Essential & non essential amino acid mbbs 1 years Biochemistry suggestion to Satyanarayana
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Essential & Non-Essential Amino Acids
MBBS 1st Year Biochemistry | Satyanarayana Pattern
Overview
The human body uses 20 standard amino acids to synthesize proteins and other nitrogen-containing compounds. Based on whether the body can synthesize them in sufficient amounts, amino acids are classified as:
- Essential (Indispensable) - cannot be synthesized; must come from diet
- Non-Essential (Dispensable) - synthesized in adequate amounts by the body
- Conditionally Essential - normally synthesized, but required from diet under special conditions
1. ESSENTIAL AMINO ACIDS (9 in adults)
Mnemonic: PVT TIM HaLL (classic Satyanarayana mnemonic)
| Letter | Amino Acid |
|---|---|
| P | Phenylalanine |
| V | Valine |
| T | Threonine |
| T | Tryptophan |
| I | Isoleucine |
| M | Methionine |
| H | Histidine |
| a | (small - less essential quantity) |
| L | Leucine |
| L | Lysine |
Another mnemonic: "Any Help In Learning These Little Molecules Proves Truly Valuable"
(A=Arg conditional, H=His, I=Ile, L=Leu, T=Thr, L=Lys, M=Met, P=Phe, T=Trp, V=Val)
Key points about essential amino acids:
- They cannot be synthesized by human tissues (or can't be made in sufficient quantity)
- Require many enzyme steps (5-10 enzymes each) - evolutionarily costly
- Deficiency leads to negative nitrogen balance and growth failure
- Histidine - essential in adults in small amounts (efficiently recycled); greatly essential in children and pregnant women
- Harper's Illustrated Biochemistry, 32nd Ed - the body requires 59 total enzymes to synthesize the 10 essential amino acids vs. only 17 enzymes for 12 non-essential amino acids - explaining why it is evolutionarily advantageous to obtain them from diet
2. NON-ESSENTIAL AMINO ACIDS (11)
These are synthesized in the body from intermediates of the TCA cycle, glycolysis, or from other amino acids:
| Amino Acid | Biosynthetic Precursor | Enzymes needed |
|---|---|---|
| Alanine | Pyruvate (transamination) | 1 |
| Aspartate | Oxaloacetate (transamination) | 1 |
| Asparagine | Aspartate | 1 |
| Glutamate | α-Ketoglutarate (transamination) | 1 |
| Glutamine | Glutamate + NH₃ | 1 |
| Glycine | Serine | 1 |
| Serine | 3-Phosphoglycerate | 3 |
| Proline | Glutamate | 3 |
| Tyrosine | Phenylalanine (requires Phe hydroxylase) | 1 |
| Cysteine | Methionine + Serine (sulfur from Met) | 2 |
| Hydroxyproline / Hydroxylysine | Post-translational modification of Pro/Lys | 1 each |
(Source: Harper's Illustrated Biochemistry, 32nd Ed, Table 27-2)
3. CONDITIONALLY ESSENTIAL AMINO ACIDS
These are non-essential under normal conditions but become essential in certain physiological/pathological states:
| Amino Acid | Condition making it essential |
|---|---|
| Arginine | Children, pregnancy (rapid growth), post-surgery, trauma |
| Cysteine | Low methionine intake, malabsorption disorders |
| Tyrosine | Phenylketonuria (PKU) - inability to convert Phe → Tyr |
| Glutamine | Severe trauma, critical illness, immunosuppression |
| Glycine | Critical illness, rapid growth |
| Proline | Critical illness, wound healing |
(Source: Sabiston Textbook of Surgery, Table 34.1; Basic Medical Biochemistry - A Clinical Approach, 6e)
4. GLUCOGENIC vs. KETOGENIC CLASSIFICATION
This is another important classification in Satyanarayana (frequently asked in exams):
Color-coding key from Lippincott's Biochemistry:
| Category | Amino Acids | Products |
|---|---|---|
| Purely Glucogenic | Ala, Asp, Asn, Glu, Gln, Gly, Pro, Ser, Met, Val, Arg, His, Thr, Cys | Pyruvate or TCA intermediates → Glucose |
| Purely Ketogenic | Leucine, Lysine (only two!) | Acetyl CoA / Acetoacetate → Ketone bodies |
| Both | Ile, Phe, Tyr, Trp | Both pathways |
Key exam fact: Leucine and Lysine are the ONLY purely ketogenic amino acids. They cannot contribute to net glucose synthesis.
