• Metabolism - glycolysis, TCA cycle, oxidative phosphorylation, fatty acid oxidation
• Proteins & Enzymes - structure, enzyme kinetics, inhibition
• Nucleic Acids - DNA replication, transcription, translation
• Vitamins & Cofactors
• Hormones & Signal Transduction
• Genetics & Molecular Biology

1. You send me a question + its answer
2. I'll break it down completely - explain every term, the underlying concept, why the answer is correct, and any memory tricks if useful
3. We go one by one so nothing is overwhelming

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🧬 COMPLETE BEGINNER'S GUIDE TO AMINO ACIDS

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PART 1: What Are Amino Acids? (The Big Picture)

1. An amino group (-NH₂) - contains nitrogen, this is the "amino" part
2. A carboxyl group (-COOH) - this is an acid group, the "acid" part
3. An R group (also called a "side chain" or "radical") - this is what makes each amino acid different from all the others

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PART 2: The General Structure of an Amino Acid

        COOH       ← Carboxyl group (acid end)
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H₂N -- C -- H     ← This central carbon is called the ALPHA-CARBON (α-carbon)
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        R          ← The side chain (unique for each amino acid)

1. The amino group (-NH₂)
2. The carboxyl group (-COOH)
3. A hydrogen atom (-H)
4. The R group (side chain) - this is what differs between amino acids

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PART 3: L-Isomers - Why Only "L"?

• L-form (Latin: "levo" = left)
• D-form (Latin: "dextro" = right)

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PART 4: Classification by Chemical Nature (Shape of R group)

🔵 A) ALIPHATIC Amino Acids (open-chain, no rings)

• Glycine (Gly) - R = just H. Smallest amino acid.
• Alanine (Ala) - R = CH₃ (one carbon group)
• Valine (Val) - R = CH(CH₃)₂ (branched)
• Leucine (Leu) - R = CH₂-CH(CH₃)₂ (branched chain)
• Isoleucine (Ile) - R = branched chain with an extra methyl group

• Serine (Ser) - R = CH₂-OH
• Threonine (Thr) - R = CH(OH)-CH₃

• Aspartic acid (Asp) - has an extra -COOH in its R group
• Glutamic acid (Glu) - same but with a longer chain before the second -COOH

• Lysine (Lys) - has an extra -NH₂ at the end of its R chain
• Arginine (Arg) - has a guanidinium group (very strongly basic)

• Cysteine (Cys) - R = CH₂-SH (has a thiol/sulfhydryl group -SH)
• Cystine - two cysteines linked together by a disulfide bond (-S-S-)
• Methionine (Met) - R = CH₂-CH₂-S-CH₃ (has a sulfur with a methyl group attached)

🔴 B) AROMATIC Amino Acids (have a benzene/phenol ring)

• Phenylalanine (Phe) - has a benzene ring (plain, no substituents)
• Tyrosine (Tyr) - has a benzene ring with an -OH group attached (phenol ring) - this makes it more polar than Phe

🟢 C) HETEROCYCLIC Amino Acids (ring contains atoms OTHER than carbon)

• Histidine (His) - has an imidazole ring (5-membered ring with 2 nitrogen atoms)
• Tryptophan (Trp) - has an indole ring system (a double ring)

🟡 D) SPECIAL CASE - Proline (Pro)

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PART 5: Classification by Leninger (Side Chain Properties - Polarity & Charge)

What is polarity?

• Polar = the molecule has uneven distribution of electric charge, like a magnet with + and - ends. Water is polar. Polar things dissolve in water = hydrophilic ("water-loving")
• Non-polar = charge is evenly distributed. These don't dissolve in water = hydrophobic ("water-fearing")

The 4 Groups (Leninger's Classification):

• Acidic amino acids (Asp, Glu) lose an H⁺ from their extra -COOH group, becoming negatively charged (-COO⁻)
• Basic amino acids (His, Arg, Lys) gain an H⁺ on their amino group, becoming positively charged (-NH₃⁺)

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PART 6: Functions of Specific Amino Acids

🔶 Methionine (Met)

• Contains sulfur in its R group
• It is the "start" amino acid - every protein synthesis in your body begins with methionine (it is the first amino acid inserted)
• It donates its methyl (-CH₃) group to many biochemical reactions - this process is called transmethylation and requires methionine to first be activated to SAM (S-adenosylmethionine). SAM is like a "methyl group taxi" in the body
• Essential amino acid - your body cannot make it, you MUST eat it

🔶 Cysteine (Cys)

• Contains a thiol group (-SH) - the -SH is very reactive
• Two cysteines can link together through their -SH groups to form a disulfide bond (-S-S-) → this cross-links protein chains and stabilizes 3D protein structure (important in antibodies, insulin, keratin in hair)
• The linked form of two cysteines is called cystine
• Cysteine is a conditionally essential amino acid - your body can make it, but only if it has enough methionine

🔶 Tyrosine (Tyr)

• Has a benzene ring with an -OH group (phenol group)
• It is the starting material (precursor) for several important molecules:
  • Thyroid hormones (T3 and T4 - regulate metabolism)
  • Catecholamines (dopamine, adrenaline/epinephrine, noradrenaline) - neurotransmitters and stress hormones
  • Melanin - the pigment that gives skin and hair its color
• Can be phosphorylated (a phosphate group can be attached) - this is how many cell signals are turned on/off
• Conditionally essential - your body can make it FROM phenylalanine (but if you don't have phenylalanine, you can't make tyrosine either)

🔶 Histidine (His)

• Has an imidazole ring in its side chain
• The imidazole ring can accept or donate a proton (H⁺) very easily near physiological pH (~6-7). This is why histidine is special - it acts as a "proton shuttle" in enzyme active sites
• It is found in the active sites of many enzymes (e.g., serine proteases, carbonic anhydrase)
• The heme group in hemoglobin is held in place by histidine residues
• Partially essential - the body cannot fully synthesize enough of it, especially in growing children

🔶 Glutamic Acid (Glu)

• One of the two acidic amino acids (along with aspartate)
• Has a negatively charged R group at physiological pH
• In the brain, it is the main excitatory neurotransmitter (it stimulates neurons to fire)
• Plays a key role in the urea cycle and transamination reactions - it shuttles nitrogen between amino acids
• Forms glutamine (by adding an -NH₂ group) which is a major nitrogen carrier in blood
• Monosodium glutamate (MSG) - the flavor enhancer used in cooking is its sodium salt

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PART 7: Essential vs. Non-Essential Amino Acids

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PART 8: General Chemical Properties of Amino Acids

A) Acid-Base Properties (Ionogenicity)

• The carboxyl group (-COOH) can donate H⁺ → becomes -COO⁻ (carboxylate, negative charge) = acts as an ACID
• The amino group (-NH₂) can accept H⁺ → becomes -NH₃⁺ (ammonium, positive charge) = acts as a BASE

H₃N⁺ - C(R)(H) - COO⁻    ← This is a zwitterion (both charged, net = 0)

B) Formation of a Peptide Bond

• The -COOH (carboxyl group) of one amino acid
• The -NH₂ (amino group) of the next amino acid
• A water molecule (H₂O) is released in the process (this is called a condensation reaction or dehydration reaction)

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PART 9: Example - Forming the Tetrapeptide Phe-Asp-Met-Val

• A free -NH₂ group at the left end (called the N-terminus)
• A free -COOH group at the right end (called the C-terminus)
• 3 peptide bonds (-CO-NH-) connecting the 4 amino acids
• 3 water molecules were released in total

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QUICK SUMMARY TABLE - All 20 Amino Acids

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