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Chapter 7: Problem 23

The control of which enzyme exerts the greatest control of glycolysis? a. hexokinase b. phosphofructokinase c. glucose--6-phosphatase d. aldolase

Short Answer

Expert verified
b. phosphofructokinase

Step by step solution

01

Understand glycolysis

Glycolysis is a series of reactions that extract energy from glucose by splitting it into two molecules of pyruvate. This process involves several enzymes that regulate different steps.
02

Identify key regulatory steps

Key regulatory steps in glycolysis are those catalyzed by enzymes whose activity is tightly controlled. These steps usually involve large changes in free energy and are often points of no return.
03

Analyze the given enzymes

Consider the roles of the given enzymes in glycolysis: (a) Hexokinase phosphorylates glucose to form glucose-6-phosphate. (b) Phosphofructokinase catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. (c) Glucose-6-phosphatase is involved in gluconeogenesis, not glycolysis. (d) Aldolase splits fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
04

Determine the enzyme with the greatest control

Phosphofructokinase (PFK) is known as a major regulatory point in glycolysis. It is subjected to allosteric regulation by various molecules such as ATP and AMP, making it a rate-limiting step.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

glycolysis
Glycolysis is the metabolic pathway that converts glucose into pyruvate, releasing energy and producing ATP in the process. It is the first step in cellular respiration and takes place in the cytoplasm of the cell. The process involves ten enzyme-catalyzed reactions and can be split into two stages: the energy investment phase and the energy payoff phase. During the energy investment phase, ATP is consumed, while during the energy payoff phase, ATP and NADH are produced.
Glycolysis is essential because it provides energy quickly and does not require oxygen. This makes it especially important during anaerobic conditions, like in heavy muscle activity. The end product, pyruvate, can further enter the mitochondria for aerobic respiration or be converted to lactate in anaerobic conditions.
phosphofructokinase
Phosphofructokinase (PFK) is a crucial enzyme in glycolysis. It catalyzes the phosphorylation of fructose-6-phosphate into fructose-1,6-bisphosphate. This reaction is vital because it is one of the key regulatory steps in glycolysis and involves a significant change in free energy.
The activity of PFK is tightly regulated, making it a major control point of glycolysis. It is responsive to the cell's energy demands, and its regulation plays a significant role in maintaining proper glucose metabolism.
enzyme regulation
Enzyme regulation in glycolysis is vital for controlling the rate and flow of the pathway. Different enzymes in glycolysis are regulated by various mechanisms to ensure efficiency and responsiveness to the cell's needs. These regulatory mechanisms include:
  • Allosteric regulation
  • Feedback inhibition
  • Covalent modification
  • Availability of substrates
Regulation of the key enzymes ensures that glycolysis is appropriately adjusted according to the cellular energy status and other metabolic needs.
allosteric regulation
Allosteric regulation is a form of enzyme regulation where the enzyme's activity is modulated by the binding of a regulatory molecule at a site other than the active site, known as the allosteric site. This can either inhibit or activate the enzyme.
In glycolysis, PFK is a prime example of an enzyme regulated allosterically. Molecules such as ATP, citrate, and AMP serve as allosteric regulators for PFK:
  • ATP acts as an allosteric inhibitor, signaling that the cell has sufficient energy and slowing down glycolysis.
  • AMP acts as an allosteric activator, indicating low energy levels and speeding up glycolysis.
This allosteric regulation ensures glycolysis is attuned to the cell's energy demands.
rate-limiting step
In metabolic pathways like glycolysis, the rate-limiting step is the slowest step, effectively controlling the overall rate of the pathway. It often involves significant regulation and high energy changes.
In glycolysis, the reaction catalyzed by phosphofructokinase (PFK) is considered the rate-limiting step. This step is highly regulated and involves the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. Because PFK activity is modulated by various factors reflecting the cell's energy status, it ensures that glycolysis proceeds efficiently under different conditions.
Understanding the rate-limiting step is crucial for comprehending how cells regulate energy production and maintain metabolic balance.

