Overview of intermediary metabolism

What are the 3 main fuels used in intermediary metabolism?

Carbohydrates, lipids, and proteins

What processes use ATP?

Biosynthesis, detoxification, muscle contraction, active ion transport, thermogenesis

What does ATP release?

ATP → ADP + Pi + Heat + CO2 (energy release)

What processes govern fuel utilisation in carbohydrates? How is ATP produced?

Carbohydrates - oxidised : glycolysis → TCA → oxidative phosphorylation

What processes govern fuel utilisation in lipids? How is ATP produced?

Lipids: B-oxidation → Acetyl-CoA → TCA

What processes govern fuel utilisation in proteins? How is ATP produced?

Proteins : amino acids → transamination → TCA intermediates

Why is metabolism called “controlled combustion”?

Because enzymes control oxidation reactions to release energy gradually instead of as heat or light

Why the need for different fuels?

Different cells use different fuels (depending on their structure, function and metabolic capacity) for our everyday processes

What fuel does erythrocytes use exclusively?

Glucose initially phosphorylated by hexokinase then pyruvate is converted via anaerobic glycolysis to lactate and leaves cell with H+ ion = small ATP

How can lactate produced by RBC be converted back to glucose?

Other tissues like liver convert it back to glucose through Cori cycle - through gluconeogenesis

Why can’t erythrocytes not generate ATP?

Have no mitochondria as they have more room for haemoglobin and maintains flexibility to access capillaries

How much energy does the brain need per day?

20% of total body’s requirements (120g of glucose per day)

What is the brain’s preferred energy source? And why?

Glucose - due to blood brain barrier

What alternate fuel can the brain use during fasting?

Ketone bodies (increases when glucose availability is reduced)

How does the brain utilise glucose for ATP?

Main pathway is anaerobic glycolysis to pyruvate

Pyruvate dehydrogenase complex (PDC) removes a carbon (pure ate decarboxylation) and converts pyruvate to acetyl-CoA

Enters TCA cycle - full oxidation and ATP

What fuels do muscles use?

Glucose (when available in abundance), fatty acids (if they increase in availability), sometimes amino acids

How does muscle utilise glucose for ATP?

Aerobic glycolysis to pyruvate

Pyruvate decarboxylation to acetyl CoA

Full oxidation in TCA (breaks down to CO and ATP, NADH, FADH2)

How much energy does muscle need per day?

25-30% daily energy expenditure

What are the 3 key glucose pathways in the liver?

Glycogenesis, glycolysis, and pentose phosphate pathway

What is the liver’s role?

Helps maintain blood glucose - by storing when there’s too much or releasing if it’s low

What happens when glucose enters livers cells?

Phosphorylated to form glucose-6-phosphate (G6P) - serves as a substrate for multiple pathways

What happens in glycogenesis pathway of G6P?

G6P is converted to glycogen (stored form of glucose in liver

Store excess glucose to prevent high blood sugar

What happens in glycolysis pathway of G6P?

G6P → pyruvate → acetyl-CoA

Acetyl-CoA can be used to make: fatty acids and cholesterol and thus dispose of excess glucose by converting it into fats and cholesterol for storage

What happens in pentose phosphate pathway of G6P?

G6P → ribose-5-phosphate + NADPH

Ribose 5 P is used to make nucleotides (DNA/RNA building blocks) and NADPH to provide reducing power for lipogenesis (fat synthesis)

Purpose of Pentose phosphate pathway?

To generate building blocks for cells and help in making fatty acids

What is the normal blood glucose concentration in humans?

4.5 and 5.5 mmol/L

What is the blood glucose concentration when fasting?

2.8-7.0 mmol/L

Why do glucose levels rise when fasting?

Body releases glucose for energy to maintain bodily functions - regulated by hormones like glucagon

What happens to your blood glucose and fuel after a meal (post-prandial)?

Concentration will increase to the upper end of the range and may go slightly above it

This will promote the storage of any fuel that exceeds immediate demand

Why can’t all glucose remain in the blood?

Excess glucose increases osmolarity and causes cellular damage (glycation)

Needs to be used and stored for later use

What happens when there is too much glucose in blood?

Blood becomes hypertonic compared to fluid inside cells

Water leaves cells (osmosis - low to high) → cells shrink (crenate) → causes cellular dehydration → harder for cells to function properly

How is high blood glucose prevented?

Storage of excess glucose in form of glycogen to keep blood sugar and water balance stable (isotonic)

Isotonic

Balanced = no net movement = normal

Hypertonic

Higher solute concentration than cell = water moves out of cell and shrinks

Hypotonic

Lower solute concentrations than cell = water moves in and swells

What is glycation?

Non-enzymatic process where glucose solution or molecule attaches to a protein, lipid, or nice lid acid

What happens to proteins in glycation?

It changes protein structure - might lose its normal function and stops them being degraded properly from cell’s systems so they accumulate

What do glycated proteins form?

Advanced Glycation End-products (AGEs) - sticky, cross linked molecules that build up in tissues

What do AGEs cause?

Implicated in disease processes - diabetic complications, chronic inflammation, aging-related tissue stiffness, atherosclerosis

Glycogenesis

Synthesising glycogen from glucose for storage in liver and muscles

Glycolysis

Metabolic pathway that breaks down a glucose molecule into 2 pyruvate = ATP and NADH

Glycogenolysis

Metabolic process of breaking down stored glycogen into glucose (provides body with energy)

What happens in the liver after a meal rich in carbohydrates?

1. Blood glucose increases → insulin released

2. Glucose enters liver cells via transport proteins

3. Glucokinase phosphorylates glucose → glucose-6-phosphate

4. Glucose-6-phosphate → glycogen (via glycogen synthase + branching enzyme)

5. Glycogen is stored in liver and muscle for later use

How does glycogen serve as an energy reserve in the liver?

