Harper's Biochemistry Chapter 14 - Overview of Metabolism

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Questions and Answers

What is the primary source of energy for the central nervous system?

Amino acids

Glycerol

Glucose (correct)

Fatty acids

What hormonal factors primarily control the formation and mobilization of glycogen and triacylglycerol reserves?

Somatostatin and glucagon

Thyroid hormones and insulin

Insulin and glucagon (correct)

Adrenaline and cortisol

What happens to dietary glucose that is not taken up by the liver?

It is stored exclusively as fat.

It is oxidized by muscle or stored as glycogen. (correct)

It is eliminated from the body.

It is converted directly into amino acids.

Which statement about fatty acid oxidation during low glucose availability is correct?

Fatty acid release from adipose tissue is suppressed. Signup and view all the answers

Which molecule is produced during the catabolism of carbohydrates, proteins, and fats that enters the citric acid cycle?

Acetyl-CoA Signup and view all the answers

What role does insulin play in glucose metabolism?

It increases storage of glucose in the liver and muscles. Signup and view all the answers

What does oxidative phosphorylation primarily produce?

ATP Signup and view all the answers

Which tissue primarily converts excess glucose to glycogen for storage?

Muscle tissue Signup and view all the answers

Which type of metabolic pathway is primarily responsible for the synthesis of larger molecules?

Anabolic pathways Signup and view all the answers

Which pathway is involved in breaking down fatty acids for energy production?

Beta-oxidation Signup and view all the answers

What role do hormones primarily play in metabolism?

They modulate flux through metabolic pathways. Signup and view all the answers

Which pathway is generally considered a catabolic process?

Glycolysis Signup and view all the answers

Which process describes the generation of ATP from the breakdown of glucose?

Oxidative phosphorylation Signup and view all the answers

What is the primary source of muscle protein synthesis?

Plasma amino acids Signup and view all the answers

In a closed system reaction, what happens when a fixed quantity of 'A' is added?

The reaction proceeds to the right until equilibrium is reached Signup and view all the answers

What condition is described when the net flux from left to right occurs in a metabolic reaction?

Steady state Signup and view all the answers

Which organelle is central to carbohydrate, lipid, and amino acid metabolism?

Mitochondrion Signup and view all the answers

What is one of the key processes the liver performs during metabolism?

Gluconeogenesis Signup and view all the answers

What is the fate of amino acids during prolonged starvation?

They are oxidized for energy production Signup and view all the answers

Which of the following statements is true regarding metabolic pathways in the liver?

Simultaneous anabolic processes occur together Signup and view all the answers

What is the primary energy yield in kJ/g from the oxidation of fat?

37 kJ/g Signup and view all the answers

Which macronutrient can directly contribute to blood glucose levels?

Carbohydrate Signup and view all the answers

What characterizes nonequilibrium reactions in metabolic pathways?

They often have reactants present far from equilibrium concentrations Signup and view all the answers

What role do ketones play during long-term fasting?

They serve as a significant energy substrate for muscle oxidation Signup and view all the answers

Which enzyme is responsible for hydrolyzing glucose-6-phosphate to release glucose into the bloodstream?

Glucose-6-phosphatase Signup and view all the answers

What role does muscle glycogen primarily serve in metabolism?

Source of glucose-6-phosphate for energy Signup and view all the answers

Which substance is required for energy demanding anabolic processes in the liver?

NADPH Signup and view all the answers

During tissue metabolism in stable individuals, what is true about the rates of protein catabolism and anabolism?

They are equal over a 24-hour period Signup and view all the answers

What is the respiratory quotient (RQ) for fat metabolism?

0.71 Signup and view all the answers

What is the primary characteristic of anabolic pathways?

They require reducing equivalents or ATP to support their processes. Signup and view all the answers

What is a significant consequence of the lack of glucose-6-phosphatase in muscle tissue?

Muscle glycogen cannot release glucose to the bloodstream Signup and view all the answers

What is the energy yield of protein catabolism in kJ/g?

17 kJ/g Signup and view all the answers

In what condition would ketogenesis occur as a result of fatty acid metabolism?

Low carbohydrate availability Signup and view all the answers

Which pathway utilizes glucose-6-phosphate primarily for energy production in muscles?

Glycolysis Signup and view all the answers

Which process is primarily associated with catabolic pathways?