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Most popular questions from this chapter

What happens when a chemical is reduced during a reaction? a. The compound is reduced to a simpler form. b. An electron is added to the chemical. c. A hydrogen atom is removed from the substrate. d. acts as a catabolic reaction

Which of the following fermentation methods can occur in animal skeletal muscles? a. lactic acid fermentation b. alcohol fermentation c. mixed acid fermentation d. propionic fermentation

A. [Extension] Living systems require free energy to carry out cellular functions, and employ various strategies to capture, use, and store free energy. Explain the advantage that the higher energy efficiency per kg of the Krebs cycle provides to you compared to a metabolism based on glycolysis alone. Your explanation should make use of all the following facts: \(\bullet\)\triangle \mathrm{G}\( for glycolysis is \)-135 \mathrm{kJ}\( per mole of glucose \)\bullet\( \triangle G\) for aerobic respiration is - 2880 \(\mathrm{kJ}\) per mole glucose \(\bullet\) the basal metabolic rate of mammals is often represented as \(-300 \mathrm{kJ} / \mathrm{day} \cdot \mathrm{m}^{0.75}\) \(\bullet\) the molar mass of glucose is 180 \(\mathrm{g} / \mathrm{mole}\) B. Explain the bioenergetic difference between aerobic and anaerobic respiration in terms of the difference between free-energy production and power. Your explanation should make use of all the following facts: \(\cdot\) power is the rate of free-energy production \(\cdot\) cancer cells derive most of their free energy from glycolysis \(\cdot\) enzymes of the citric acid (Kreb's) cycle form coordinate complexes on the cytoskeleton within the mitochondria C. The life cycle of the human parasite Trypanosoma brucei is divided between the body of the tsetse fly and the human blood stream. The parasite causes 鈥渟leeping sickness鈥� in Sub-Saharan Africa. Within the human bloodstream, the parasite depends on glycolysis, with enzymes compartmentalized in a membrane-bound organelle called the glycosome. In the insect host, the parasite utilizes glycolysis as well as substrate-level and oxidative phosphorylation. Explain the advantage of a life cycle in the human host that employs anaerobic respiration with a rate of free-energy production that is enhanced by compartmentalization in the glycosome and a life cycle in the insect host that is aerobic. D. Predict the advantages of a biological system that uses both glycolysis and oxidative phosphorylation. Your prediction should make use of all the following facts: \(\cdot\) signaling can be used to detect low-oxygen environments and to regulate response \(\cdot\) some cells, such as muscle and blood cells, must function in both low- and high-oxygen environments \(\cdot\) glycolysis is reversible \(\cdot\) the citric acid cycle is not reversible \(\cdot\) thermoregulation is needed for homeostasis

Red blood cells (RBCs) do not perform aerobic respiration, but they do perform glycolysis. Why do all cells need an energy source and what would happen if glycolysis were blocked in a red blood cell? a. Cells require energy to perform certain basic functions. Blocking glycolysis in RBCs causes imbalance in the membrane potential, leading to cell death. b. Cells need energy to perform cell division. Blocking glycolysis in RBCs interrupts the process of mitosis leading to nondisjunction. c. Cells maintain the influx and efflux of organic substances using energy. Blocking glycolysis stops the binding of \(\mathrm{CO}_{2}\) to the RBCs, causing cell death. d. Cells require energy to recognize attacking pathogens. Blocked glycolysis inhibits the process of recognition, causing invasion of the RBCs by a pathogen.

Glucose catabolism pathways are sequential and lead to the production of ATP. What is the correct order of the pathways for the breakdown of a molecule of glucose as shown in the formula? \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}+\) energy \(\begin{aligned} \text { a. } & \text { oxidative phosphorylation } \rightarrow \text { citric acid cycle } \\ & \rightarrow \text { oxidation of pyruvate } \rightarrow \text { glycolysis } \end{aligned}\) \(\begin{aligned} \text { b. the oxidation of pyruvate } & \rightarrow \text { citric acid cycle } \\ & \rightarrow \text { glycolysis } \rightarrow \text { oxidative phosphorylation } \end{aligned}\) c. glycolysis \(\rightarrow\) oxidation of pyruvate \(\rightarrow\) citric acid cycle \(\rightarrow\) oxidative phosphorylation d. citric acid cycle \(\rightarrow\) glycolysis \(\rightarrow\) oxidative phosphorylation \(\rightarrow\) oxidation of pyruvate
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