Fetal hemoglobin is associated with a left-shift due to its greater binding affinity for oxygen. The Michaelis constant, K_m, is defined as the substrate concentration at which the reaction rate is 0.5 * V_max. A low K_m indicates high substrate affinity. 

The correct answer is pyrimidine complex. A pyrimidine refers to a type of nucleotide base. Enzymes commonly have the suffix -ase at the end of their name.

Enzymes are unable to shift the equilibrium of a reaction. This is a commonly confused enzyme concept, but it should be known that, chemically speaking, only adding or removing reactants and/or products can shift the equilibrium of a reaction. Although this concept is mainly seen in chemisty, known as Le Chatelier's principle, this principle is surprisingly helpful and applicable to many fields of science, including biology. All other answer choices are functions of enzymes.

The rate of enzymatic activity can be increased in a few ways. Enzymes have optimal levels of acidity and temperature at which they function best. If this optimal level is exceeded, the enzyme will denature. Enzymes of the small intestine are adjusted to a relatively alkaline environment and will denature in acidic environments. Decreasing the temperature would decrease the rate of catalyzation. By increasing the amount of substrate, the enzyme will function faster. The level at which an enzyme's rate of catalyzation can no longer be made faster by the addition of substrate is referred to as its V_max.

Reaction rates are generally impacted by the concentration of the reactants in the reaction. The Km is the amount of substrate needed to attain a concentration resulting in half of the maximum rate achievable by an ezyme catalyzed reaction.

The maximum rate will be limited by the number of enzyme moleules available, and will be reached when the enzyme active sites are saturated.

The basement membrane and sub-basement structures are predominately made of protein (connective tissue). To infiltrate this region, a protease would be most appropriate. The remaining choices are all enzymes, but would not be capable of digesting appropriate proteins.

Cysteine has a sulfide (-SH) in its side chain, which can form covalent disulfide bonds. These bonds are often integral in creating protein tertiary structures.

Secondary peptide structure refers to alpha-helices and beta-sheets, which are formed by hydrogen bonding. A supercoiled helix is due to the secondary structure of a peptide. Primary structure is the sequence of amino acids, tertiary structure is the three-dimensional arrangement of the protein, and quatenary structure arises when more than one peptide subunit interacts.

Disulfide bonds are covalent attachments created between two sulfur atoms. Cysteine is usually the amino acid that creates these connections.

Pepsin is used to break down protein in the stomach by hydrolyzing some peptide bonds. Pepsin can operate best at a pH of 2 (same pH as the stomach). Adding HCl to the solution will bring the solution's pH closer to this optimal pH.