Biological Macromolecules, Protein Structure, Lipids, and General Biology Review

Overview of Macromolecules and Functional Groups

• Carbohydrate Monomers and Polymers:
  • Monosaccharides represent simple sugars and serve as the single monomeric units of carbohydrates.
  • Polysaccharides (complex sugars) are formed when multiple monosaccharides combine via covalent bonds into complex molecular structures.
• Nucleic Acid Monomers:
  • Nucleotides are the single monomer units that combine to build nucleic acids, specifically DNA and RNA.
• Key Functional Groups in Biological Macromolecules:
  • Hydroxyl group: $-OH$
  • Carbonyl group: $-C=O$ or $-CHO$
  • Amine group: $-NH_2$
  • Sulfhydryl group: $-SH$
  • Phosphate group: $-PO_4^{-}$ or $-PO_4^{3-}$
  • Methyl group: $-CH_3$
  • Carboxyl group: $-COOH$ (crucial for structural assembly in proteins and fatty acids)

Biological Functions of Proteins

• Enzyme Catalysis:
  • Catalysts accelerate chemical reactions (e.g., converting reactant $A$ to product $B$) by tuning up reaction amplitude and speed, increasing efficiency without being consumed.
  • Biological catalysts are called enzymes. Structurally, enzymes are biological proteins.
  • Specific enzyme examples and mechanisms:
  • DNA polymerase: Enzyme utilized in Polymerase Chain Reaction (PCR) to synthesize and replicate multiple copies of DNA strands.
  • Hydrolases: Enzymes that catalyze hydrolysis reactions, breaking covalent bonds between molecules through the addition of a water molecule ($H_2O$).
  • Phosphatases: Enzymes responsible for cleaving and removing phosphate groups from substrates.
• Defense and Immune Recognition:
  • Antibodies generated by the immune system are specialized defense proteins (white blood cells are the cellular units that synthesize these antibody proteins).
  • Cell-surface proteins function in self-versus-non-self recognition, enabling the immune system to identify foreign material that must be evicted from the body.
• Cellular Transport:
  • Transport proteins embedded in biological membranes form selective channels or pores to move specific atoms, ions, or molecules across cell membranes based on molecular size or pH gradients.
  • Specific transport examples:
  • Aquaporin: Channel protein that selectively permits water molecules to pass through membrane pores into or out of the cell.
  • Ion-gated channels: Protein channels that open or close in response to specific chemical signals or gradients to allow the regulated passage of ions such as sodium ($Na^+$), potassium ($K^+$), or calcium ($Ca^{2+}$).
• Structural Support:
  • Hard, resilient anatomical structures are formed from fibrous support proteins.
  • Example: Keratin is the primary structural protein found in hair and nails.
• Motion and Muscle Contraction:
  • Cellular and muscular movement relies on the combined action of two contractile proteins: actin and myosin.
• Regulation and Gene Expression:
  • Regulatory proteins govern key processes such as DNA replication and gene expression.
  • Transcription factors: Specialized proteins that bind to DNA, direct where and when the double helix should open up, and signal the start of transcribing specific genes into RNA.
• Ion and Molecule Storage:
  • Storage proteins bind and hold essential ions and metallic elements within tissues or fluids.
  • Examples: Calcium-binding storage proteins; hemoglobin is the iron-storing protein within red blood cells responsible for oxygen transport.

