Integration of metabolism

Question 1

Examples of tissues

Answer 1

Skeletal muscle

Brain

Heart

Liver

Question 2

Energy intake needs to be tightly coordinated with…

Answer 2

Energy expenditure

Question 3

Metabolic features of tissues - brain

Answer 3

• The brain requires a continuous supply of glucose, the brain cannot metabolise fatty acids ( not good at metabolising other molecules, bad in times like fasting)
• ketone bodies (e.g. β-hydroxybutyrate) can partially substitute for glucose
• Too little glucose (hypoglycaemia) causes faintness and coma
• Too much glucose (hyperglycaemia) can cause irreversible damage

Question 4

Metabolic features of skeletal muscle

Answer 4

• Accounts for 40% of total body weight
• ATP requirements vary depending on exercise undertaken
• Light contraction – requirements met by OxPhos, O, gluc and fatty acids used as energy source.
• Vigorous contraction - O2 becomes a limiting factor
• glygogen breakdown (muscles)
• lactate formation

Question 5

Metabolic features of heart

Answer 5

• The heart must beat constantly.
• It is designed for completely aerobic metabolism, and is rich in mitochondria.
• The heart utilises TCA cycle substrates, e.g. free fatty acids, ketone bodies
• Loss of O2 supply to the heart is devastating
• Leads to cell death and myocardial infarction

(energy demand >>> energy supply)

Question 6

Metabolic features of liver

Answer 6

• Undertakes a wide repertoire of metabolic processes: eg glycolysis, transamination and gluconeogenesis.
• Is highly metabolically active
• Can interconvert nutrient types
• plays a central role in maintaining blood [glucose] at 4.0-5.5 mM
• is a glucose storage organ (glycogen)
• plays a key role in lipoprotein metabolism
• (transport of triglycerides & cholesterol)

Question 7

Metabolic features of muscle

Answer 7

(40 % of total body weight)

Can have periods of very high ATP requirement during vigorous contraction

Relies upon carbohydrate and fatty acid oxidation

Question 8

Metabolic features of brain and nervous tissue

Answer 8

(2 % of total body weight)

Uses 20 % of resting metabolic rate as it has a continuous high ATP requirement

Cannot utilise fatty acids as a fuel source

Question 9

Metabolic features of adipose tissue

Answer 9

(15 % of total body weight)

Long term storage site for fatty acids in the form of triglycerides.

Question 10

Metabolic features of heart

Answer 10

(1 % of total body weight)

10 % of resting metabolic rate

Can oxidise fatty acids and carbohydrate

Question 11

Metabolic features of liver

Answer 11

(2.5 % of total body weight)

20 % of resting metabolic rate; the body’s main carbohydrate store (glycogen)

Source of blood glucose.

Question 12

Carbohydrate metabolism

Answer 12

• Carbohydrates are broken down into simple sugars, enter glycolytic pathway leading to the production of pyruvate.
• Decarboxylation and reduction of pyruvate produces acetyl CoA which can enter the TCA cycle. This cycle produces reduced co-factors which are reoxidised by the electron transport chain which in turn is coupled to ATP production (Oxidative phosphorylation).
• Excess glucose-6-phosphate can be used to generate glycogen in liver and muscle (red arrow). Excess Acetyl CoA can be used to generate fatty acids, which are stored as triglycerides in adipose tissue (red arrow).
• During extreme exercise, the ATP demands of the muscle outstrip the oxygen supply needed for aerobic respiration and lactate is produced (blue arrow).
• During fasting, rather than enter the TCA, much of the acetyl CoA produced results in ketone body production (purple arrow).
• Pyruvate and other TCA cycle intermediates can also be a source of some amino acids. The backbone of these molecules can be used to used to make nucleotides.
• Glucose-6-phosphate via the pentose phosphate pathway can also be used as a source for nucleotide production in a pathway that generates the bulk of the NADPH needed for anabolic pathways e.g. cholesterol synthesis.

Question 13

When does hypoglycaemia occur and how do we avoid it?

Answer 13

During fasting, if plasma glucose concentrations fall below 3mM.

In the short term, to avoid hypoglycaemia the body can:

• breakdown of liver glycogen stores occurs to maintain plasma glucose levels.
• releases free fatty acids from adipose tissue.
• convert Acetyl CoA into ketone bodies via the liver.

Question 14

Why do we need to generate glucose

Answer 14

Both fatty acids and ketone bodies can be used by muscle, making more of the plasma glucose available for the brain. However, within 12-18 hr all glycogen stores are typically exhausted, hence the need for another pathway to generate glucose – gluconeogenesis.



