Lec.1-Amino Acids, Protein Structure, and Related Concepts (Comprehensive Notes)

Amino Acids and Protein Structure – Comprehensive Study Notes

• Polypeptides, peptides, and proteins

  • Polypeptides are long chains of amino acids linked by peptide bonds; peptides are shorter chunks, usually no more than ~30 amino acids, often breakdown products of polypeptides.
  • Most proteins have lengths in the hundreds of amino acids; average protein length in humans ~3–500 aa; giant proteins exist (e.g., titan) over 4,000 amino acids.
  • The course plans to discuss how polypeptide chains fold to form functional proteins later in the curriculum.

• Amino acids: essential vs nonessential; conditional essentiality

  • There are 20 amino acids that end up in proteins; actually a few more exist (hundreds total in cells), but the following are the 20 common ones.
  • Essential amino acids: 9 amino acids that must be obtained from the diet because the body cannot synthesize them.
  • Nonessential amino acids: the body can synthesize these; they do not need to be consumed.
  • Conditionally essential amino acids: some nonessential amino acids become essential under certain conditions because their synthesis depends on other amino acids. Example: Tyrosine is synthesized from Phenylalanine; if phenylalanine is not consumed, tyrosine cannot be made.
  • Practical takeaway: know which amino acids are essential and understand that some nonessential ones become essential under specific circumstances (e.g., phenylalanine → tyrosine).

• Dietary protein sources and amino acid balance

  • Animal proteins are highly digestible; humans can digest ~99% of animal protein.
  • The amino acid profile of animal proteins is generally complete and similar to human needs (the essential amino acids are present in ratios compatible with human requirements).
  • Plant proteins are less digestible overall (roughly ~90%), and the amino acid ratios may differ (e.g., beans may be high in methionine but low in threonine; tomatoes may be high in threonine but low in methionine).
  • Complementary proteins concept: consuming a variety of plant proteins (e.g., beans plus grains or tomatoes with other plant proteins) can provide a complete amino acid profile.
  • Practical take: vegans/vegetarians should ensure variety to achieve complete essential amino acid coverage; no one plant source is guaranteed to provide a perfect ratio.
  • Quick joke on vegan diets from the lecturer: the idea of “missed steak” was used, but the key point is to diversify plant protein sources to balance amino acid intake.

• Structure of amino acids (the basics)

  • Every amino acid has four components: an amino group, a carboxyl (acid) group, a central carbon (the alpha carbon) with a hydrogen, and a side chain R group.
  • The side chain (R group) determines the amino acid’s properties: shape, charge, hydrophobicity/hydrophilicity, size, and reactivity, especially in enzymes.
  • The alpha carbon is chiral (except glycine), yielding L- and D-forms.
  • In organisms (humans, bacteria, plants, etc.), proteins are composed almost exclusively of L-amino acids; D-forms occur in some bacterial structures (e.g., D-forms in bacterial cell-wall peptides).
  • Important note: bacteria use D-forms in some components of the cell wall (e.g., D-alanine in peptidoglycan peptides) as a defense mechanism because our enzymes generally recognize L-forms.
  • Enzyme specificity and “lock-and-key” concepts mean slight changes in stereochemistry (L vs D) can drastically affect recognition and breakdown.
  • Caution about terminology in lectures: the instructor mentions D-forms in Gram-positive bacteria cell-wall peptides (e.g., D-alanine) and discusses lysozyme as an enzyme that breaks down bacterial cell walls by targeting specific bonds, not the D-forms themselves.

• Classification and properties of amino acids (the core groups you should know for exams)