Amino Acid Structure, Chirality, and Categories

• Basic Amino Acid Monomer Structure:
  • Proteins are linear polymers assembled from monomeric units called amino acids.
  • The generalized structure of an amino acid features a central alpha-carbon ($C$) bound to four distinct chemical entities:
  1. An amine group ($-NH_2$)
  2. A carboxyl group ($-COOH$)
  3. A hydrogen atom ($-H$)
  4. A variable side chain ($-R$ group)
• Carbon Chirality (Asymmetry):
  • A carbon atom is defined as chiral (asymmetric) when it is covalently bonded to four entirely different atoms or chemical groups.
  • Glycine: The simplest amino acid, where the variable $-R$ group is a hydrogen atom ($-H$). Because the central carbon is attached to two identical hydrogen atoms, glycine lacks asymmetry and is non-chiral (achiral).
  • All other $19$ standard amino acids possess a chiral central carbon because their $-R$ groups differ completely from $-H$, $-NH_2$, and $-COOH$.
• Optical Stereoisomers ($D$ and $L$ Forms):
  • Chiral molecules exist as optical stereoisomers that rotate plane-polarized light either to the right ($D$-form, dextrorotatory) or to the left ($L$-form, levorotatory).
  • Except for achiral glycine, all amino acids can exist in $D$ or $L$ stereoisomeric forms.
  • Biological systems almost exclusively utilize $L$-form amino acids to construct functional proteins ($D$-form amino acids are extremely rare in living organisms).
• Chemical Classification of the $20$ Amino Acids by $-R$ Group Properties:
  • Nonpolar Amino Acids: $-R$ groups consist primarily of nonpolar hydrocarbons (e.g., alanine, valine, leucine, isoleucine). Leucine and isoleucine both contain four-carbon $-R$ chains, but isoleucine attaches at its second carbon rather than forming a linear terminal chain.
  • Polar Uncharged Amino Acids: $-R$ groups contain electronegative atoms (such as hydroxyl $-OH$ groups) capable of forming hydrogen bonds while maintaining no net formal charge.
  • Charged Acidic Amino Acids: $-R$ groups contain additional carboxyl groups ($-COOH$), causing them to carry a net negative charge at physiological pH.
  • Charged Basic Amino Acids: $-R$ groups contain extra amine groups ($-NH_2$), causing them to carry a net positive charge at physiological pH.
  • Aromatic Amino Acids: $-R$ groups contain cyclic organic ring structures with alternating double bonds (e.g., benzene ring derivatives).
  • Special Exception Amino Acids:
  • Proline: Features an $-R$ group that loops around to bond covalently with its own backbone amine group, forming a rigid ring that creates structural kinks in polypeptide chains.
  • Cysteine and Methionine: Contain sulfur ($-S-$) atoms in their $-R$ groups. The sulfhydryl groups of cysteine are critical for forming covalent disulfide bonds.

Higher-Order Protein Structures, Motifs, and Domains

• Peptide Bond Formation:
  • Amino acids link sequentially via condensation/dehydration synthesis reactions, which require the removal of a water molecule ($H_2O$).
  • The carboxyl group ($-COOH$) of one amino acid reacts with the amine group ($-NH_2$) of another to form a covalent amide linkage termed a peptide bond ($-CO-NH-$).
  • Dipeptide: Two amino acids linked together by a single peptide bond.
  • Polypeptide: A continuous, linear chain of multiple amino acids joined by peptide bonds.
• Four Levels of Protein Structural Organization:
  • Primary Structure: The unique, linear sequence of amino acids linked together by covalent peptide bonds along the polypeptide backbone.
  • Secondary Structure: Localized folding or coiling of the polypeptide backbone, stabilized exclusively by hydrogen bonds between backbone carbonyl and amine groups.
  • Alpha-helix (\null\alpha-helix): A tightly coiled, helical rod structure stabilized by hydrogen bonds between nearby backbone groups.
  • Beta-pleated sheet (\null\beta-sheet): A planar, sheet-like structure formed when adjacent polypeptide segments fold back and forth in a zigzag alignment, held together by inter-strand hydrogen bonds.
  • Tertiary Structure: The overall, fully folded three-dimensional shape of a single polypeptide chain (globular protein shape). It is stabilized by interactions between variable $-R$ side chains:
  • Hydrogen bonding between polar $-R$ groups.
  • Disulfide bridges: Strong covalent bonds ($-S-S-$) formed between sulfur atoms of adjacent cysteine residues.
  • Ionic bonds: Electrostatic attractions between oppositely charged acidic and basic $-R$ groups.
  • Van der Waals interactions: Weak attractions between closely packed nonpolar atoms.
  • Hydrophobic interactions: Forces that drive nonpolar, hydrophobic $-R$ groups to aggregate inward away from surrounding aqueous environments.
  • Quaternary Structure: The structural arrangement formed when two or more independent polypeptide chains (each in their tertiary structure) assemble into a multi-subunit protein complex.
• Protein Structural Motifs and Domains:
  • Motifs: Specific, recurring combinations of secondary structures (such as \null\alpha-helices and \null\beta-sheets) found across different protein families that help characterize and predict the function of newly discovered proteins.
  • Example 1: Beta-alpha-beta motif (two parallel \null\beta-sheets connected by an \null\alpha-helix).
  • Example 2: Helix-turn-helix motif (two \null\alpha-helices joined by a short polypeptide turn).
  • Domains: Distinct, modular globular units within a protein complex that fold independently and carry out specific functional roles. In quaternary complexes, individual folded polypeptides act as separate domains.