Question 15

General strategy for gluconeogenesis

Answer 15

• The overall aim of pathway is to generate glucose from pyruvate.
• Non-carbohydrate precursors enter gluconeogenesis pathway at the points shown (green arrows), namely lactate, amino acids and glycerol.
• Lactate is generated by skeletal muscle during strenuous exercise, when rate of glycolysis exceeds the rate of the TCA cycle and the electron transport chain. Lactate can be taken up by the liver and utilised to regenerate pyruvate by lactate dehydrogenase (LDH), also known as the Cori cycle.
• Amino acids can be derived from the diet or during times of starvation, e.g. from the breakdown of skeletal muscle.
• Triglyceride hydrolysis yields fatty acids and glycerol, the glycerol backbone being used to generate dihydroxyyacetone phosphate (DHAP).
• The red arrows denote key steps which must be bypassed by non-glycolytic enzymes.

Question 16

Bypass reactions of gluconeogenesis

Answer 16

Glycolysis has three essentially irreversible reactions, catalysed by the kinases hexokinase, phosphofructokinase and pyruvate kinase. Gluconeogenesis is therefore not a simple reversal of glycolysis, as these three reactions have to be bypassed.

Four additional enzymes needed for gluconeogenesis.

The first reaction catalysed by pyruvate carboxylase occurs in the mitrochondria, whereas the remaining reactions are cytosolic.

The four “additional” high energy bonds are required to turn an energetically unfavourable process into an energetically favourable one:

ΔG for the straight reversal of glycolysis would be +90 kJ/mol which is energetically unfavourable.

ΔG for gluconeogenesis is -38 kJ/mol.

Question 17

Proteins and fats as fuel sources

Answer 17

Deamination of all 20 aa - just seven molecules, namely pyruvate, acetyl CoA, acetoacetyl CoA, α-ketoglutarate, succinyl CoA, fumerate and oxaloacetate. Urea is lost as a waste product.

The glucogenic amino acids are so-called because skeletons can give rise to glucose via gluconeogenesis (dashed line). Ketogenic amino acids give rise to skeletons which cannot enter gluconeogenesis but can be used to synthesis fatty acids and ketone bodies.

Triglycerides are broken down into fatty acids and glycerol. Glycerol can be converted to DHAP and enter the gluconeogenic pathway upstream.

Fatty acids cannot be converted into glucose by gluconeogenesis. 2C atoms enter the TCA cycle as acetyl CoA by combining with oxaloacetate to form citrate. As the cycle progresses, two C atoms are sequentially lost as CO2 before oxaloacetate is eventually regenerated. Hence, no net synthesis of oxaloacetate or pyruvate is possible from acetyl CoA .

Fatty acids can be converted into ketone bodies and used by tissues such as muscle and brain.

Question 18

Protein as fuel source diagram

Answer 18

Question 19

Fat as fuel source diagram

Answer 19

Question 20

Energy stores and energy consumption - Aerobic respiration

Answer 20

• During moderate levels of exercise, where oxygen supply is adequate, the ATP demands of muscle can be met by oxidative phosphorylation using glucose and other substrates as fuels.
• Glucose is transported from the blood into muscle cells where it can undergo metabolism by glycolysis and the TCA cycle to ultimately generate ATP by the re-oxidation of cofactors.
• As muscle contracts, the demand for ATP increases e.g. requirements of muscle actomyosin ATPase and cation balance. Increased demand for glucose is met by an increase in the number of glucose transporters on the membranes of muscle cells.
• Adrenalin plays a key role in meeting the demand for ATP by increasing the rate of glycolysis in muscle, increasing the rate of gluconeogensis by the liver (red dashed arrows) and increasing the release of fatty acids from adipocytes.

Question 21

Energy stores and energy consumption - Anaerobic respiration :

Answer 21

• Under anaerobic conditions, the demands of the contracting muscle for ATP cannot be met by oxidative phosphorylation and similarly, the transport of glucose from the blood cannot keep up with the demands of glycolysis.



• Glycogen within the muscle is therefore broken down to meet these demands. To replenish NAD+ levels and maintain glycolysis, pyruvate is taken up by the liver and converted into lactate by lactate dehydrogenase. Lactate can then be used by the liver to generate glucose by gluconeogenesis.

Question 22

Control of metabolic pathways:

Answer 22

Control of metabolic pathways is typically centred around reactions that are irreversible steps. At these points, increases in the rate of enzyme activity greatly increases the rate of the downstream steps. For the greatest levels of control it is desirable that these control steps are reasonably early in the pathway.