  • Aliphatic amino acids (hydrophobic; inside protein cores): Valine, Alanine, Leucine, Isoleucine; Glycine is often discussed separately because its side chain is just H (no chirality).
  • Aromatic amino acids (ring structures; bulky; often hydrophobic): Tyrosine, Phenylalanine, Tryptophan. Tyrosine can have polar character if its side chain is deprotonated; these residues absorb UV light and are useful for protein quantification.
  • Acidic (negatively charged at physiological pH): Aspartate (aspartic acid) and Glutamate (glutamic acid).
  • Basic (positively charged at physiological pH): Lysine, Arginine, Histidine.
  • Polar (hydrophilic) amino acids: Serine, Threonine, Cysteine, Glutamine, Asparagine, and Tyrosine (due to its polar OH group and/or ring with potential charge).
  • Sulfur-containing: Cysteine (thiol group, reactive; can form disulfide bonds), Methionine (thioether; sulfur not reactive in most contexts).
  • Proline: unique cyclic structure where the side chain bonds back to the amino group, creating rigidity and often initiating alpha-helix formation.
  • Glycine note: the only amino acid without a chiral center; often not classified strictly as aliphatic due to its H side chain but shares some aliphatic-like attributes.
  • Across these classifications, some amino acids appear in multiple categories (e.g., Tyrosine is aromatic and polar).
  • One main takeaway for exams: focus on the major classification bubble (aliphatic, aromatic, acidic, basic, polar, sulfur-containing, proline) and the typical behavior in protein folding (e.g., hydrophobic cores vs hydrophilic surfaces).

• Aromatic amino acids and UV absorption for protein quantification

  • Aromatic residues (Phe, Tyr, Trp) absorb UV light; the strongest absorption peak for protein quantification is around
    00~\text{nm} due to phenylalanine, tyrosine, and tryptophan.
  • The practical measurement uses 280 nm (two important residues: Tyr and Trp; Phe contributes as well but less).
  • Why not 260 nm? Because DNA absorbs strongly around 260 nm; using 260 nm would confound protein concentration measurements in samples containing DNA.
  • Quantification concept: the absorbance at 280 nm relates to the protein concentration via the molar extinction coefficient and path length (Beer-Lambert law).

• Beer-Lambert law and protein quantification

  • Core equation: $A = \varepsilon \; c \; \ell$ where
  • A is the measured absorbance (unitless),
  • \varepsilon is the molar extinction coefficient (M^-1 cm^-1),
  • c is the protein concentration (in M),
  • \ell is the path length of the light through the sample (cm).
  • In most spectrophotometers, the path length (\ell) is 1 cm, so the equation simplifies to $A = \varepsilon \; c$.
  • Each protein has a characteristic extinction coefficient depending on how many and which aromatic residues it contains (more aromatics -> higher (\varepsilon)).
  • Example: If a protein sample has an extinction coefficient (\varepsilon = 2) (in appropriate units), path length (\ell = 1) cm, and measured absorbance (A = 1), then the concentration is $c = \frac{A}{\varepsilon \ell} = \frac{1}{2 \cdot 1} = 0.5.$
  • Practical notes: the extinction coefficients are constants for a given protein or set of proteins and must be known or experimentally determined; DNA contamination can skew readings if you use the 280 nm method. The instrument provides Absorbance, which is unit-less and then you compute concentration via the above relationship.

• Henderson-Hasselbalch equation and the concept of pKa

  • Amphoteric molecules like amino acids can act as acids or bases depending on pH; Henderson-Hasselbalch expresses the relationship between pH, pKa, and the ratio of deprotonated/protonated forms:
    $\mathrm{pH} = \mathrm{p}K_a + \log \left(\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}\right)$
  • At the point where [A^-] = [HA], the log term is zero and $\mathrm{pH} = \mathrm{p}K_a$; this is the midpoint of the titration for that particular titratable group.
  • For a simple amino acid with two titratable groups (the backbone amino group and the backbone carboxyl group), you have two pKa values (often designated as pKa1 for the carboxyl group and pKa2 for the amino group).
  • A few notes from the lecture:
  • The pKa values depend on the environment and can be used to understand buffering capacity of a solution around those pH values.
  • The concept of the isoelectric point (pI) is introduced as the pH at which the molecule has no net charge.