Protein Folding, Chaperone Proteins, and Denaturation

• Chaperone Proteins (Chaperonins):
  • Chaperone proteins are specialized helper proteins responsible for assisting unfolded or misfolded proteins in folding into their correct native tertiary structures.
  • Mechanism of Action: In bacteria like Escherichia coli, the chaperonin complex forms a cylindrical, symmetrical barrel structure. An unfolded or misfolded polypeptide enters the central interior cavity. Using cellular energy derived from adenosine triphosphate ($ATP$) hydrolysis, the chaperonin provides an isolated environment that forces the protein into its correct fold before releasing it.
  • Disease Implications: Genetic mutations that swap amino acid residues can prevent proper protein folding. Insufficient chaperone activity or misfolded proteins lead to cellular defects and disorders such as cystic fibrosis.
• Protein Denaturation:
  • Denaturation is the unfolding of a protein's secondary, tertiary, or quaternary structure, resulting in the complete loss of its biological function without breaking the primary peptide bonds.
  • Environmental Causes of Denaturation:
  • Increases in temperature (thermal denaturation).
  • Alterations in pH (extreme acidity or alkalinity).
  • Fluctuations in ambient ionic/salt concentrations.
  • Practical Examples:
  • Heating milk or creamer forms a skin layer on top due to the unfolding and clumping of denatured milk proteins.
  • Fermented milk becomes chunky because produced acids alter the pH, causing proteins to denature and aggregate.
• Anfinsen's Ribonuclease Refolding Experiment:
  • Hypothesis: The primary amino acid sequence contains all necessary structural information required to direct spontaneous folding into a protein's active native tertiary conformation.
  • Experimental Procedure: The enzyme ribonuclease was treated with a chemical reducing agent to break its hydrogen bonds and covalent disulfide bridges ($-S-S-$), fully denaturing and unfolding it into a linear chain.
  • Results: Upon removing the reducing agent and restoring optimal physiological conditions, ribonuclease spontaneously refolded back into its functional, native tertiary structure, supporting the hypothesis.

