designed for undergraduate level teaching of nitrogen metabolism focusing on amino acid metabolism in biochemistry. this is third in the series of three lectures. ideal for MBBS level teaching

1. DEGRADATION OF AMINO
ACIDS-II
Kshema Thakur, PhD
Molecular Biology & Biochemistry

4. Synthesis of S-adenosyl methionine
and homocysteine

5. Degradation and resynthesis of methionine.
[Note: The resynthesis of methionine from homocysteine is the only reaction in which THF
both carries and donates a methyl group. In all other reactions, SAM is the methyl group
carrier and donor.]

6. Homocysteine and heart attacks
 Elevations in plasma homocysteine levels (normal <15 μmol/l) promote oxidative damage,
inflammation, and endothelial dysfunction.
 Apparently it is believed to react with collagen to produce reactive free radicals in addition to
interfere with incorporation of collagen cross links.
 Also implicated in aggregation of LDL particles.
 Is an independent risk factor for occlusive vascular disease.
 Epidemiologic studies have shown that plasma homocysteine levels are inversely related to plasma
levels of folate, B12, and B6—the three vitamins involved in the conversion of homocysteine to
methionine or cysteine.
 Supplementation with these vitamins has been shown to reduce circulating levels of homocysteine.
 However, in patients with established cardiovascular disease, vitamin therapy does not decrease
cardiovascular events or death.
 Elevated homocysteine or decreased folic acid levels in pregnant women are
associated with increased incidence of neural tube defects (improper closure,
as in spina bifida) in the fetus.
 Periconceptual supplementation with folate reduces the risk of such defects.

7. DEGRADATION OF
CYSTEINE

8.  The homocystinurias are a group of disorders involving defects in the
metabolism of homocysteine.
 autosomal recessive illnesses, characterized by high plasma and urinary
levels of homocysteine and methionine and low levels of cysteine.
 Most common cause of homocystinuria is defective cystathionine β-
synthase (Type I).
 Homozygotes exhibit ectopia lentis (displacement of the lens of the eye),
skeletal abnormalities, a tendency to form thrombi (blood clots),
osteoporosis, and neurological deficits.
 The patients have high level of homocysteine in serum and usually die of
myocardial infarction, stroke or pulmonary embolism.
 Patients can be responsive or nonresponsive to oral administration of
pyridoxine (vitamin B6)—a coenzyme of cystathionine β-synthase.
 Vitamin B6–responsive patients usually have a milder and later onset of
clinical symptoms compared with B6-nonresponsive patients. Treatment
includes restriction of methionine intake and supplementation with vitamins
B6, B12, and folate.
Enzyme deficiency in
homocystinuria
Homocystinuria Type II: defective N5-N10 – Methylene THF reductase
Homocystinuria Type III: defective N5-Methyl THF homocysteine methyltransferase.
There is impaired synthesis of methylcobalamin
Homocystinuria Type IV: defective intestinal absorption of vitamin B12
HOMOCYSTINURIAS

9. CYSTINURIA
 Cystine-lysinuria
 1 in 7000
 Defective reabsorption of amino acids
 Carrier system for cysteine, ornithine, arginine and lysine (COAL) in
kidney.
 Defect leads to excretion of all these in urine.
 Cystine being relatively insoluble leads to formation of stones in kidney
and urinary tract on precipitation.
 Diagnosis: cyanide nitroprusside test
 Treatment: restricted ingestion of dietary cystine and high intake of fluids
CYSTINOSIS
 Cystine storage disease
 There is a deposition of cystine crystals in reticuloendothelial system
throughout the body – spleen, lymph nodes, liver, kidney, bone marrow etc.
 There is a lysosomal deposition of cystine
 Impairment in renal function is observed and characterized by amino aciduria
 The patients usually die in 10 years of age