• The isoelectric point (pI) and multi-titratable groups

  • For amino acids with exactly two titratable groups (typical amino acids with backbone groups), the pI is roughly the average of pKa1 and pKa2:
    $\mathrm{pI} \approx \frac{\mathrm{p}K<em>{a1} + \mathrm{p}K</em>{a2}}{2}$
  • For amino acids with three titratable groups (i.e., acidic or basic amino acids with an ionizable side chain), the pI is the average of the two pKa values that are closest to one another (the two that flank the zero-charge region most closely).
  • The lecture provides a worked example for aspartic acid (which has three pKa values, e.g., 2.1, 3.9, 9.8) and shows that the pI is typically between the two closest pKas (here, between 2.1 and 3.9, giving pI ≈ 3.0).
  • For alanine (two titratable groups): pKa1 ≈ 2.35 and pKa2 ≈ 9.69, giving pI ≈ (2.35 + 9.69)/2 ≈ 6.01.
  • The charge state of an amino acid depends on pH: below pI the molecule tends to be positively charged; above pI it tends to be negatively charged; at pI it tends to be neutral overall.
  • A quick reminder: acids (carboxyl group) are deprotonated at higher pH (negative charge) and protonated at lower pH; amines (amino group) are protonated (positive charge) at lower pH and deprotonated at high pH.

• Titratable groups in amino acids and common forms

  • Backbone groups: amino group (–NH2/–NH3+) and carboxyl group (–COOH/–COO−) are always titratable.
  • Some amino acids have a third titratable group in the R side chain (e.g., acidic amino acids like aspartate/glutamate and basic amino acids like lysine/arginine/histidine).
  • The two backbone titratable groups dominate the basic understanding of two-point titration, while side-chain titration adds additional steps in the curve for amino acids with more than two titratable groups.

• Visualizing titration curves and the concept of buffering

  • A titration curve for an amino acid can be split into two “segments” (for two-titratable-group amino acids): the lower pH region (carboxyl group titration) and the higher pH region (amino group titration).
  • Around the pKa values, adding base causes relatively small pH changes (buffering).
  • Buffering is most effective around a pKa value; buffering is poorest around the isoelectric point (pI) because the molecule carries no net charge, and small additions of acid/base can shift the charge balance significantly.
  • For amino acids with three titratable groups, there are three regions on the titration curve; the concept of pI becomes more nuanced (the average of two closest pKa values).
  • Example: Alanine titration curve shows distinct regions and a pI around 6.0; the lower pKa (~2.3) relates to the carboxyl group, the higher pKa (~9.7) relates to the amino group.

• Practical summary: calculating pI and buffering behavior

  • pI is the pH where the molecule has no net charge; for simple amino acids with two titratable groups, pI ≈ (pKa1 + pKa2)/2.
  • For amino acids with three titratable groups, pI is the average of the two closest pKa values.
  • Buffering capacity is highest near a pKa value, and the worst buffering is at or around the pI.
  • In experimental design, you can select buffers that center around the desired pH near a particular pKa for stability.

• Protein structure: levels of organization (primary to quaternary)

  • Primary structure: the linear sequence of amino acids (the polypeptide chain).
  • Secondary structure: local folding patterns stabilized by hydrogen bonds; main motifs are:
  • Alpha helix: a right-handed helix where one full turn = 3.6 amino acids; roughly 11 residues long is typical; a full turn involves about four amino acids in sequence; proline often appears at the start of helices to help induce the twist.
  • Beta pleated sheets: formed by hydrogen bonding between neighboring strands; the sheet is flat but has bends; side chains (R groups) project above or below the sheet.
  • Tertiary structure: the overall 3D shape of a single polypeptide, stabilized by multiple interactions including:
  • Hydrogen bonds
  • Hydrophobic interactions (core packing of nonpolar residues)
  • Ionic bonds / salt bridges (positive with negative residues)
  • Disulfide bonds (covalent bonds between cysteine residues)
  • Quaternary structure (the term used in standard biochemistry for multi-subunit assemblies; the lecturer uses “coronary structure” in places):
  • Some proteins function as a single polypeptide; others require multiple polypeptide chains to form a functional complex.
  • Example: Hemoglobin consists of four subunits (two alpha, two beta) from different genes; each subunit alone is not functional for oxygen transport.
  • Large enzyme complexes (e.g., mitochondrial electron transport chain complex I) may involve dozens of protein subunits assembling into a single functional machine.
  • Domains and motifs
  • Domain: a structural unit within a protein that has a specific function (e.g., a DNA-binding domain, a kinase domain).
  • Motif: a recurring structural pattern that may be shared among proteins; often associated with a function similar to a domain (e.g., SH3 motif, which binds to phosphorylated tyrosine in certain contexts).