Structure and Classification of Lipids

• General Properties of Lipids:
  • Lipids are a heterogeneous group of organic molecules defined by their hydrophobic nature and insolubility in water.
  • Hydrophobicity is caused by a high proportion of nonpolar carbon-hydrogen ($C-H$) covalent bonds.
  • Major categories: Fats, oils, waxes (e.g., beeswax, candle wax), terpenes, steroids (e.g., cholesterol), and fat-soluble vitamins.
• Structure of Neutral Fats (Triglycerides / Triacylglycerols):
  • A triglyceride molecule is composed of $1$ molecule of glycerol covalently bound to $3$ fatty acid chains.
  • Glycerol: A three-carbon alcohol molecule ($C_3H_8O_3$) possessing a hydroxyl group ($-OH$) on each carbon atom.
  • Fatty Acid: A long, unbranched hydrocarbon chain (typically $14$ to $20$ carbons in length) ending in a terminal carboxyl group ($-COOH$).
• Saturated versus Unsaturated Fatty Acids:
  • Saturated Fatty Acids:
  • Hydrocarbon chains contain only single carbon-carbon ($C-C$) bonds.
  • Every carbon atom is fully saturated with hydrogen atoms.
  • The linear chains pack tightly in parallel arrangements, resulting in high melting points.
  • Typically solid at room temperature (predominantly found in animal fats).
  • Unsaturated Fatty Acids:
  • Hydrocarbon chains contain one or more double carbon-carbon ($C=C$) bonds.
  • Monounsaturated fatty acids contain a single $C=C$ double bond; polyunsaturated fatty acids contain multiple $C=C$ double bonds.
  • $C=C$ double bonds introduce physical kinks/bends in the hydrocarbon tail, preventing tight packing.
  • Results in lower melting points; typically liquid at room temperature (predominantly found in plant oils).
• Cis versus Trans Unsaturated Isomers:
  • Cis Configuration: Hydrogen atoms attached to double-bonded carbons reside on the same side of the double bond, creating a prominent physical kink in the chain.
  • Trans Configuration: Hydrogen atoms attached to double-bonded carbons reside on opposite sides of the double bond, maintaining a straight chain structure similar to saturated fats.
  • Health Impact of Trans Fats: Industrial partial hydrogenation converts cis plant oils into trans fats (e.g., historical production of margarine). Trans fats pack tightly, require significantly more metabolic energy/heat to break down, elevate bad LDL cholesterol levels, and increase cardiovascular disease risks. Consequently, health authorities like the FDA removed trans fats from approved safe food categories.

Phospholipids and Biological Membranes

• Molecular Structure of Phospholipids:
  • Constructed from a $3$-carbon glycerol backbone attached to:
  1. Two hydrophobic fatty acid tail chains (covalently bound to two backbone carbons).
  2. One hydrophilic phosphate group ($-PO_4^{-}$) attached to the third carbon (often modified with an additional polar $-R$ group).
  • Amphipathic Nature: Phospholipids possess both a hydrophilic (polar/water-loving) head region and two hydrophobic (nonpolar/water-fearing) tail regions.
• Behavior and Assemblies in Aqueous Solutions:
  • Micelles: Spherical clusters formed when phospholipids or detergents are mixed in water. Hydrophobic fatty acid tails aggregate inward to isolate themselves from water (trapping nonpolar grease or lipids), while hydrophilic phosphate heads project outward to interact with water.
  • Phospholipid Bilayer: A continuous two-layered sheet forming the structural foundation of all biological cell membranes. Hydrophilic phosphate heads face outward toward the aqueous extra-cellular and intra-cellular environments, while hydrophobic fatty acid tails face inward toward each other, completely shielded from water.
• Membrane Protein Associations:
  • Transmembrane Proteins: Intrinsic proteins that span entirely across the hydrophobic interior of the lipid bilayer, creating regulated pores or channels (e.g., aquaporins).
  • Surface Proteins: Peripheral proteins bound exclusively to the outer or inner hydrophilic surfaces of the membrane bilayer.