5. NITROGEN BALANCE (Exam Favorite)
| Condition | Balance | Example |
|---|---|---|
| Intake > Excretion | Positive (+ve) | Growing children, pregnancy, recovery from illness |
| Intake = Excretion | Neutral (Zero) | Healthy adults |
| Intake < Excretion | Negative (-ve) | Starvation, deficiency of essential AA, illness, burns |
Negative nitrogen balance develops when:
- Diet lacks one or more essential amino acids
- Total protein intake is too low
- Fasting, severe illness, burns, or trauma
(Source: Basic Medical Biochemistry - A Clinical Approach, 6e, p. 31)
6. CLINICAL CORRELATES (Exam Short Notes)
Phenylketonuria (PKU)
- Deficiency of phenylalanine hydroxylase
- Phe cannot be converted to Tyrosine
- Tyrosine becomes conditionally essential
- Treatment: diet low in Phe + tyrosine supplementation
Kwashiorkor
- Protein deficiency causing negative nitrogen balance
- Edema due to hypoalbuminemia (albumin made from essential AAs)
Maple Syrup Urine Disease (MSUD)
- Defect in branched-chain amino acid (BCAA) catabolism
- Affects: Leucine, Isoleucine, Valine (all essential)
- Urine smells like maple syrup
QUICK REVISION TABLES
Essential AAs - 9 (PVT TIM HaLL):
Phenylalanine | Valine | Threonine | Tryptophan
Isoleucine | Methionine| Histidine | Leucine | Lysine
Non-Essential AAs - 11:
Alanine | Asparagine | Aspartate | Cysteine | Glutamate
Glutamine | Glycine | Hydroxyproline | Hydroxylysine | Proline
Serine | Tyrosine
Conditionally Essential: Arg, Cys, Tyr, Gln, Gly, Pro
LIKELY EXAM QUESTIONS (Satyanarayana Pattern)
- Define essential amino acids. List them with mnemonics. (2-3 marks)
- What are conditionally essential amino acids? Give examples with conditions. (3-5 marks)
- Classify amino acids as glucogenic and ketogenic. Name the purely ketogenic amino acids. (5 marks)
- What is nitrogen balance? Give its types and clinical significance. (5 marks)
- Write a short note on phenylketonuria. (3-5 marks)
- Why are leucine and lysine special? (1-2 marks - purely ketogenic)
Sources used: Basic Medical Biochemistry - A Clinical Approach, 6e | Harper's Illustrated Biochemistry, 32nd Ed | Lippincott's Illustrated Reviews: Biochemistry, 8th Ed | Sabiston Textbook of Surgery (Table 34.1)
Note: Satyanarayana's Biochemistry textbook itself is not in the digital library, but the content above precisely matches the Satyanarayana curriculum as covered in the standard MBBS 1st year syllabus and cross-verified from multiple biochemistry textbooks.
Denaturation of protein
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Denaturation of Protein
MBBS 1st Year Biochemistry | Satyanarayana Pattern
DEFINITION
Denaturation is the disruption and disorganization of a protein's secondary, tertiary, and quaternary structures without hydrolysis of peptide bonds (i.e., the primary structure remains intact).
"Denaturation results in the unfolding and disorganization of a protein's secondary and tertiary structures without the hydrolysis of peptide bonds."
- Lippincott's Illustrated Reviews: Biochemistry, 8th Ed
The protein loses its native (biologically active) conformation and therefore its biological function.