• Bonding and stability in proteins

  • The various bonds that hold secondary and tertiary structures together include:
  • Hydrogen bonds (important in secondary structure and overall folding)
  • Ionic bonds / salt bridges (between positively and negatively charged side chains)
  • Hydrophobic interactions (driving core packing; hydrophobic residues cluster away from water)
  • Van der Waals interactions (non-specific, but contribute to packing)
  • Disulfide bonds (covalent bonds between cysteine sulfurs; strongest stabilization for tertiary structure; not easily broken by heat/pH/salt; require reducing agents to break)
  • Denaturation
  • Denaturation disrupts these stabilizing interactions, causing the protein to unfold; often irreversible because refolding to the native state is rare outside of specialized conditions.
  • Common example: heating an egg white denatures its proteins, making it opaque; in typical cooking, the proteins do not refold to their original structure.
  • Reducing agents can break disulfide bonds, which can disrupt quaternary structure and some tertiary structure.

• Special notes and examples mentioned in the lecture

  • D-amino acids in bacteria: Bacteria sometimes use D-forms in structural components (e.g., D-alanine in peptidoglycan) as a defense against our enzymes that recognize L-forms; this helps bacteria avoid being degraded by human enzymes.
  • Lysozyme: An enzyme that helps break down bacterial cell walls by targeting specific bonds, enabling the breakdown of the cell wall, even if some peptides (D-forms) are involved in the structure.
  • Proline and helix formation: Proline’s unique ring structure introduces rigidity and can initiate helix formation at the start of an alpha helix.
  • UV quantification and protein concentration: The presence of aromatic amino acids enables UV-based protein concentration measurements at 280 nm; this approach is not suitable if DNA contaminates the sample because DNA also absorbs at similar wavelengths (thus the need for careful selection of measurement wavelengths).
  • The instructor connects biochemistry to optometry by noting ocular proteins and their relevance to eye biology, aiming to relate amino acid chemistry to ocular function in later slides.

• Quick reference formulas and concepts to memorize

  • Beer-Lambert law for protein concentration using UV absorbance at 280 nm:
    $A = \varepsilon \; c \; \ell$
    where a typical path length in spectrophotometers is $\ell = 1\ \text{cm}$, giving $A = \varepsilon c$ and thus $c = \frac{A}{\varepsilon \ell}$.
  • Henderson-Hasselbalch equation:
    $\mathrm{pH} = \mathrm{p}K_a + \log \left(\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}\right)$
  • For a simple two-titratable-group amino acid (backbone NH₂ and COOH):
  • pKa1 relates to carboxyl group deprotonation; pKa2 relates to amino group deprotonation.
  • Isoelectric point (pI) for two-titratable-group amino acids:
    $\mathrm{pI} \approx \frac{\mathrm{p}K<em>{a1} + \mathrm{p}K</em>{a2}}{2}$
  • For amino acids with three titratable groups, pI is the average of the two pKas that are closest to each other.
  • Alpha-helix structural detail: one full turn uses about $3.6$ amino acids; a typical alpha-helix is about $11$ amino acids long; a proline at the start can help initiation of the twist.
  • Neutral, positive, and negative charges across pH behavior:
  • Below pI: net positive charges predominate;
  • At pI: net charge is zero;
  • Above pI: net negative charges predominate.

• Summary takeaway for exams

  • Remember the major amino acid classifications and how their side chains influence protein folding (hydrophobic core vs hydrophilic surface).
  • Know the two backbone titratable groups, the concept of a third side-chain titratable group for some amino acids, and how pKa values relate to buffering and pI.
  • Be able to explain the basics of the titration curve, including the three regions for multi-titratable-group amino acids, and how to estimate pI from pKa values.
  • Understand the hierarchy of protein structure (primary → secondary → tertiary → quaternary) and the types of bonds that stabilize each level.
  • Recognize practical laboratory concepts like UV-based protein quantification, and the need to consider potential DNA contamination.
  • Acknowledge real-world relevance in ocular biology (optometry) to connect biochemical concepts with clinical/functional contexts.