Tutor profile: Julie S.
What is an enzyme that is critical for biological and physiological processes.
One extremely important (and my personal favorite) biological enzymatic complex is the pyruvate dehydrogenase complex (PDC). This complex is known for its role in bridging the major product of glycolysis to the next major set of reactions in aerobic respiration, the citric acid cycle. PDC is a large three-enzyme complex that catalyzes the conversion of pyruvate to acetyl CoA via three major reactions. Here I will discuss in detail the role of the first two enzymes (E1 & E2) of this complex, and briefly mention the third (E3), as it has more of a resetting function in this pathway. The PDC usually exists in its active, holoenzyme form as a tetramer, meaning the 3 enzymes together will form a complex which is connected to 3 other complexes of the same composition. The 3 enzymes are considered apoenzymes without their prosthetic groups, each of which is described below (TPP, lipoamide, and FAD) (Poole, 1998). Beginning with the pathway that processes glucose, glycolysis takes place in the cytoplasm of liver cells, the final major product is pyruvate. Pyruvate must now enter the cell’s mitochondria in order to begin the citric acid cycle. However, pyruvate must first be converted to acetyl CoA before it can enter the cycle. The E1 portion of the PDC is known as pyruvate dehydrogenase component and contains the prosthetic group (or coenzyme) thiamine pyrophosphate (TPP). The purpose of TPP on E1 is to oxidatively decarboxylate (remove a carbon) from pyruvate. By combining the carbanion of TPP with pyruvate, this reaction generates CO2 and hydroxyethyl-TPP. The hydroxyethyl-TPP is then transferred to the E2 portion of the PDC, dihydrolipoyl transacetylase, whose prosthetic group is lipoamide. First, an oxidation occurs and hydroxyethyl-TPP is split to produce the carbanion form of TPP (see above) and acetyllipoamide via a thioester bond. Then, acetyllipoamide transfers its acetyl group to Coenzyme A which ultimately gives the desired product, Acetyl CoA, which can then enter the citric acid cycle. Finally, lipoamide is reset in a reaction with dihydrolipoyl dehydrogenase, which regenerates lipoamide’s oxidized form, using its prosthetic group FAD. Importantly, this reaction also produces NADH. References Poole, R. K., & Poole, R. K. (1998). In Advances in Microbial Physiology (p. 6). essay, Academic Press.
Subject: Basic Chemistry
What is the difference between isotopes, ions, and how can you tell?
In the periodic table, elements are listed according to their atomic number. This number correlates to their number of protons. In neutral forms of an element, (elemental forms) there is an equal number of protons to electrons within an atom. An atom has a nucleus where it holds the protons and neutrons, and a limbo-like space around the nucleus called orbitals. An atom has a set number of orbitals that holds electrons. When electrons are added or removed from these spaces, it creates an ion, or a charged particle. Adding or subtracting the electrons creates a negative or positive charge, respectively. Can you change the number of protons? Kind of- but this changes the element. You won't see Lithium with 4 protons, but instead now a Beryllium atom! Finally, what about those pesky neutrons? They can be added as well! When you add a neutron to the nucleus of an atom, you increase the weight of the atom without changing its elemental identity. This is called an isotope. You can always identify this information by learning the periodic table!
Why does the relative refractory period of an action potential in a frog muscle inversely depend upon the stimulus intensity upon the muscle?
The relative refractory period is longer when the stimulus intensity is not stronger than the previous stimulus that caused the first action potential. There are less Na+ channels open during this period and so a stronger stimulus will be needed to push the membrane to a high enough depolarization for an AP.