Control can be at several levels including:



• product inhibition
• under the influence of signalling molecules such as hormones

Question 23

Glucose metabolism in liver and muscle

Answer 23

• Blood glucose concentrations are typically maintained at around 4mM. Following a meal, blood glucose levels will rise. Hexokinase catalyses the first irreversible step in the glycolysis pathway.
• Muscle and liver contain suitably different forms (isoforms) of this enzyme. Both isoforms catalyse the same reaction. However they are maximally active at different concentrations of glucose. We can compare the relative activities of enzymes by using parameters such as the Michaelis constant (KM) which is the concentration of substrate at which an enzyme functions at a half-maximal rate (Vmax).
• The KM of Hexokinase I found in muscle is 0.1mM (left hand graph) which means it is active at low concentrations of glucose and is essentially operating at maximal velocity at all times. Hexokinase I is also highly sensitive to inhibition by the product glucose-6-phosphate. This means that under anaerobic conditions when the rate of the TCA cycle drops, and glycolysis therefore slows, Hexokinase I is inhibited by accumulating levels of glucose-6-phosphate.
• Hexokinase IV found in liver behaves a little differently, having a much higher KM of around 4mM and therefore is much less sensitive to blood glucose concentrations (right hand graph). It is also less sensitive to the inhibitory effects of glucose-6-phosphate (G-6-P).
• Glucose 6-phosphatase (found in the liver but not in muscle) can catalyse the reverse reaction to hexokinase, generating glucose from glucose-6-phosphate.

Question 24

Hormonal control of blood glucose levels:

Answer 24

• Insulin ; secreted when glucose levels rise: it stimulates uptake and use of glucose and storage as glycogen and fat.
• Glucagon ; secreted when glucose levels fall: it stimulates production of glucose by gluconeogenesis and breakdown of glycogen and fat.
• (both are secreted by islets of the pancreas)
• Adrenalin (or epinephrine): strong and fast metabolic effects to mobilise glucose for “flight or fight”.
• Glucocorticoids: steroid hormones which increase synthesis of metabolic enzymes concerned with glucose availability.

Question 25

On having a meal, blood glucose levels initially rise which is controlled by increased secretion of…..

This has several effects including…

Answer 25

1. insulin (and reduced glucagon) from islets.
2. increased glucose uptake by liver – used for glycogen synthesis and glycolysis (acetyl-CoA produced is used for fatty acid synthesis).

increased glucose uptake and glycogen synthesis in muscle.

increased triglyceride synthesis in adipose tissue.

increased usage of metabolic intermediates due to a general stimulatory effect on the body’s synthesis and growth.

Question 26

After a meal blood glucose levels start to fall and are controlled by:

Answer 26

• increased glucagon secretion (and reduced insulin) from islets.
• glucose production in liver resulting from glycogen breakdown and gluconeogenesis.
• utilisation of fatty acid breakdown as alternative substrate for ATP production (important for preserving glucose for brain).
• [NB adrenalin has similar effects on liver, but also stimulates skeletal muscle towards glycogen breakdown and glycolysis, and adipose tissue towards fat lipolysis to provide other tissues with alternative substrate to glucose]

Question 27

After prolonged fasting (i.e. longer than can be covered by glycogen reserves):

Answer 27

• glucagon/insulin ratio increases further
• adipose tissue begins to hydrolyse triglyceride to provide fatty acids for metabolism
• TCA cycle intermediates are reduced in amount to provide substrate for gluconeogenesis
• protein breakdown provides amino acid substrates for gluconeogenesis
• ketone bodies are produced from fatty acids and amino acids in liver to substitute partially the brain’s requirement for glucose

Question 28

Diabetes mellitus:

Answer 28

Diabetes mellitus is a disorder of insulin release and signalling, resulting in an impaired ability to regulate blood glucose concentrations.



There are two main types of diabetes mellitus:



Type I diabetes in which individuals fail to secrete enough insulin (β-cell dysfunction).

Type II diabetes in which individuals fail to respond appropriately to insulin levels (insulin resistance).



The overall effect is that metabolism is controlled as if the person is undergoing starvation, regardless of dietary glucose uptake.

Question 29

Complications of diabetes include:

Answer 29

• hyperglycaemia with progressive tissue damage (e.g. retina, kidney, peripheral nerves)
• increase in plasma fatty acids and lipoprotein levels with possible cardiovascular complications
• increase in ketone bodies with the risk of acidosis
• hypoglycaemia with consequent coma if insulin dosage is imperfectly controlled

Question 30

Glucagon is important in protection against hypoglycaemia.

Answer 30

• A major site of action is the liver where glucagon stimulates gluconeogenesis and glycogenolysis.
• Insulin deficiency and relative excess of glucagon leads to increased hepatic output of glucose and, thus, hyperglycaemia.