the inborn errors of metabolism?

• First put forth in 1902 by Archibald Garrod

• Linked alcaptonuria (alkaptonuria), benign condition that involved black urine, to inherited metabolic defect

• Discovered absence of homogentisate oxidase stops metabolism of Phenylalanine & tyrosine

  • Result, accumulation of homogentisate occurs

What are patient with alcaptonuria more prone to develop?

Form of arthritis

What happens with a blockage of enzymes w/in the urea cycle?

• Urea cycle disorders occur in 1 of every 10,000 births

• Synteshis of urea in liver is major route of NH4+ removal & there are no alternative pathways to synthesize urea

• Leads to: hyperammonemia (high NH4+) which can lead to coma & irreversible brain damage

• Possible cause is high conversion of alpha-keto glutamate to Glu & Gln

What is Glu?

Excitatory AA

What can happen w/ an argininosuccinase deficiency?

• Can be partly bypassed by providing a surplus of Arginine in diet & restricting protein intake

• Changes urea cycle to linear elimination

• 2 Ns are eliminated per Arginine molecule administered

  • Argininesuccinate substitutes for urea in removal of NH4+

What happens with Carbamoyl phosphate synthase I deficiency/ ornithine transcarbamoylase deficiency?

• Impairs formation of citrulline & arginosuccinate

• Excess N accumulates in Gly & Gln

• Challenge: remove Gly & Gln

• Solution: supplement protein-restricted diet with large amounts

• Inorganic pyroP

  • Hydrolyzed, energy released, drives rxn forward

• Conjugates substitute for urea cycle

• 1st step is same activation seen w/ fatty acid beta oxidation

What is sodium phenylbutyrate (Buphenyl)?

• Orphan drug to treat above deficiencies

• Condition occurs in <200,000 patients

• Metabolized to phenyl acetate

• One round of beta-oxidation

What is the advantage & adverse effects of sodium phenylbutyrate (Buphenyl)?

• Lacks unpleasant odor of phenyl acetate, however both have unpleasant taste

• Adverse: irregular menses or amnorrhea, decreased appetite (could be due to condition), disagreeable body odor (attributed to phenyl acetate

What is phenylketonuria?

• Due to genetic defect in phenylaline metabolism, specifically absence/deficiency of phenylalanine hydroplane. Incidence is 1 in 20,000

• Can cause severe retardation

  • Newborn infants w/ appear normal at birth but are severely affected at age 1 if not treated

• Biochemical basis for retardation is due to excess phenylalanine competing w/ other AAs for transport across the BBB, resulting in deficit in AAs required (requires certain amount of lipids to enter BBB, AAs are water soluble and need transporter)

What is the therapy for phenylketonuria?

• Low phenylalanine diet, must be started very soon after birth to prevent irreversible brain damage

  • Early testing is essential, simple blood test can detect

What are glycogenic AAs?

• Can funnel parts or all of carbon skeletons to form glucose

• 13 AAs are solely glucogenic

What are ketogenic AAs?

• Give rise to ketone bodies & cannot form glucose

• Only Lys & Leu are solely ketogenic

What are both ketogenic & glucogenic?

• Ile, Thr, Trp, Phe, & Tyr (large- broken “in half” during metabolism)

• Some carbons emerge as acetyl CoA/ acetoacetyl CoA while others emerge in CAC

What are aspects of glucogenic AAs?

• While glucogenic AAs can form glucose, there are differences in cost/gain of individual AAs

• CAC begins w/ condensation of oxaloacetate & acetyl CoA

• The FULL cycle contains 8 steps & forms 10 ATP/acetyl CoA

• When glucogenic AAs enter CAC, they only use some of these steps & form less than 10 ATP depending on their starting point

What are the 5 entry points for gluconeogenic AAs? What is the gain/cost of ATP to form 1 molecule of glyceraldehyde 3-P?

1. Start w/ alpha-ketoglutarate

• 2 NADH + ATP + FADH2= 7.5 minus cost (4.5) = 3 ATP

2. Start w/ succinyl CoA

• NADH + ATP + FADH2= 5 minus 4.5 = 0.5 ATP

3. Start w/ fumarate

• NADH = 2.5 minus 4.5 = -2 ATP

4. Start w/ oxaloacetate

• -4.5 ATP

5. Start w/ pyruvate

• -5.5 ATP

How many carbons can each glucogenic AAs contribute to molecule of glucose?

Three, therefore takes 2 glucogenic AAs to form 1 glucose molecule