10. BRANCHED CHAIN AMINO ACIDS
 Although much of the
catabolism of amino acids
takes place in the liver, the
three amino acids with
branched side chains
(leucine, isoleucine, and
valine) are oxidized as fuels
primarily in muscle, adipose,
kidney, and brain tissue.
 These extrahepatic tissues
contain an aminotransferase,
absent in liver, that acts on
all three branched-chain
amino acids to produce the
corresponding α-keto acids

11. METABOLIC DEFECTS OF BRANCHED
CHAIN AMINO ACIDS
 Maple syrup disease (Maple Syrup Urine Disease/MSUD)
 Rare, autosomal recessive disorder where 1 in 1,85,000 are affected
 Also known as branched chain ketonuria
 A characteristic maple syrup or burnt sugar odor is imparted in the urine
 Partial or complete defect of branched chain α-keto acid dehydrogenase complex enzyme
 These amino acids and their corresponding α-keto acids accumulate in the blood, causing a
toxic effect that interferes with brain functions.
 If untreated, the disease leads to mental retardation, physical disabilities, and even death.
 Infants with classic MSUD show symptoms within the first several days of life.
Characterized by feeding problems, vomiting, dehydration, severe metabolic acidosis.
• There is impairment in transport and function of other amino acids
• Protein biosynthesis is reduced
• Branched chain amino acids competitively inhibit glutamate dehydrogenase
 Diagnosis: enzyme analysis or urinary analysis for branched chain amino acids and keto
acids.
 Treatment: diet low in or free from branched chain amino acids
 Patients with the rare thiamine-dependent variant of MSUD achieve increased activity of
branched-chain α-keto acid dehydrogenase if given large doses of this vitamin

12. Intermittent branched chain ketonuria
Less severe variant of MSUD. Impairment in the activity of α-keto acid
dehydrogenase and not total blockade. Dietary planning can overcome
the disorder
Isovaleric acidemia
Associated with leucine metabolism. Defective isovalery CoA
dehydrogenase
High amount of isovalerate is observed in urine
Cheesy odor in breath and body fluids
Acidosis and mild retardation are observed.
Hypervalinemia
Associated with valine.
Transamination of valine is selected impaired.
Increased plasma concentration of valine are observed.
METABOLIC DEFECTS OF BRANCHED
CHAIN AMINO ACIDS (contd)

13. Histidine, proline, arginine
form glutamate
Type I
hyperprolinemia
Type II
hyperprolinemia
Hyperargininemia
Agmatine: derivative of arginine naturally produced in
brain. Has modulatory action on various molecular
targets such as neurotransmitter systems, nitric oxide
synthesis, ion channels and polyamine metabolism.

14. Biosynthesis of nitric oxide
NADPH, FMN, FAD, HEME &
Tetrahydrobiopterin
 Synthesised in brain
 Has a very short half life of about 5 sec
 Is endothelium derived release factor (EDRF) causing
smooth muscle relaxation.
 Promotes synthesis of cGMP and hence mediates its
action via cGMP and Protein Kinase G.
Functions:
1. Vasodilator and causes smooth muscle relaxation
2. Regulates blood flow and blood pressure. NO synthase
inhibitors elevate blood pressure
3. Inhibits platelet aggregation and adhesion
4. Acts as neurotransmitter
5. Mediates bactericidal action of macrophages
6. Involved in erection of penis

15. Histidine metabolism
Histidinemia
Folate deficiency
blocks this
reaction and
leads to elevated
excretion of
FIGLU in urine

16. HISTIDINE METABOLISM
Histidine is a source of one carbon pool
Histidine load test is commonly employed to assess folate deficiency.
Histamine:
Decarboxylation of histidine to histamine is catalyzed by a broad-
specificity aromatic L-amino acid decarboxylase that also catalyzes the
decarboxylation of dopa, 5-hydroxytryptophan, phenylalanine, tyrosine,
and tryptophan.
Histidinemia
1 in 20,000
Elevated histidine in plasma
Elevated excretion of imidazole pyruvate and histidine in urine
Mental retardation and defective speech are observed
No treatment improves the condition