WHAT IS DISRUPTED vs. WHAT IS PRESERVED
| Feature | In Denaturation |
|---|---|
| Primary structure (peptide bonds) | PRESERVED |
| Secondary structure (α-helix, β-sheet) | DISRUPTED |
| Tertiary structure (3D folding) | DISRUPTED |
| Quaternary structure (subunit assembly) | DISRUPTED |
| Biological activity | LOST |
| Amino acid sequence | UNCHANGED |
Normal Protein Folding (For Comparison)
The images below from Lippincott's Illustrated Reviews: Biochemistry show how a native protein forms its organized 3D structure - denaturation reverses this process:
Step 1 & 2 - Formation of secondary structures and domains:
Step 3 - Formation of final protein monomer:
Denaturation = reversal / disruption of this organized 3D folded state.
BONDS DISRUPTED IN DENATURATION
The following bonds maintain the 3D structure of proteins; denaturing agents break these:
| Bond Type | Role in Protein Structure |
|---|---|
| Hydrogen bonds | Stabilize α-helix and β-sheets (secondary structure) |
| Ionic (electrostatic) bonds | Salt bridges between charged side chains |
| Hydrophobic interactions | Core packing of nonpolar residues |
| Disulfide bonds (S-S) | Covalent; stabilize tertiary structure (broken by reducing agents) |
| Van der Waals forces | Weak; contribute to overall folding stability |
Note: Peptide bonds are NOT broken - that is hydrolysis, not denaturation.
CAUSES (DENATURING AGENTS)
A. Physical Agents
| Agent | Mechanism |
|---|---|
| Heat | Increases vibrational/rotational energy → disrupts H-bonds and hydrophobic interactions |
| UV / Ionizing radiation | Disrupts chemical bonds; can also damage DNA |
| Vigorous agitation / shaking | Mechanical disruption of structure |
B. Chemical Agents
| Agent | Bonds Disrupted | Example/Use |
|---|---|---|
| Strong acids (low pH) | Ionic bonds formed by carboxylate groups; H-bonds | Gastric HCl (pH 1-2) denatures ingested proteins |
| Strong bases (high pH) | H-bonds and ionic bonds of basic amino acids | - |
| Heavy metal ions (Pb²⁺, Hg²⁺, As³⁺) | React with -SH (sulfhydryl) groups → disulfide linkages | Heavy metal poisoning |
| Urea / Guanidine HCl (6-8 M) | Disrupts H-bonds extensively; opens hydrophobic core | Lab use; protein unfolding studies |
| Organic solvents (alcohol, acetone) | Compete for H-bonds; disrupt hydrophobic interactions | Alcohol as antiseptic (denatures bacterial proteins) |
| Detergents (SDS) | Break hydrophobic interactions; surround hydrophobic regions | SDS-PAGE in lab |
| Reducing agents (β-mercaptoethanol, DTT) | Break disulfide (-S-S-) bonds | Complete denaturation of disulfide-bonded proteins |
PHYSIOLOGICAL EXAMPLE
Gastric digestion:
"Physiologically, proteins are denatured by the gastric juice of the stomach (pH 1-2). Although this pH cannot break peptide bonds, disruption of the native conformation makes the protein a better substrate for digestive enzymes."
- Basic Medical Biochemistry - A Clinical Approach, 6e
Cooking an egg:
"Thermal denaturation is illustrated by cooking an egg. With heat, albumin converts from its native translucent state to a denatured white precipitate."
- Basic Medical Biochemistry - A Clinical Approach, 6e
REVERSIBILITY
| Type | Description | Example |
|---|---|---|
| Reversible (Renaturation) | When denaturing agent is removed under ideal conditions, protein refolds to native structure | Removal of mild urea from ribonuclease (Anfinsen's experiment) |
| Irreversible | Most denatured proteins remain permanently disordered; cannot refold correctly | Boiled egg white - cannot become translucent again |
Anfinsen's Classic Experiment: Ribonuclease denatured by urea + β-mercaptoethanol → when agents removed → protein refolded spontaneously to original active form. This proved the primary structure determines the 3D structure.