Review of Cell Biology, Taxonomy, and Scientific Principles

• Universal Characteristics of Living Organisms:
  1. Composed of cellular units.
  2. Exhibit growth (increasing cell size or reproducing to increase population numbers).
  3. Respond to environmental stimuli.
  4. Adapt to environmental changes.
  5. Consume and transform energy.
  6. Capable of reproduction.
• Prokaryotic versus Eukaryotic Organization:
  • Prokaryotic Cells (e.g., Domain Bacteria and Domain Archaea):
  • Lack internal membrane-bound organelles (e.g., mitochondria, Golgi apparatus, endoplasmic reticulum).
  • Lack a membrane-bound nucleus; genomic DNA exists as a free-floating circular strand within a region called the nucleoid.
  • Consist predominantly of single-celled organisms.
  • Eukaryotic Cells (e.g., Protists, Fungi, Plants, Animals):
  • Possess a true membrane-bound nucleus housing genomic DNA.
  • Contain compartmentalized membrane-bound organelles.
  • Protists are unicellular eukaryotic organisms.
  • Universal Feature: ALL living cells (prokaryotic and eukaryotic alike) possess a protective plasma membrane.
• Historical Scientific Principles and Taxonomy:
  • Spontaneous Generation: The hypothesis that life spontaneously arises from non-living matter is scientifically false.
  • Biological Taxonomy:
  • Developed by Carl Linnaeus.
  • Naming convention uses Binomial Nomenclature: Genus name followed by the species name.
  • Hierarchical classification organizes biological diversity into progressively narrower groups to define specific ecological niches.
  • Three-Domain System (established by Carl Woese):
  1. Domain Bacteria
  2. Domain Archaea
  3. Domain Eukarya
• The Scientific Method and Experimental Control:
  • Hypothesis: A testable, falsifiable, educated guess formulated to explain an observation, which can be evaluated through repeatable experimentation.
  • Independent Variable: The single factor intentionally manipulated or tested by an experimenter.
  • Dependent Variable: The measured response or outcome that changes in direct response to the independent variable.
  • Standardized Variables / Controls: Environmental conditions kept strictly constant across all experimental groups to prevent confounding factors (e.g., zeroing/blanking a spectrophotometer before taking measurements).
  • Eliminating Scientific Bias: Utilizing blinded protocols where samples are anonymized or coded prior to testing (e.g., analyzing genetic heart defects in fruit flies without knowing whether a sample is a control or a mutant until after data collection).

Fundamentals of Chemistry and Atomic Structure

• Atomic Structure and Composition:
  • Element: A pure chemical substance that cannot be broken down or reduced into simpler substances by ordinary chemical processes.
  • Four Most Abundant Elements in Living Organisms: Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), and Nitrogen ($N$).
  • Atomic Number: The exact number of protons located within an atom's nucleus (defines element identity).
  • Atomic Mass: The total combined sum of protons and neutrons in an atom's nucleus.
  • Bohr Model: A classical structural diagram depicting an atom with central protons/neutrons surrounded by electrons traveling in discrete concentric circular orbits/shells.
• Isotopes and Atomic Weight Calculations:
  • Isotopes: Atoms of the same chemical element that contain identical numbers of protons and electrons, but differing numbers of neutrons.
  • Because neutrons carry no electrical charge, isotopes exhibit identical chemical properties but differ in overall atomic mass.
  • The atomic mass listed on the periodic table represents a weighted average (mean) mass of all naturally occurring isotopes of that element based on their relative abundance.
• Electron Orbitals and Valence Dynamics:
  • Each individual atomic orbital holds a maximum of $2$ electrons.
  • Valence Electrons: Electrons located in the outermost shell of an atom. Valence electrons govern chemical reactivity and the formation of chemical bonds.
• Quantitative Chemical Formulas and Constants:
  • Empirical Formula: The simplest whole-number ratio of individual atoms present in a chemical compound (e.g., $(CH_2O)_n$ represents the empirical formula for carbohydrates).
  • Avogadro's Number: $6.022 \times 10^{23}$ particles (atoms or molecules) contained in exactly $1\, \text{mole}$ of a substance.
• Distinction Between Major Chemical Bonds:
  • Ionic Bond: Formed via the complete transfer of electrons from one atom to another, creating positively charged cations and negatively charged anions (e.g., sodium chloride, $NaCl$).
  • Ionic bonds are extremely strong in dry environments, but weak in aqueous solutions because polar water molecules dissociate ionic interactions.
  • Covalent Bond: Formed when two atoms share pairs of valence electrons that orbit around both atomic nuclei (e.g., bonds within $H_2O$).
  • Covalent bonds are highly stable and strong in aqueous biological environments.