I have my university exam of 1 st year of subject biochemistry These are the important question give me answer of this "Products of phenylalanine and tyrosine metabolism" and remeber it's a university examination The answer should be easy to memorize and follow the pattern of answer writing as in University paper
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Products of Phenylalanine and Tyrosine Metabolism
[University Biochemistry - 1st Year Examination Answer]
Introduction
Phenylalanine and tyrosine are aromatic amino acids. Phenylalanine is an essential amino acid, while tyrosine is non-essential (since it is synthesized from phenylalanine). Both amino acids share a common degradative pathway and give rise to several important biological products.
Step 1: Conversion of Phenylalanine to Tyrosine
The first and key reaction is the hydroxylation of phenylalanine to tyrosine, catalyzed by phenylalanine hydroxylase (PAH), a mixed-function oxidase.
- Coenzyme required: Tetrahydrobiopterin (BH4), synthesized from GTP
- BH4 is oxidized to dihydrobiopterin (BH2), then regenerated by dihydropteridine reductase (DHPR) using NADH
- Deficiency of PAH → Phenylketonuria (PKU)
Step 2: Degradative Pathway of Tyrosine (Catabolic Products)
The degradation of tyrosine proceeds stepwise as shown:
| Step | Compound | Enzyme | Associated Disease if Deficient |
|---|---|---|---|
| 1 | Phenylalanine → Tyrosine | Phenylalanine hydroxylase (PAH) | PKU |
| 2 | Tyrosine → p-Hydroxyphenylpyruvate | Tyrosine aminotransferase (TAT) + PLP | Tyrosinemia II |
| 3 | p-Hydroxyphenylpyruvate → Homogentisate | 4-Hydroxyphenylpyruvate dioxygenase (+ Vitamin C) | Transient tyrosinemia of newborn |
| 4 | Homogentisate → Maleylacetoacetate | Homogentisate oxidase | Alkaptonuria |
| 5 | Maleylacetoacetate → Fumarylacetoacetate | Maleylacetoacetate isomerase | - |
| 6 | Fumarylacetoacetate → Fumarate + Acetoacetate | Fumarylacetoacetate hydrolase | Tyrosinemia I |
Final catabolic products: Fumarate (glucogenic) + Acetoacetate (ketogenic)
- This is why phenylalanine and tyrosine are classified as both glucogenic and ketogenic amino acids.
Step 3: Other Important Products of Tyrosine (Anabolic/Specialized Products)
Tyrosine is not just degraded - it is also a precursor for several key biological molecules:
A. Catecholamines (Neurotransmitters and Hormones)
Tyrosine
↓ (Tyrosine hydroxylase, BH4 required)
L-DOPA (3,4-dihydroxyphenylalanine)
↓ (DOPA decarboxylase, PLP)
Dopamine
↓ (Dopamine β-hydroxylase, Vitamin C)
Norepinephrine (Noradrenaline)
↓ (PNMT - phenylethanolamine-N-methyltransferase, SAM)
Epinephrine (Adrenaline)
B. Thyroid Hormones
- Tyrosine residues in thyroglobulin are iodinated to form:
- Monoiodotyrosine (MIT)
- Diiodotyrosine (DIT)
- Coupling: DIT + DIT → Thyroxine (T4)
- Coupling: MIT + DIT → Triiodothyronine (T3)
C. Melanin (Skin Pigment)
Tyrosine
↓ (Tyrosinase, Cu2+ dependent)
DOPA
↓
Dopaquinone
↓
Melanin (black/brown pigment)
- Deficiency of tyrosinase → Albinism (lack of skin pigmentation)
Summary Table of All Products
| Category | Product(s) |
|---|---|
| Catabolic end-products | Fumarate + Acetoacetate |
| Neurotransmitters | Dopamine, Norepinephrine, Epinephrine |
| Thyroid hormones | T3 (triiodothyronine), T4 (thyroxine) |
| Skin pigment | Melanin |
| Non-essential amino acid | Tyrosine (from phenylalanine) |
Associated Disorders (Important for Exams)
| Disorder | Enzyme Defect | Key Feature |
|---|---|---|
| PKU (Phenylketonuria) | Phenylalanine hydroxylase | Intellectual disability; musty odor of urine; fair skin |
| Alkaptonuria | Homogentisate oxidase | Dark urine, ochronosis (darkening of cartilage), arthritis |
| Tyrosinemia I | Fumarylacetoacetate hydrolase | Liver failure in infancy |
| Tyrosinemia II | Tyrosine aminotransferase | Eye and skin lesions, neurological problems |
| Albinism | Tyrosinase | Lack of melanin pigment |
Memory Tips
- "Phe gives Tyr, Tyr gives All" - Phenylalanine gives Tyrosine, Tyrosine gives All products (catecholamines, thyroid hormones, melanin, fumarate, acetoacetate)
- "DOPA - DAN - E" - DOPA → Dopamine → Adrenaline (Noradrenaline in between)
- PKU = PAH Kills Understanding (PAH deficiency leads to intellectual disability)
- Alkaptonuria = urine turns Black on standing (homogentisate oxidizes)
Sources: Basic Medical Biochemistry - A Clinical Approach 6e (Lieberman & Peet); Biochemistry 8th ed - Lippincott Illustrated Reviews; Harper's Illustrated Biochemistry 32nd Ed
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