Download MBBS Biochemistry PPT 39 Atabolism Ii Lecture Notes

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Specific Learning Objectives

Catabolic pathways of four aa to Succinyl-CoA

Degradation and Resynthesis of Methionine

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Disorder related to Methionine degradation

Degradation of Branched chain amino acids

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Disorder related to Branch chain aa degradation
Summary of Amino acid Catabolism

Fig18.15: Lehninger Principles of Biochemistry by David L Nelson

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Genetic disorders related to Amino-acid catabolism

Table 18.2: Lehninger Principles of Biochemistry by David L Nelson
Catabolic pathways of four aa to Succinyl-CoA

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Cystathione

Fig18.27: Lehninger Principles of Biochemistry by David L Nelson

Cont--

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Catabolism of isoleucine, methionine, and valine to propionyl-CoA

Propionyl-CoA, product of odd-chain fatty acid degradation, is

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converted, to succinyl-CoA by a series of reactions involving

participation of biotin and coenzyme B12
Degradation and Resynthesis of Methionine

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Biosynthesis of S-adenosylmethionine from methionine
and ATP is catalyzed by methionine adenosyltransferase

(MAT)

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Fig 20.8. Lippincott's Illustrated Reviews, Biochemistry, 6th Ed

Disorder related to Methionine degradation

Hyper Homocysteinemia

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Elevations in plasma homocysteine (Hcy) as a result of rare deficiencies

in cystathionine -synthase of transsulfuration pathway causes

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homocysteine to accumulate and remethylation leads to high levels of

methionine in patients

Elevated homocysteine and decreased folic acid levels in pregnant

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women are associated with increased incidence of neural tube defects

(improper closure, as in spina bifida) in fetus
Cont--

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Lens of eye is frequently dislocated after age of 3, and other ocular

abnormalities occur

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Mental retardation is frequently first indication of this deficiency

Attempts at treatment include restriction of methionine intake and feeding

of betaine (or its precursor, choline)

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In some cases significant improvement by feeding pyridoxine (vit B6)

Supplementation with folate reduces risk of such defects

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Degradation of Branched chain amino

acids
Mitochondrial branched-chain -keto acid dehydrogenase complex

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consists of five components:

E1: thiamin pyrophosphate (TPP)-dependent branched chain -

ketoacid decarboxylase

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E2: dihydrolipoyl transacylase (contains lipoamide)
E3: dihydrolipoamide dehydrogenase (contains FAD)
Protein kinase
Protein phosphatase

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Catabolism of Leucine to Acetoacetate and Acetyl-CoA

Methylcrotonyl CoA carboxylase

-keto acid decarboxylase complex

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Methylglutaconyl-CoA hydralase

Isovaleryl CoA dehydrogenase

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HMG CoA lyase

Fig 29.20 and 29.21. Harper's Illustrated Biochemistry 30th Edition
Catabolism of Valine to -Aminoisobutyrate and Succinyl-CoA

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Enoyl CoA hydratase

-hydroxyisobutyryl CoA hydrolase

-keto acid decarboxylase complex

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Methacrylyl CoA dehydrogenase

Fig 29.20 and 29.23. Harper's Illustrated Biochemistry 30th Edition

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Catabolism of Isoleucine to Acetyl-CoA and Propionyl-CoA

Amino acid transaminase

-hydroxybutyryl CoA dehydrogenase

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-keto acid decarboxylase complex

Fig 29.20 and 29.22. Harper's Illustrated Biochemistry 30th Edition
Disorder related to Branch chain aa degradation

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Biochemical defect in maple syrup urine disease (MSUD) involves -keto

acid decarboxylase complex (thiamine pyrophosphate, FAD, NAD, lipoate

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and CoA)

Plasma and urinary levels of leucine, isoleucine, valine, and their -keto

acids and -hydroxy acids (reduced -keto acids) are elevated and

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accumulated in blood and spil over into urine

This condition is called MSUD or branched-chain ketonuria suggests

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maple syrup, or burnt sugar.

Signs and symptoms of MSUD include fatal ketoacidosis,

neurological derangements, mental retardation, and a maple syrup

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odor of urine

Early diagnosis by enzymatic analysis is essential to avoid brain

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damage and early mortality by replacing dietary protein by an aa

mixture that lacks leucine, isoleucine, and valine
Two Clinical-cases discussed

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Reference Books

18

1) Lehninger Principles of Biochemistry, 6th Ed.

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2) Harper's Il ustrated Biochemistry-30th edition
3) Biochemistry, Lippincott's Il ustrated Reviews, 6th Ed
4) Text Book of Medical Biochemistry by Chatterjee & Rana Shinde, 8th Ed
5) DM Vasudevan' s Textbook of Biochemistry for Medical Students, 6th Ed
6) Gregory S. Ducker and Joshua D Rabinowitz. Cell Metab. 2017 Jan

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10;25(1):27-42
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