Helps maintain blood glucose levels between meals

How does glycogen serve as an energy reserve in muscle?

Provides energy for muscle contraction during activity

How does the liver release glucose during fasting?

1. Low blood sugar → glucagon is released

2. Glucagon activates glycogen phosphorylase and de-branching enzyme → glycogen breaks down

3. Glucose-6-phosphatase converts G6P → glucose

4. Glucose exits liver cells into blood to maintain normal blood sugar

What does glycolysis produce?

Convert 1 molecule of glucose (6 carbons) into 2 molecules of pyruvic acid (3 carbons each), producing ATP (energy) and NADH

Where is hexokinase found in?

Most tissues - brain, heart, muscle, adipose

Where is glucokinase?

In liver and pancreas

What happens during glycolysis?

1. Glucose → glucose-6-phosphate

2. Glucose-6-phosphate → fructose-6-phosphate

3. Fructose-6-phosphate → fructose-1,6-biphosphate

4. Fructose-1,6-biphosphate → DHAP + G3P

5. G3P → PEP

6. PEP → pyruvic acid

How much energy net gain is there from 1 glucose molecule?

2 ATP (4 made, 2 used early), 2 NADH, 2 pyruvic acid

What 3 enzymes are importantly for glycolysis?

Hexokinase or glucokinase, phosphofructokinase (PFK), pyruvate kinase (PK)

What do the enzymes do in glycolysis?

Unidirectional

Catalyse non-reversible reactions

Subject to regulation

Control pathway direction

Gluconeogenesis

Synthesises glucose from non-carbohydrate sources like lactate, glycerol, amino acids from liver and kidneys - (when dietary carbs are unavailable) during fasting

Glycolysis

Breaks down glucose for energy

What are key enzymes in gluconeogenesis?

Glucose-6-phosphatase, fructose-1,6-biphosphatase, pyruvate carboxylase/PEPCK

What is an allosteric enzyme?

Can change it’s activity by binding an “effector” molecule to a site other than the active site, known as allosteric site

What does allostery achieve in metabolism?

Adjusts enzyme activity according to cellular energy levels

What are activators of phosphofructokinase?

AMP, Pi, fructose 1,6-biphsophate, fructose 6-phosphate

What do activators of phosphofructokinase do?

Speeds up glycolytic rate

What are inhibitors of phosphofructokinase?

ATP, phosphocreatine, citrate, phosphoenolpyruvate, hydrogen ions

What do inhibitors of phosphofructokinase do?

Slows glycolysis rate down

What are allosteric modulators?

Molecules that bind to an enzyme at a site other than its active site (called allosteric site) and change its activity

What happens when an allosteric modulator binds to an enzyme?

It changes the enzyme’s shape, which can either activate or inhibit the enzyme’s function

What is the main purpose of allosteric regulation in metabolism?

To control the speed of metabolic pathways (like glycolysis) according to the cell’s energy needs

What does “energy status of the cell” refer to?

The balance between ATP (energy-rich) and ADP/AMP (energy-poor) molecules inside the cell

Which molecules indicate high energy status in a cell?

High ATP concentration and low ADP/AMP levels

Which molecules indicate low energy status in a cell?

High ADP or AMP concentration and low ATP levels

what are the free fatty acids (FFA) known as?

Non-esterified fatty acids (NEFA)

How are free fatty acids transported in blood?

Travel in aqueous blood plasma bound to albumin

How are fatty acids stored?

Triacylglycerol or triglycerides

What proprieties do fatty acids have?

Amphipathic properties - charged nature makes it hydrophilic (or polar)

What is a fatty acid composed of?

Carboxylic head, methyl tail

What are triacyglycerols?

Storage form of lipids made of glycerol (as a ‘backbone’) + 3 fatty acids - from condensation

What are the two sides of lipid liver metabolism?

Lipogenesis and lipolysis + b-oxidation

Lipogenesis

Fat synthesis from acetyl-CoA - builds up fat stores (when energy and glucose are abundant) mainly in liver and adipose tissue

Lipolysis and B-oxidation

Fat breakdown of stored triglycerides into fatty acids and glycerol through hydrolysis for energy - happens where energy is needed or glucose is low

What happens during lipogenesis?

1. Glucose enters liver via GLUT2 transporters

2. Glycolysis converts glucose → pyruvate → acetyl-CoA (in mitochondria)

3. Acetyl-CoA moves into cytosol from mitochondria to form malonyl-CoA (by acetyl-CoA carboxylase enzyme)

4. Malonyl-CoA extends a growing fatty acid chain using fatty acid synthase

5. Fatty acids + glycerol-3-phosphate (esterification - combined to form)→ triaglycerol (TAG) - store fat

6. TAGs are packaged into VLDL (very low density lipoproteins) and sent into blood to deliver fat to adipose tissue as energy reserve for future use

What does insulin activate in lipogenesis?

Acetyl-CoA carboxylase - for increased malonyl-CoA for more fatty acid synthesis

Fatty acid synthase (FAS) - makes new fatty acids

Glucose uptake and glycolysis → provides acetyl-CoA and NADPH

What happens during lipolysis and b-oxidation?

1. Glucagon activated enzymes that break down stored fats

2. Fatty acids enter mitochondria for b-oxidation: CPT-1 allows long chain fatty acids to cross into mitochondria and fatty acids are converted → acetyl-CoA

3. Acetyl-CoA can: enter TCA cycle for ATP production or be used to form ketone bodies when carb availability is low

Which processes are influenced by insulin?

Glycolysis, glycogenesis, lipogenesis, protein synthesis

What processes are influenced by glucagon?

Gluconeogenesis, glycogenolysis, lipolysis, protein breakdown