Breakdown of larger molecules through oxidative reactions. Signup and view all the answers

What is true regarding the caloric requirements of growing animals?

Growing animals have a higher energy requirement to support growth. Signup and view all the answers

How does the resting or basal metabolic rate contribute to daily energy expenditure?

It contributes to around 60% of daily energy expenditure. Signup and view all the answers

What role do amphibolic pathways play in metabolism?

They act as intermediates in both anabolic and catabolic processes. Signup and view all the answers

What macronutrient typically provides the highest percentage of caloric intake in a human diet?

Carbohydrates (40-60%) Signup and view all the answers

What is the main consequence of protein breakdown exceeding synthesis?

Protein wasting and increased risk of death. Signup and view all the answers

What is the role of hormonal regulation in metabolism?

It can affect both catabolic and anabolic pathways. Signup and view all the answers

What metabolic pathway primarily produces ATP?

Catabolic pathways through oxidative reactions. Signup and view all the answers

How does the composition of one's diet affect energy metabolism?

Diet composition influences the type and amount of fuels oxidized. Signup and view all the answers

Which organ is primarily involved in the synthesis of bile salts and plasma proteins?

Liver Signup and view all the answers

In prolonged starvation, which substrate does the brain primarily utilize for energy?

Ketone bodies Signup and view all the answers

What is the major metabolic pathway occurring in fast twitch muscle fibers?

Anaerobic glycolysis Signup and view all the answers

Which metabolic byproduct is primarily exported from erythrocytes?

Lactate Signup and view all the answers

Which enzyme is a key player in gluconeogenesis in the kidney?

Glycerol kinase Signup and view all the answers

Which statement accurately describes anabolic pathways?

They are endothermic and require reducing equivalents. Signup and view all the answers

What is a key consequence of a caloric excess in metabolism?

Development of abnormal metabolism. Signup and view all the answers

Which macronutrient generally provides the lowest percentage of caloric intake in a human diet?

Proteins Signup and view all the answers

What can be a significant outcome of prolonged protein breakdown exceeding protein synthesis?

Protein wasting and potential emaciation. Signup and view all the answers

What does the term 'amphibolic pathways' refer to in metabolism?

Pathways that can function in both anabolic and catabolic reactions. Signup and view all the answers

How does the energy requirement for metabolic fuels change with the physical activity level of an adult?

Energy requirements increase as activity level increases. Signup and view all the answers

During fasting states, what primary factor influences the variation in the mix of carbohydrates, lipids, and proteins being oxidized?

Duration and intensity of physical work. Signup and view all the answers

In metabolic processes, what primarily differentiates catabolic pathways from anabolic pathways?

Catabolic pathways are exothermic reactions. Signup and view all the answers

What percentage of glucose is typically taken up by the liver in the fed state?

10 to 15% Signup and view all the answers

What is the primary source of long-chain fatty acids aside from dietary intake?

Lipogenesis Signup and view all the answers

Which nutrient is primarily converted to pyruvate via glycolysis?

Glucose Signup and view all the answers

What happens to triacylglycerol from chylomicrons prior to uptake by the liver?

It undergoes hydrolysis by lipoprotein lipase. Signup and view all the answers

During fasting, which metabolic process does the liver primarily switch to?

Production of glucose Signup and view all the answers

What is the process by which glucose is synthesized from glycogen?

Glycogenolysis Signup and view all the answers

What role does glycerol play when released into circulation from hydrolyzed triacylglycerol?

It serves as a substrate for gluconeogenesis. Signup and view all the answers

What is the fate of glucose that is not absorbed by the liver?

It is oxidized by the brain and other tissues. Signup and view all the answers

What is primarily released into circulation when adipose tissue triacylglycerol is hydrolyzed?

Free fatty acids and glycerol Signup and view all the answers

What type of protein structure do enzymes catalyzing nonequilibrium reactions primarily exhibit?

Allosteric proteins Signup and view all the answers

What type of nutrients are carried by chylomicrons aside from triacylglycerol?

Lipid-soluble vitamins Signup and view all the answers

What is the result of the irreversible reaction catalyzed by pyruvate dehydrogenase?

Synthesis of acetyl-CoA Signup and view all the answers

How does positive feedforward allosteric activation function in metabolic regulation?

