Visualized in the vertical plane with dots representing hydrogen bonds.
Hydrogen bonding is not confined to a single side; as the helix turns, hydrogen bonds appear at different positions, contributing to the overall strength of the alpha helix.
Pattern in the amino acid sequence: every fourth amino acid participates in the hydrogen bonding pattern characteristic of the alpha helix.
Some amino acids contribute more to this pattern and are highlighted in orange/circled in the figure.
Beta pleated sheet contrast: relies on a sequence of amino acids but does not show a nice, regular mathematical pattern like the alpha helix.
Key takeaway: this regular pattern of hydrogen bonding in alpha helices versus the less regular pattern in beta sheets helps determine secondary structure.
Aesthetic and educational appeal: secondary structures are often shown as ribbons or other traditional models; the alpha helix often illustrated with a regular helical pattern.
MCAT relevance: this topic is a common MCAT question area; staying with secondary structure concepts makes exam preparation easier.
Mutations and secondary structure:
Missense mutation can substitute one amino acid for another.
If a missense mutation changes an amino acid to proline in a helix, it can disrupt the helix (proline acts as a helix breaker).
Why proline breaks helices: the side chain is small but rigid and forms a kink; there is limited capacity for participating in hydrogen bonding at that position, which disrupts the local helical geometry.
The term “proline kink” is used to describe the disruption caused by proline.
If a mutation disrupts the helix within the active site of an enzyme, catalysis can be severely impaired; if the mutation occurs outside the active site, the effect may be less drastic.
Real-world implications:
Enzymes rely on proper folding and secondary structure to create functional active sites.
Location of mutations (active site vs. exterior) matters for functional outcomes.
Beta pleated sheets
Beta sheets arise from a different pattern of hydrogen bonding that is not as easy to express as a single, regular sequence pattern as in alpha helices.
Extensive hydrogen bonding occurs, contributing to stability, but without the same regular periodicity as alpha helices.
Overall takeaway: beta sheets provide structural stability through interstrand hydrogen bonds, but their pattern is less uniform than that of alpha helices.
Secondary structure overview and educational relevance
Alpha helices vs beta sheets are two primary forms of protein secondary structure.
The presence and arrangement of hydrogen bonds, as well as the participating amino acids, determine which structure predominates.
Secondary structure visualization (e.g., ribbons) is a traditional and helpful way to conceptualize these motifs.
MCAT-style emphasis: questions often test understanding of how mutations can affect secondary structure and function, especially in contexts like enzyme active sites.
Mutations and protein function in context
Missense mutation: a single amino acid substitution.
If a missense mutation replaces an amino acid with proline within an alpha helix:
The helix can be disrupted due to proline’s unique conformational properties.
This disruption can alter the local structure and potentially broader protein function.
Location matters:
Active site mutations: likely detrimental to catalysis and function of the enzyme.
Outside the active site: may have a milder or different effect, depending on how the mutation alters folding, stability, or interactions.
Sulfide bonds (disulfide bridges)
Cysteine has a thiol side chain (R–SH).
Possible disulfide bridge formation between cysteine residues:
Intramolecular disulfide bonds: occur within a single polypeptide chain.
Intermolecular disulfide bonds: occur between two different polypeptide chains.
Relevance to keratin and nails: disulfide bonds contribute to the durability and resistance of these proteins.
Practical takeaway: covalent cross-links are a major factor in the mechanical stability of certain proteins.
Aside on imagery: the depiction includes “hooks”; the instructor jokingly notes not to over-interpret as animal-specific features (e.g., not goat, possibly horse in some contexts).
Ionic bonding and salt bridges in proteins
Ionic interactions occur between acidic and basic side chains; these are often referred to as salt bridges in a biochemical context.
Real-world examples include:
Insulin: two chains (alpha and beta) connected by ionic interactions forming a salt bridge; this helps stabilize the subunit interface.
Hemoglobin: multiple salt bridges help hold together the subunits; red dots in the illustration highlight these salt-bridging interactions.
The general point: ionic bonds/salt bridges contribute to quaternary structure stability and subunit interactions in multi-subunit proteins.
A quick primer on amino acid structure (four components and roles)
Exercise prompt discussed in class: draw and label the four components of an amino acid and explain the role of each in protein function.
Four components around the central carbon (alpha carbon):
Amino group (-
NH_2)
Carboxyl group (-COOH)
Hydrogen atom (-H)
Side chain (R group)
Roles in function:
Carbon (alpha carbon) is the central point to which all four groups attach.
Amino group: contributes to peptide bond formation (acts as a nucleophile in condensation reactions) and participates in acid-base chemistry.
Carboxyl group: participates in peptide bond formation as the acyl component; involved in the formation of the peptide backbone.
Side chain (R group): determines the chemical nature of the amino acid (polar, nonpolar, charged, etc.) and largely dictates the protein’s folding, structure, and function.
The interplay of these groups around the chiral alpha carbon gives amino acids their properties and the overall 3D structure of proteins.
Practical note: discussing the full set of about 20 amino acids can be lengthy; the exercise suggests that listing a representative set could be done quickly, but a protein like Titin illustrates the scale of protein diversity.
Titin: a paradigmatic large structural protein
Titin is enormous, with polypeptides ranging from about 27,000 to 33,000 amino acids.
Its size makes it one of the largest known proteins, contributing to its substantial mass.
Function: acts as an elastic spring in muscle, contributing to passive elasticity and structural integrity of muscle fibers.
The protein is structural rather than enzymatic and does not have a typical active site.
Weight remark from the transcript: the protein weighs a lot, with a stated range of about 900 to 38 (note: units not specified in the transcript; likely intended as very large molecular weight measures such as kDa or Da).
Proteins: general functions and real-world relevance
Proteins serve a variety of roles beyond enzymes, including structural components and signaling molecules.
Oxidative phosphorylation is described as the opposite of photosynthesis in the transcript (a framing used in the course); both are fundamental energy-related processes in cells.
Real-world examples of protein function across the body include enzymatic activity, structural roles (e.g., keratin in hair, nails), and involvement in energy production pathways.
Quick takeaways and connections
Secondary structure (alpha helices vs beta sheets) is largely determined by hydrogen bonding patterns and the amino acid sequence.
Mutations can disrupt structure and function, with effects depending on location (active site vs other regions) and the specific amino acid substituted (e.g., proline as a helix breaker).
Covalent (disulfide) bonds and ionic (salt bridge) interactions provide strong stabilization for certain proteins and complexes (keratin/nails, insulin, hemoglobin).
Amino acids contain four groups around a central carbon, and the side chain (R group) largely dictates function and folding.
Titin exemplifies the magnitude of some proteins and their specialized structural roles in muscle.
The study of protein structure and bonding has direct implications for understanding disease, enzyme function, and biophysical properties relevant to health and medicine.