Download MBBS Biochemistry PPT 39 Atabolism Ii Lecture Notes

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Catabolism of Carbon Skeletons of aa and

related disorders-II

Specific Learning Objectives

Catabolic pathways of four aa to Succinyl-CoA

Degradation and Resynthesis of Methionine

Disorder related to Methionine degradation

Degradation of Branched chain amino acids

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

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

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

Cystathione

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

Cont--

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

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

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

participation of biotin and coenzyme B12
Degradation and Resynthesis of Methionine

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

(MAT)

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

Disorder related to Methionine degradation

Hyper Homocysteinemia

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

in cystathionine -synthase of transsulfuration pathway causes

homocysteine to accumulate and remethylation leads to high levels of

methionine in patients

Elevated homocysteine and decreased folic acid levels in pregnant

women are associated with increased incidence of neural tube defects

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

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

abnormalities occur

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)

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

Supplementation with folate reduces risk of such defects

Degradation of Branched chain amino

acids
Mitochondrial branched-chain -keto acid dehydrogenase complex

consists of five components:

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

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

Catabolism of Leucine to Acetoacetate and Acetyl-CoA

Methylcrotonyl CoA carboxylase

-keto acid decarboxylase complex

Methylglutaconyl-CoA hydralase

Isovaleryl CoA dehydrogenase

HMG CoA lyase

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

Enoyl CoA hydratase

-hydroxyisobutyryl CoA hydrolase

-keto acid decarboxylase complex

Methacrylyl CoA dehydrogenase

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

Catabolism of Isoleucine to Acetyl-CoA and Propionyl-CoA

Amino acid transaminase

-hydroxybutyryl CoA dehydrogenase

-keto acid decarboxylase complex

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

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

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

and CoA)

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

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

accumulated in blood and spil over into urine

This condition is called MSUD or branched-chain ketonuria suggests

maple syrup, or burnt sugar.

Signs and symptoms of MSUD include fatal ketoacidosis,

neurological derangements, mental retardation, and a maple syrup

odor of urine

Early diagnosis by enzymatic analysis is essential to avoid brain

damage and early mortality by replacing dietary protein by an aa

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

Reference Books

18

1) Lehninger Principles of Biochemistry, 6th Ed.
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

10;25(1):27-42
Thank you

This post was last modified on 05 April 2022