It promotes synthesis of the enzyme's end product Signup and view all the answers

What significant loss occurs during the entry of two-carbon units from acetyl-CoA into the citric acid cycle?

Two carbon atoms are released as CO2 Signup and view all the answers

What is the role of calmodulin in enzyme activity regulation?

It acts as a calcium-dependent activator Signup and view all the answers

In metabolic reactions, what does negative feedback allosteric inhibition accomplish?

Inhibition of enzyme activity in response to product concentration Signup and view all the answers

Which of the following processes is generally involved in the regulation of enzyme synthesis?

Nuclear production of mRNA Signup and view all the answers

What defines the mechanisms of control in an enzyme-catalyzed reaction?

Functional conformational changes of proteins Signup and view all the answers

What mechanism primarily explains the energy balance in a weight stable individual?

Net zero energy balance through equal anabolism and catabolism Signup and view all the answers

Which metabolic process is directly influenced by the availability of cofactors or cosubstrates?

Enzyme-catalyzed reactions Signup and view all the answers

What type of feedback mechanism allows rapid response to metabolic changes in the cell?

Feed-forward control Signup and view all the answers

After fasting, what is the primary source of glucose production in the human body?

Hepatic glycogen stores through glycogenolysis Signup and view all the answers

What can lead to the development of obesity if caloric intake consistently exceeds energy expenditure?

Storage of excess energy as triacylglycerol Signup and view all the answers

During prolonged fasting, what process provides glucose to glucose-dependent tissues?

Gluconeogenesis from amino acids and lactate Signup and view all the answers

Which statement best describes the role of adipose tissue during fasting?

It releases nonesterified fatty acids for energy. Signup and view all the answers

What is the primary factor that influences the metabolic fate of amino acids during energy deficit?

Conversion to glucose via gluconeogenesis Signup and view all the answers

What occurs to glucose levels in the body after a meal?

Net anabolism as glucose is stored in glycogen Signup and view all the answers

What is mainly synthesized from fatty acids during times of low glucose availability?

Ketone bodies for energy supply Signup and view all the answers

Which process primarily occurs when there is consistent caloric deficiency over time?

Dominance of catabolic pathways over anabolic pathways Signup and view all the answers

What characterizes the transition from anabolism to catabolism after meal intake ceases?

Increased reliance on nonesterified fatty acids Signup and view all the answers

Match the following amino acids with their classification:

Histidine = Essential amino acid Alanine = Nonessential amino acid Lysine = Essential amino acid Glutamate = Nonessential amino acid Signup and view all the answers

Match the following metabolic processes with their definitions:

Transamination = The process of transferring an amino group to form new amino acids Deamination = The removal of an amino group resulting in ammonia Glycolysis = The breakdown of glucose in the cytosol to produce energy Citric Acid Cycle = A series of reactions in the mitochondria generating energy from acetyl-CoA Signup and view all the answers

Match the following substrates with their primary sites of action:

Glucose = Cytosol Acetyl-CoA = Mitochondrion Amino acids = Tissue protein Urea = Liver Signup and view all the answers

Match the following end products with their corresponding metabolic pathways:

Ammonia = Deamination Ketone bodies = Fatty acid metabolism Carbohydrates = Glycolysis Citric acid = Krebs cycle Signup and view all the answers

Match the following metabolic intermediates with their roles:

Glutamate = Amino nitrogen donor in transamination Acetyl-CoA = Key player in the citric acid cycle NH3 = Toxic byproduct of amino acid catabolism CO2 = End product of citric acid cycle Signup and view all the answers

Match the following types of metabolic pathways with their descriptions:

Anabolic pathways = Build larger molecules from smaller precursors Catabolic pathways = Break down larger molecules into smaller units Amphibolic pathways = Participate in both anabolic and catabolic processes Oxidative reactions = Involve the transfer of electrons and can release energy Signup and view all the answers

Match the following macronutrients with their percentage contribution to the human diet:

Carbohydrates = 40-60% Lipids = 30-40% Proteins = 10-15% Alcohol = Variable content Signup and view all the answers

Match the following factors with their impact on metabolic requirements:

Physical activity = Increases caloric requirements Growing animals = Have higher energy needs relative to size Fasting state = Alters energy source utilization Basal metabolic rate = Accounts for ~60% of daily energy expenditure Signup and view all the answers

Match the following metabolic outcomes with their associated triggers:

