Biochem MCAT BrSc Flashcards
Name two methods of phosphoryl group transfer that form ATP.
(1) Oxidative phosphorylation is the production of ATP from ADP by ATP synthase. This occurs immediately after the electron transport chain. (2) Substrate-level phosphorylation is the transfer of a phosphoryl group from a substrate directly to ADP. An example of such a substrate is 1,3-bisphosphoglycerate.
The phosphorylation of glucose into glucose-6-phosphate has a ΔGº of +13.8 kJ/mol. However, this reaction proceeds as the initial step of glycolysis. What process is responsible?
This situation is the result of COUPLING the reaction with another, more favorable, process. Specifically, this reaction is coupled with the hydrolysis of ATP. glucose + Pi → glucose-6-phosphate + H2O ΔGº = +13.8 kJ / mol ATP + H2O → ADP + Pi ΔGº = -30.8 kJ / mol Together, these coupled reactions have a ΔGº of (+13.8 kJ / mol) + (-30.5 kJ / mol) = -16.7 kJ / mol. With such a negative value, this process will be spontaneous.
In what part(s) of a eukaryotic cell do oxidative and substrate-level phosphorylation occur, respectively?
(1) Oxidative phosphorylation occurs only in the mitochondria. Specifically, ATP synthase is located along the inner mitochondrial membrane.
(2) Substrate-level phosphorylation can occur in the cytosol (as in glycolysis) or the mitochondrial matrix (as in the Krebs cycle).
Why are enzymes generally ineffective catalysts over a broad temperature range?
Enzymes must be at a certain optimal temperature to maintain their structure. The structure of an enzyme, especially in relation to its active site, is essential to its function as a catalyst. Like other proteins, enzymes denature, or lose their original conformations, above a certain temperature. However, reactions generally progress more slowly at low temperatures, further narrowing the range of optimal activity.
What name is given to the phosphate bonds in ATP?
These are phosphoanhydride bonds, which exist between phosphate groups on molecules like ATP (shown) and ADP.
The name of these bonds comes from the reaction that forms them: dehydration (removal of H2O).
Predictably, they can be broken by the reverse reaction, hydrolysis (addition of H2O).
What type of chemical reaction most closely relates to the biological molecules FAD and NAD+?
While the breaking of bonds usually requires energy, hydrolysis of a phosphoanhydride bond in ATP is extremely exergonic. What structural features of ATP explain this phenomenon?
For a biological oxidation-reduction reaction to proceed spontaneously, what should be the signs of its ΔG and E values?
Its ΔG value should be negative, while its E value, or reaction potential, should be positive.
Note t
hat, as in most biological reactions, nonspontaneous reactions can occur if coupled with spontaneous processes. In that case, the sum of their ΔG values must still be negative.
What class of substrate is common to maltase, sucrase and lactase?
The substrates of these enzymes are all disaccharides.
Maltase breaks down maltose into two glucose molecules.
Sucrase cleaves sucrose into glucose and fructose, and
lactase breaks down lactose into glucose and galactose.
Is FADH2 the reduced or oxidized form of flavin adenine dinucleotide?
FADH2 is the reduced form, meaning that it is currently carrying electrons. FAD+ is the oxidized form.
When FADH2 donates its electrons to another molecule, as in the electron transport chain, it is acting as a reducing agent.
Is NAD+ the reduced or oxidized form of nicotinamide adenine dinucleotide?
NAD+ is the oxidized form, meaning that it is not currently carrying electrons. NADH is the reduced form.
When NAD+ accepts electrons to become NADH, it is acting as an oxidizing agent.
What is the functional significance of the active site?
The active site of an enzyme is the structural component where enzyme-substrate interactions take place.
In other words, the active site is the catalytic region of an enzyme. It is structured to facilitate the binding of its substrate, often through the presence of certain amino acid residues.
How many total electrons can be accepted by eight molecules of NAD+?
Eight molecules of NAD+, if reduced to NADH, would hold a total of sixteen electrons.
Remember that both common electron carriers (NAD+ and FAD) can accept two electrons per molecule.
How does the lock-and-key model explain enzyme-substrate specificity?
The lock-and-key model posits that a substrate will fit perfectly into the active site of its corresponding enzyme, without any conformational changes taking place.
In this model, the active site is the “lock” and its complementary substrate is the “key.”
What is the difference between cofactors and coenzymes?
Cofactors are a broad group of compounds that are required for the proper functioning of enzymes. Cofactors can also be inorganic substances, such as ions.
Coenzymes are a class of small, organic cofactors.
