Introduction to METABOLISM OF INDIVIDUAL AMINO ACIDS
- Protein metabolism denotes the various biochemical processes responsible for the synthesis of proteins and amino acids (anabolism), and the breakdown of proteins by catabolism.
- Simplest amino acid and is the only amino acid which does not exhibit optical isomerism
- Non essential amino acid and glycogenic
- Glycine in Proteins
- Commonly occuring amino acid in protein structure.
- The structural protein collagen has a high content of glycine.
Synthesis of Glycine
Biologically important compounds synthesized from glycine
- Synthesis of purine ring
- Synthesis of heme
- Formation of Glutathione
- Glycine as conjugating agent
- Glycine as neurotransmitter
- Formation of creatine
CREATINE AND CREATINE PHOSPHATE
- Creatine 0.2-0.6 mg/dl
- Creatinine 0.6-1.5 mg/dl
- Creatine 0-50 mg/day
- Creatinine 1-2 g/day
Metabolic errors of Glycine
- Characterized by urinary excretion of 0.6g to 1g of glycine per day and a tendency to form oxalate renal stones
- Results from defect in renal tubular re absorption.
- Arises from deamination of glycine, forming glyoxylate (oxalate semialdehyde).
- Urinary excretion of oxalate is unrelated to dietary intake of oxalate.
- Progressive bilateral calcium oxalate urolithiasis, nephrocalcinosis and re-current infection of the urinary tract are followed by early mortality from renal failure or hypertension.
- Alanine can be synthesized by the transamination of pyruvate by glutamate-pyruvate aminotransferase (also called alanine transaminase, ALT).
- Non essential
- glutamate + pyruvate <——-> a-KG + alanine
Also formed from
Glucose – Alanine Cycle
- Glucose get converted to pyruvate in muscle tissue to generate energy.
- Amino group from Glutamate is transferred to pyruvate to form alanine.
- Alanine is exported to the liver via the blood stream where it is deaminated to pyruvate.
- Must remove toxic ammonia it will convert to urea.
- Pyruvate is get convert to glucose which is returned to the muscle for fuel.
BRANCHED CHAIN AMINO ACIDS
- A branched-chain amino acid (BCAA) is an amino acid having aliphaticside-chains with a branch.
- Among the proteinogenic amino acids, there are three BCAAs: leucine, isoleucine and valine.
- Valine is glucogenic; Leucine is ketogenic While Isoleucine is both ketogenic and glucogenic.
- All the three are essential amino acids.
- Leucine is the major ketogenic amino acid
- These amino acid serves as an alternate source of fuel for brain
- Especially under the condition of starvation
- All the three amino acids undergo similar sequence of reactions.
- Transamination, Decarboxylation and dehydrogenation.
Metabolic Disorders of Branched Chain Amino Acid Catabolism
Maple Syrup Urine Disease
(Urine with smell of burnt sugar)
- It is also called branched chain ketonuria.
- Incidence is 1 per 1 lakh Births.
- The name originates from the characteristic smell of urine similar to burnt sugar or maple sugar due to excretion of branched chain keto acids.
- An inherited disorder of the branched chain amino acids.
- Occurs due to the absence of α-ketoacid decarboxylase or greatly reduced activity of the enzyme.
- The conversion of all three branched chain α-keto acids to CO2 and acyl CoA-thioesters does not take place.
Clinical features :
- The disease is manifested by the first week of extra uterine life.
- Infant does not take feed and may vomit, child has poor muscle tone.
- The patient may exhibit lethargy and convulsive seizures.
- Excessive brain damage occurs in the surviving children and mental retardation
- Biochemical findings
- Plasma levels of the branched chain amino acids leucine, isoleucine , valine and their corresponding α– keto acids are greatly enhanced.
- Increased excretion of the branched chain amino acids and their corresponding keto acids in urine is noticed. The condition is called branched chain ketonuria.
- The urine has a characteristic odour which resembles that of maple syrup or burnt sugar.
- The defect occurs due to impaired activity of isovaleryl CoA dehydrogenase involved in the leucine metabolism.
- Isovaleryl – CoA accumulates, is hydrolysed to isovalerate and is excreted in the urine and sweat.
- Symptoms include ‘cheesy odor’ of the breath and body fluids, vomiting, acidosis and coma precipitated by excessive ingestion of protein.
- A disorder that occurs due to the defect in the Propionyl – CoA carboxylase deficiency that catalyses the conversion of α Methylacetoacetyl – CoA to Acetyl – CoA and Propionyl – CoA during catabolism of isoleucine.
Other Biochemistry Notes
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