Protein wasting = Excessive protein breakdown Energy cost of growth = Requires higher caloric intake Exothermic reactions = Release energy during catabolism Endothermic pathways = Require energy input for synthesis Signup and view all the answers

Match the following statements with the appropriate metabolic process:

Triacylglycerol synthesis = Occurs from carbohydrates in anabolic pathways Larger molecules breakdown = Primarily occurs in catabolic pathways Net protein breakdown = Must be balanced by protein synthesis Daily metabolism = Requires constant intake of metabolic fuels Signup and view all the answers

Study Notes

Metabolism Overview

Metabolism encompasses the interconversion of chemical compounds within the body.
It involves individual molecule pathways within a cell, interrelationships between cells within an organ, and between organs, and the regulation of metabolites.
Metabolic pathways are categorized as anabolic, catabolic, and amphibolic.
Anabolic pathways synthesize large molecules from smaller precursors, requiring energy.
Catabolic pathways break down large molecules into smaller ones, releasing energy.
Amphibolic pathways participate in both anabolic and catabolic functions.

Metabolic Fuels: Major Dietary Products

Dietary composition dictates the general plasma amino acid levels.
Muscle tissue accounts for about half of body mass.
Muscle is a significant protein store and can provide amino acids for gluconeogenesis.
Ketones can contribute significantly to muscle substrate oxidation during prolonged fasting.

Subcellular Compartmentation of Metabolism

Compartmentation within organelles allows for integration and regulation of metabolic pathways.
The Liver performs many anabolic processes, such as gluconeogenesis, lipogenesis, and VLDL synthesis, all requiring energy.
The mitochondrion plays a crucial role as the center of carbohydrate, lipid, and amino acid metabolism.

Non-Equilibrium Reactions: Regulating Metabolic Flux

In equilibrium, forward and reverse reactions occur at equal rates, resulting in no net flux.
In vivo, there is a net flux due to the continuous supply of substrate and removal of product.
The pathway is in a steady state when reaction rates and substrate, product, and intermediate concentrations are constant.
Metabolic pathways often have non-equilibrium reactions to regulate metabolite flux.

Energy Yields, Oxygen Consumption & Carbon Dioxide Production

Different metabolic fuels have varying energy yields, oxygen consumption, and carbon dioxide production.
Carbohydrates provide ~16 kJ/g energy, consume 0.829 L/g O2, and produce 0.829 L/g CO2.
Proteins provide ~ 17kJ/g energy, consume 0.966 L/g O2, and produce 0.782 L/g CO2.
Fats provide ~37 kJ/g energy, consume 2.016 L/g O2, and produce 1.427 L/g CO2.

Glucose Metabolism

Glucose not taken up by the liver is used by peripheral tissues for oxidation or storage.
Glucose is stored as glycogen in muscle and as triacylglycerol in adipose tissue.
Muscle glycogen cannot be directly converted to plasma glucose due to the absence of glucose-6-phosphatase in muscle.

Fasting Overview

During fasting, the body mobilizes stored energy reserves.
The liver plays a central role in maintaining blood glucose levels.
Glycogen stores in the liver are depleted within 12-18 hours.
Gluconeogenesis becomes the primary source of glucose.
Amino acids from muscle breakdown and glycerol from lipolysis are used for gluconeogenesis.
Ketones are produced from fatty acid metabolism and become an important energy source.
Net protein breakdown occurs during prolonged fasting, leading to muscle wasting.

Anabolic and Catabolic Pathways

Complex molecules are built from smaller precursors, for example, protein from amino acids, triacylglycerol and glycogen from carbohydrates.
Anabolic pathways are endothermic and require reducing equivalents or ATP to fuel them
Catabolic pathways break down larger molecules, often involving oxidative reactions
Catabolic pathways are exothermic and produce reducing equivalents and ATP
Amphibolic pathways are involved in reaction types that involve both anabolic and catabolic pathways

Human Energy Requirements

A 70-kg adult requires 8-12 MJ (1920-2900kcal) per day, depending on physical activity.
Caloric needs increase with the size of the animal.
Growing animals have proportionally higher needs to support growth.
Humans obtain their caloric requirements from:
- Carbohydrates (40-60%)
- Lipids (mainly triacylglycerol, 30-40%)
- Protein (10-15%)
- Alcohol

Metabolism in the Fed State

The liver is the main point of uptake for glucose and amino acids
In the fed state, approximately 10-15% of glucose is taken up by the liver.
The majority of glucose is used to synthesize glycogen (glycogenesis).
A small fraction of glucose is used to synthesize fatty acids (lipogenesis).
Excess glucose is broken down via glycolysis to produce pyruvate for oxidation in the citric acid cycle.

