Biochemistry Study Guide: Cell Structure, Protein Analysis, Acid-Base Chemistry, and Chromatography

Cell Structure and Macromolecular Organization

Volume Occupancy in Bacterial Cells

Bacterial cells, such as Escherichia coli, have defined dimensions and contain various macromolecular structures. Understanding the spatial organization within the cell is crucial for appreciating cellular function.

• Protective Envelope: The cell envelope surrounds the bacterium and occupies a fraction of the total cell volume. The volume of a cylindrical envelope can be calculated using $V = ext{πr}^2h$.

• Ribosome Occupancy: Ribosomes are abundant in rapidly growing cells, and their collective volume can be compared to the total cell volume to determine the percentage they occupy.

• DNA Length and Packaging: The molecular weight of E. coli DNA is approximately $3.1 \times 10^9$ daltons. Each nucleotide pair contributes 0.34 nm to the DNA length. The total length of DNA can be calculated and compared to cell dimensions to understand DNA packaging.

Example: If the cell diameter is 0.8 μm and the DNA length is several millimeters, the DNA must be highly compacted to fit inside the cell.

Acid-Base Chemistry in Biochemistry

Weak Acids and pKa Values

Many biological molecules are weak acids or bases, and their ionization state depends on the pH of the environment. The pKa value indicates the pH at which half of the molecules are ionized.

• Aspirin Absorption: Aspirin (acetylsalicylic acid) has a pKa of 3.6. Its absorption in the body depends on its ionization state, which is influenced by the pH of the stomach (pH ~1.5) and small intestine (pH ~6.0).

• Ionization and Membrane Permeability: Non-ionized (uncharged) molecules cross cell membranes more readily than ionized (charged) molecules.

Example: Aspirin is more non-ionized in the acidic stomach, favoring absorption there compared to the more basic small intestine.

Buffer Systems and pH Control

Biological systems use buffers to maintain pH within narrow limits. The phosphate buffer system is a common example.

• Phosphate Buffer: $\text{H}_3\text{PO}_4$ is a triprotic acid with three pKa values: 2.14, 6.86, and 12.4.

• Henderson-Hasselbalch Equation: Used to calculate the ratio of conjugate base to acid at a given pH:

$\text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)$

Example: To achieve pH 7.0, use the pKa closest to 7 (6.86) and solve for the ratio $[\text{HPO}_4^{2-}]/[\text{H}_2\text{PO}_4^-]$.

Control of Blood pH by Respiratory Rate

CO2 and Blood pH Regulation

The partial pressure of CO2 ($pCO_2$) in the lungs can be rapidly altered by changing breathing rate and depth, affecting blood pH.

• Hypoventilation: Increases $pCO_2$, leading to decreased blood pH (acidosis).

• Hyperventilation: Decreases $pCO_2$, leading to increased blood pH (alkalosis).

• Lactic Acid Production: During intense exercise, muscles produce lactic acid, which can lower blood pH.

Example: Hyperventilating before a short-distance run can help buffer the acid produced during exercise.

Protein Structure and Analysis

Molecular Weight Calculation

The molecular weight of a protein can be estimated by multiplying the number of amino acid residues by the average molecular weight of an amino acid (approximately 110 Da).

• Example: A protein with 682 residues has a molecular weight of $682 \times 110 = 75,020$ Da.

Subunit Composition of Proteins

Proteins may consist of multiple subunits, which can be identified by gel electrophoresis under different conditions.

• Size-Exclusion Chromatography: Measures the native molecular mass of a protein.

• SDS-PAGE: Denatures proteins and separates subunits by mass.

• Disulfide Bond Reduction: Further separates subunits if they are linked by disulfide bonds.

Example: If a protein shows three bands of 180, 160, and 60 kDa after SDS-PAGE, and two bands of 160 and 60 kDa after reduction, it likely contains three subunits.

Protein Purification and Enzyme Activity

Purification Table Analysis

Protein purification involves multiple steps, each affecting yield and specific activity. The following table summarizes typical data:

Procedure	Total Protein (mg)	Activity (units)
Crude extract	10,000	68,000
Precipitation (salt)	5,000	65,000
Precipitation (pH)	4,000	56,000
Affinity chromatography	12	42,000
Ion-exchange chromatography	70	40,000
Size-exclusion chromatography	18	40,000

• Specific Activity: Calculated as units of activity per mg protein.

• Purification Effectiveness: Steps that increase specific activity are more effective.

Example: Affinity chromatography greatly increases purity by reducing total protein while retaining high activity.

Chromatography and Peptide Separation

Cation-Exchange Chromatography

Cation-exchange chromatography separates peptides based on their net charge at a given pH. Peptides with more positive charge bind more strongly and elute later.

• Peptide Composition: The relative abundance of acidic (Asp, Glu) and basic (Lys, Arg, His) residues determines net charge.

• Elution Order: Peptides with higher net positive charge elute last.

Example: At pH 7.0, a peptide rich in Lys and Arg will elute after a peptide rich in Asp and Glu.

Protein Sequence Analysis

Sequence Comparison and Motifs

Proteins such as molecular chaperones have conserved signature sequences that can be analyzed to identify invariant residues and functional motifs.

• Invariant Residues: Amino acids that are conserved across species indicate important functional roles.

• Motif Identification: Sequence logos visually represent the frequency of amino acids at each position in a motif.

Example: The ATP-binding motif in chaperones is identified by conserved residues such as Gly and Lys at specific positions.

Chromatography of Peptides

Different types of chromatography (anion-exchange, cation-exchange, size-exclusion) separate peptides based on charge, size, or other properties.

• Migration Rate: Peptides with more charge migrate more slowly in ion-exchange chromatography.

• Motif Detection: Sequence analysis can reveal functional motifs such as ATP-binding sites.

Example: A peptide containing the sequence GK (Gly-Lys) is likely to contain an ATP-binding motif.

Additional info: Academic context and explanations have been expanded for clarity and completeness. All equations are provided in LaTeX format as required.