Protein Structure and Function Vocabulary

Chemical Composition of Cells and Lipids

• Saturated Fatty Acids and Esterification:

  • Saturated fats feature single covalent bonds linking all carbon atoms within the fatty acid tails.

  • Fatty acid tails are attached to glycerol, which contains alcohol groups ($-\text{OH}$), through esterification reactions.

  • Esterification forms an ester bond linking the carboxyl group ($-\text{COOH}$) of a fatty acid to an alcohol group of glycerol.

  • Carboxyl and carboxylate functional groups exhibit extensive diversity in structure and perform a wide range of biological functions.

• Cellular Mass and Biological Medium:

  • Water constitutes the primary biological medium of all cells, accounting for approximately $95\%$ to $96\%$ of cellular mass.

  • Excluding water, proteins account for approximately $50\%$ (half) of the total dry mass of most cells.

Overview of Proteins and Enzymatic Function

• Proteins as Fundamental Biological Macromolecules:

  • Biological research heavily emphasizes proteins due to their central involvement in cellular operations.

  • A key functional group of proteins is enzymes.

  • Enzymes act as biological catalysts that speed up chemical reactions in living organisms without being consumed in the process.

• Lactose Metabolism and Enzyme Function:

  • Lactase is an enzyme present in the small intestine that digests lactose, the primary sugar found in milk.

  • In individuals with lactose intolerance, endogenous lactase is non-functional or deficient, preventing the breakdown of milk sugars and proteins and causing sickness.

  • Exogenous enzyme supplements (such as lactase enzyme preparations or Lactaid) can be taken to break down milk components properly.

  • Contextual remarks mention acquiring snacks or beverages (such as a cane in the food court) during instructional discussions.

Monomers, Dipeptides, and Polypeptide Synthesis

• Monomers vs. Polymers:

  • The individual monomer subunit of a protein is a single amino acid.

  • Joining multiple amino acids together (two or more) forms a polypeptide.

  • The prefix "poly" signifies many, while "peptide" designates the linkages connecting the chain.

• Peptide Bond Formation via Dehydration Reactions:

  • A dipeptide consists of two amino acids bonded together.

  • Synthesizing a longer polypeptide chain by adding a new amino acid requires a dehydration reaction (condensation reaction).

  • In this reaction, a hydroxyl group ($-\text{OH}$) is removed from the carboxyl group ($C\text{-terminus}$) of the existing chain, and a hydrogen ion ($H^+$) is removed from the amino group ($N\text{-terminus}$) of the incoming amino acid.

  • Removing these components forms a molecule of water ($H_2O$) and creates a covalent peptide bond linking the amino acids.

Structural Termini and Chemical Properties of Amino Acid Side Chains

• Directional Termini of Polypeptides:

  • $N\text{-terminus}$ (Amino Terminus): The end of the polypeptide chain terminating in a free amino group.

  • $C\text{-terminus}$ (Carboxyl Terminus): The end of the polypeptide chain terminating in a free carboxyl group.

  • New amino acids are added to a growing chain by joining the amino end ($N\text{-terminus}$) of the incoming monomer to the carboxyl end ($C\text{-terminus}$) of the chain.

• Chemical Properties of Amino Acid Side Chains (R-Groups):

  • The unique chemical properties of side chains determine downstream macromolecular interactions and overall protein folding.

  • Nonpolar Side Chains:

  • Contain nonpolar components such as methyl groups ($-\text{CH}_3$) and hydrocarbon structures.

  • Nonpolar side chains engage in hydrophobic interactions.

  • Basic Side Chains:

  • Located on specific amino acids (such as those depicted on the lower right of amino acid charts), these side chains contain amine groups.

  • Basic side chains gain protons ($H^+$ / hydrogen ions), acquiring a positive charge at physiological pH.

  • Polar and Hydrophilic Side Chains:

  • Side chains with polar functional groups participate in hydrophilic bonding and electrostatic interactions.

• Amino Acid Sequence Notation and Protein Definition:

  • Sequences are frequently written using three-letter abbreviations instead of full chemical formulas (e.g., Met for methionine, Tyr for tyrosine, Cys for cysteine).

  • A polypeptide is the linear polymer chain.

  • A functional protein consists of one or more polypeptide chains folded, twisted, or coiled into a specific three-dimensional conformation.

Higher-Order Levels of Protein Structure

• Primary Structure:

  • Primary structure is the unique, linear sequence of amino acids in a polypeptide chain.

  • Primary structure is dictated by genetic code (DNA sequences / genes).

  • Example: A gene for brown hair contains a specific DNA sequence that codes for the synthesis of a protein responsible for producing brown pigment.

• Secondary Structure:

  • Secondary structure consists of localized coils and folds along the polypeptide backbone.

  • The polypeptide backbone comprises the repeating structural elements of peptide-bonded amino acids, excluding variable side chains.

  • Secondary structures are stabilized by hydrogen bonds between backbone constituents.

• Tertiary Structure:

  • Tertiary structure is the overall three-dimensional conformational shape of a single polypeptide subunit.

  • Driven by interactions between amino acid side chains (R-groups).

  • Disulfide Bridges:

  • Strong covalent bonds formed between sulfur-containing side chains (cysteine residues) in the polypeptide.

  • Disulfide bridges lock regions of the protein together and stabilize its tertiary structure.

Protein Conformational Modeling and Quaternary Aggregation

• Quaternary Structure:

  • Quaternary structure occurs when a protein consists of two or more aggregated polypeptide subunits.

  • Not all proteins have quaternary structure; it is unique to multi-subunit proteins.

  • Subunit assemblies include complexes composed of paired chains, such as hemoglobin models containing distinct chains (e.g., two alpha chains and two beta chains, or specialized configurations such as $2,002$ beta chains in model schematics).

• Computational Analysis and Prediction:

  • Modern biological research utilizes computer analysis, structural algorithms, and computational predictions to model complex three-dimensional protein subunit configurations and folding patterns.