4. Enzymes & Metabolism

The professor begins by stating they will continue discussing amino acids, peptides and proteins. They want to talk about a protein variant that causes sickle cell anemia, which is an interesting structural issue. They briefly recap what was covered last time on how the primary sequence of a polypeptide chain defines its folded structure. The folded structure involves secondary and tertiary interactions, such as hydrogen bonding. Some proteins can dissociate into quaternary structure with subunits that may form dimers, trimers or tetramers.

Hemoglobin is the dominant protein in red blood cells that carries oxygen and carbon dioxide for respiration. It has a heme molecule with iron that binds oxygen. Hemoglobin is a heterotetrameric protein with two alpha and two beta subunits. A single amino acid mutation from glutamic acid to valine in beta globin causes sickle cell anemia. This alters hemoglobin properties and causes red blood cells to become sickle-shaped, which can clog capillaries and cause severe pain.

The mutation occurs at position 6 in beta globin, changing a charged glutamic acid to a hydrophobic valine residue. This is caused by a single base pair mutation in the beta globin gene out of 134 million base pairs total. In normal hemoglobin, the structure cooperatively carries oxygen. With the mutation, hemoglobin molecules aggregate into fibrillar clusters, forming long inflexible chains. This distorts the discoid shape into a sickle shape that blocks capillaries and blood flow.

People heterozygous for the mutation have some normal and some sickle cell hemoglobin. People homozygous for the mutation have all disrupted hemoglobin. The heterozygous state offers an evolutionary advantage by conferring some resistance to malaria. There is overlap between the prevalence of the sickle cell trait and malaria in Africa, since the sickle shape makes red blood cells less habitable for the malaria parasite.

A structure of hemoglobin dimers reveals the valine mutation causes adjacent subunits to stick together. Valine interacts with a hydrophobic patch of phenylalanine and leucine on an adjacent subunit. Glutamic acid would be deterred from this interaction. This explains how the mutation causes hemoglobin molecules to aggregate abnormally.

Replacing glutamic acid with aspartic acid should have minimal effect since it is a similar residue. Tyrosine would likely be detrimental since it is hydrophobic. Lysine or serine may be less detrimental since lysine is charged and serine is slightly polar.

Enzymes catalyze metabolic reactions and other transformations. An isozyme is an enzyme variant from a different gene that catalyzes the same reaction. An allozyme is an enzyme variant from an allele of the same gene. Enzymes are needed to accelerate hard reactions that are too slow uncatalyzed. They lower the activation energy to speed up reactions while obeying thermodynamic laws. Exergonic reactions like catabolism release energy, while endergonic anabolic reactions require energy input.

Catalysts lower activation energy to increase reaction rates but do not change ΔG. Enzymes provide many mechanisms to lower activation energy like binding multiple substrates in proximity, distorting bonds to destabilize them, and stabilizing charged intermediates. This enables much faster reaction rates under mild physiological conditions.

Enzymes are common drug targets. Competitive inhibitors resemble the substrate and bind to the active site, while irreversible inhibitors covalently bond to the enzyme. Allosteric inhibitors bind at a different site but alter the active site. Allosteric activators bind elsewhere but enhance catalysis.

The professor recommends reading on carbohydrates for the next class on that topic. There are also useful videos on enzyme catalysis at the Protein Data Bank website.