Question

Which of the following statements about phosphofructokinase is false? a. It is located and has its main activity in the inner mitochondrial membrane. b. It can be inhibited by NADH to slow glycolysis. c. It can be inactivated by ATP at an inhibitory site on its surface. d. It can be activated by ADP at an excitatory site on its surface. e. It can cause ADP to form.

Short answer

The false statement about phosphofructokinase is (a), as it is located in the cytosol, not the inner mitochondrial membrane.

Step-by-step solution

Understanding the Role of Phosphofructokinase

Recognize that phosphofructokinase (PFK) is an enzyme that plays a key role in the regulation of glycolysis. It catalyzes the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate.

Identifying the False Statement Regarding Phosphofructokinase Location

Understand that PFK is actually located in the cytosol, not the inner mitochondrial membrane. Glycolysis as a whole occurs in the cytosol, and PFK is one of the key enzymes involved in this process.

Evaluating the Other Statements for Accuracy

Recognize that PFK can indeed be inhibited by ATP and activated by ADP; this is part of its regulation mechanism. NADH is also a known inhibitor. However, PFK doesn't directly cause ADP to form; instead, it uses ATP to phosphorylate fructose 6-phosphate, thus consuming ATP and generating ADP.

Key concepts

Glycolysis Regulation

Glycolysis is a critical energy-producing pathway that converts glucose into pyruvate, with the net production of ATP. This process is tightly regulated to meet the energy needs of the cell. A central player in this process is the enzyme phosphofructokinase (PFK), which acts as a gatekeeper at one of the key control points in glycolysis.

PFK's activity is modulated through feedback mechanisms responsive to the cell's energy status. When ATP levels are high, indicating sufficient energy, ATP binds to an inhibitory site on PFK, reducing the enzyme's activity. This is a form of negative feedback. Similarly, when the levels of ADP and AMP increase, suggesting a low energy state, these molecules bind to an activatory site on PFK, enhancing its activity. This ensures that glycolysis proceeds when the cell requires energy. Furthermore, other factors such as the levels of citrate and hydrogen ions can also influence PFK’s activity, thereby illustrating the complex regulation of glycolysis to maintain energy homeostasis.

Enzyme Inhibition and Activation

Enzymes are biological catalysts essential for numerous biochemical reactions in the body, but their activities must be controlled. Inhibition and activation are two means by which enzyme activity is regulated.

Inhibitory molecules, such as ATP in the case of phosphofructokinase, decrease enzyme activity. This is often a feedback response, preventing overproduction of an enzyme's product when it is abundant. Enzyme inhibition can be competitive, where the inhibitor mimics the substrate and binds to the active site, or non-competitive, where the inhibitor binds elsewhere but still impedes the enzyme's function.

On the flip side, activators, like ADP for PFK, increase the activity of enzymes. These molecules may bind to sites other than the active site, inducing a conformational change that improves the enzyme's ability to bind to substrates, often during times of increased metabolic demand. The precise control of enzyme activity by inhibition and activation is crucial for maintaining homeostasis within the metabolic pathways.

Metabolic Pathways in Cellular Respiration

Cellular respiration is a series of metabolic pathways whereby living cells produce energy. It comprises three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle or TCA cycle), and the electron transport chain.

During glycolysis, glucose is broken down in the cytosol to form pyruvate, which then enters the mitochondria if oxygen is available. The citric acid cycle further processes these molecules, releasing electrons that are used by the electron transport chain in the inner mitochondrial membrane to generate a substantial amount of ATP through oxidative phosphorylation.

Regulation happens at various points along these pathways, largely to ensure that energy production matches the cell's energy needs. Among the regulated enzymes is phosphofructokinase in glycolysis. Other enzymes in the citric acid cycle and electron transport chain are similarly regulated by the availability of substrates and feedback from the energy-rich molecule ATP, as well as NADH and FADH2, which are reduced during these processes.