17. LYSINE
Lysine
Synthesis of L-
carnitine

18.  An essential amino acid
 ε-amino group of lysine plays crucial role in
protein structure by forming salt bridges
Synthesis of carnitine
 Trimethyllysine from the proteolysis of proteins serves as the precursor of carnitine.
 Trimethyllysine is methylated form of lysine present in proteins, a methylation
reaction mediated by SAM.
 Lysine is methylated only when present in proteins, and never in free form.
 Carnitine serves as an important transporter of long chain fatty acids
from cytosol to mitochondria for oxidation.
 Besides endogenous synthesis, the external sources include meat, fish, poultry and
dairy products.
LYSINE

19. G
L
U
T
A
M
A
T
E
G
L
U
T
A
M
I
N
E
γ-Aminobutyric acid
(GABA)
Glutathione
N-acetylglutamate
γ-carboxyglutamate
(clotting factor)
Detoxification
Amino sugars
Pyrimidines (N3)
Purines (N3, N9)
Arginine
Proteins
Histidine
Glucose
NH3
Proteins
Urea
NH3
Overview of
glutamate
and
glutamine
metabolism

20. OVERVIEW OF GLUTAMINE
METABOLISM

21. Metabolism of γ-Aminobutyric acid (GABA)
GABA – inhibitory
neurotransmitter in brain
Opens the chloride
channels leading to
increased permeability of
post-synaptic membranes
and hence its inhibitory
effect.
Decreased GABA causes
convulsions

22. B6 deficiency reduces the production of GABA and hence
neuronal hyperexcitability and convulsions
Stiff person syndrome (SPS)
 Neurological syndrome
 Impairment in the synthesis and function of GABA
 Presence of glutamic acid decarboxylase antibodies
inhibit the enzyme activity and hence reduce the GABA
synthesis.
 Muscular rigidity, stiffness and painful muscle spasms

23. OVERVIEW OF SERINE METABOLISM

24. OVERVIEW OF THRONINE
METABOLISM

25. Metabolic
fate of
amino acids
Ketogenic
amino
acids
Glucogenic
amino
acids

26. One carbon
metabolism

32. Amino acid/derivative Major function(s)
Glycine Inhibitory neurotransmitter in
spinal cord
Glutamate Excitatory neurotransmitter
Dopamine Increases blood pressure
Norepinephrine &
epinephrine
Hormonal neurotransmitters,
increase cardiac output and
blood pressure
Serotonin Regulates cerebral activity and
behaviour
γ-Aminobutyric acid Inhibitory neurotransmitter in
brain
Amino acids and their
derivatives as neurotransmitters

33. POLYAMINES
 The compounds having more than one amino group.
 Ornithine and S-adenosylmethionine are precursors
 Interestingly, the four carbon moiety and not the methyl group of SAM is utilized in this reaction
 Ornithine decarboxylase is the regulatory enzyme.
 SAM decarboxylase reaction is a rare example not requiring PLP as coenzyme
 Polyamine oxidase of liver peroxisomes is involved in polyamine degradation
Functions:
 Involved in DNA, RNA and protein synthesis
 Essential for cell growth and proliferation
 Protein kinase is inhibited by polyamines
 Involved in stabilization of membrane structure
Clinical significance:
 Elevated excretion of polyamines in various cancers : leukemias, carcinoma of lungs,
bladder, kidney etc
 Putrescine makes for an ideal marker of cell proliferation
 Spermidine makes up for cell destruction marker

35. Summary of amino
acid metabolism

36. Summary of amino
acid metabolism

37. Sources:
1. Lehninger – Principles of Biochemistry
David L. Nelson & Michael M. Cox
2. Lipincott’s Illustrated Reviews
Richard A. Harvey & Denise A. Ferrier
3. Harper’s Illustrated Biochemistry
Robert K. Murray, Daryl K. Granner, Peter A. Mayes & Victor W. Rodwell
4. Biochemistry
U. Satyanarayan & U. Chakrapani
5. World wibe web

38. Thank you