Metabolism in the Fasted State

The liver becomes the primary producer of glucose.
Glucose is produced from stored glycogen (glycogenolysis) and newly synthesized glucose (gluconeogenesis).
The central nervous system and red blood cells are largely dependent on glucose.
Adipose tissue releases nonesterified fatty acids through the hydrolysis of stored triglycerides
Many tissues use fatty acids as fuel.
The liver synthesizes ketone bodies from fatty acids for use by muscle and other tissues.

Protein Turnover

During fasting, the liver mobilizes amino acids from protein breakdown.
Persistent protein breakdown can lead to protein wasting, emaciation, and death.
This is due to a lack of amino acids for protein synthesis.

Regulation of Metabolism

The availability of substrates, cosubstrates, and cofactors influences metabolic pathways
Enzymes catalyzing nonequilibrium reactions are often allosteric, responding quickly to changes in cellular needs.
The end product of a pathway can often provide feedback to regulate the activity of enzymes.

Major Metabolic Features of Organs

The liver is a central organ in metabolism, participating in glycolysis, gluconeogenesis, lipogenesis, beta-oxidation, the citric acid cycle, ketogenesis, lipoprotein metabolism, drug detoxification, and synthesis of bile salts, urea, uric acid, cholesterol, and plasma proteins.
The brain depends on glucose, amino acids, and ketone bodies for fuel, and is involved in amino acid metabolism and neurotransmitter synthesis. It produces lactate and waste products from neurotransmitter metabolism.
The heart uses fatty acids, ketone bodies, lactate, and chylomicron and VLDL triacylglycerol for fuel and is involved in the citric acid cycle and electron transport.
Adipose tissue stores triacylglycerol and releases fatty acids and glycerol during fasting through the processes of lipolysis and esterification.
Fast twitch muscle uses glucose and glycogen as fuel, producing lactate and amino acids during fasting.
Slow twitch muscle uses fatty acids, ketone bodies, chylomicron, and VLDL triacylglycerol as fuel, participating in the citric acid cycle and electron transport.
The kidney is involved in gluconeogenesis, using fatty acids, lactate, glycerol, and glucose as substrates to produce glucose.
Erythrocytes utilize glucose for anaerobic glycolysis and the pentose phosphate pathway, producing lactate as a byproduct.

Key Points

The body constantly cycles through anabolic and catabolic processes.
The liver plays a key role in maintaining glucose homeostasis in both fed and fasted states.
Dietary intake of carbohydrates, lipids, and proteins impacts metabolic processes.
Metabolic pathways are highly regulated by a complex interplay of factors.
Different organs have specialized metabolic functions that contribute to overall bodily homeostasis.

Essential Amino Acids

Cannot be synthesized by the body and must be supplied in the diet.
Examples include: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Nonessential Amino Acids

Can be synthesized by the body from metabolic intermediates using transamination.
Examples include: alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine.

Overview of Metabolism

Metabolic fuels are required for cell function and growth.
The resting or basal metabolic rate accounts for ~60% of daily energy expenditure in humans.

Metabolic Fuels

The human body obtains its fuel from three primary sources:
- Carbohydrates (40-60% of daily intake)
- Lipids (mainly triacylglycerol, 30-40% of daily intake)
- Protein (10-15% of daily intake)

Pathways of Metabolism

Anabolic Pathways:
- Involve the synthesis of complex molecules from simpler precursors.
- Are endothermic (require energy) and require reducing equivalents or ATP.
- Examples include protein synthesis from amino acids, and triacylglycerol and glycogen synthesis from carbohydrates.
Catabolic Pathways:
- Involve the breakdown of larger molecules.
- Commonly involve oxidative reactions.
- Are exothermic (release energy) and produce reducing equivalents and ATP.
- Examples include glycolysis, beta-oxidation of fatty acids.
Amphibolic Pathways:
- Can function in both anabolic and catabolic pathways.
- Examples include the citric acid cycle, which can be used to break down carbohydrates, lipids, and proteins.