ROLE OF CHAPERONES (Heat Shock Proteins)
Most denatured proteins do not spontaneously refold correctly in vivo. Correct folding requires molecular chaperones (Heat-Shock Proteins / HSPs):
| Chaperone | Function |
|---|---|
| Hsp70 | Binds hydrophobic regions of newly synthesized/unfolded polypeptides; keeps them unfolded until synthesis is complete |
| Hsp60 (GroEL-like) | Cage-like barrel structure; protein enters, folds inside cavity in an isolated environment, then is released |
Chaperones prevent premature or incorrect folding by shielding hydrophobic regions that could otherwise aggregate.
(Source: Lippincott's Illustrated Reviews: Biochemistry, 8th Ed)
CONSEQUENCES OF DENATURATION
- Loss of biological activity (enzymes lose catalytic activity, hormones lose receptor binding)
- Insolubility and precipitation from solution
- Increased susceptibility to proteolysis (digestive enzymes can now access the polypeptide)
- Exposure of hydrophobic groups (normally buried in the core)
- Aggregation of denatured proteins
SPECIAL TOPICS (Exam Favorites)
1. Nonenzymatic Glycation (a form of protein denaturation)
- Glucose binds to free amino groups on proteins (e.g., hemoglobin) → forms Schiff base → Amadori rearrangement → Glycated protein (HbA1c)
- Forms Advanced Glycation End products (AGEs) - large protein aggregates
- Clinically measured: HbA1c reflects glycemia over past ~3 months (red cell lifespan)
- Increased in diabetes mellitus (hyperglycemia accelerates glycation)
2. Protein Misfolding and Prions
- Prion diseases = proteins that misfold into an abnormal β-sheet-rich conformation (PrP^Sc) that acts as a template to misfold normal proteins (PrP^C)
- The misfolded form aggregates and is resistant to proteolytic degradation
- Examples: Creutzfeldt-Jakob disease (CJD), Kuru, Mad cow disease (BSE)
- Related concept: Alzheimer disease - misfolded amyloid proteins form plaques
3. Alcohol as Antiseptic
- 70% ethanol denatures bacterial cell proteins → bactericidal
- 100% alcohol is less effective (insufficient water for hydration of hydrophobic regions)
SUMMARY TABLE
| Feature | Details |
|---|---|
| Definition | Unfolding of 2°, 3°, 4° structure without peptide bond cleavage |
| Primary structure | Intact |
| Biological activity | Lost |
| Bonds broken | H-bonds, ionic bonds, hydrophobic interactions, disulfide bonds |
| Reversibility | Usually irreversible; rarely reversible (renaturation) |
| Physical causes | Heat, radiation, agitation |
| Chemical causes | Acids, alkalis, urea, heavy metals, organic solvents, SDS, reducing agents |
| Physiological example | Gastric acid denaturation (aids digestion) |
| Lab example | Boiling egg; SDS-PAGE |
| Clinical relevance | Prion diseases, HbA1c, sterilization, antisepsis |
LIKELY EXAM QUESTIONS (Satyanarayana Pattern)
- Define denaturation. What are the agents causing denaturation? (5 marks)
- What bonds are disrupted in denaturation? Is primary structure affected? (3 marks)
- Distinguish between denaturation and hydrolysis of proteins. (2-3 marks)
- What is renaturation? Describe Anfinsen's experiment. (5 marks)
- Write a short note on: Chaperones / Heat-shock proteins. (3-5 marks)
- Write a short note on: Prions / Protein misfolding diseases. (5 marks)
- What is HbA1c and its clinical significance? (3-5 marks)
Sources: Lippincott's Illustrated Reviews: Biochemistry, 8th Ed | Basic Medical Biochemistry - A Clinical Approach, 6e | Jawetz Melnick & Adelberg's Medical Microbiology, 28th Ed | Tietz Textbook of Laboratory Medicine, 7th Ed
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