S4) Energy Storage Flashcards Preview

(LUSUMA) Metabolism, Endocrinology & Haematology > S4) Energy Storage > Flashcards

Flashcards in S4) Energy Storage Deck (34)
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1
Q

Which tissues have an absolute requirement for glucose as an energy source?

A
  • Red blood cells
  • Neutrophils
  • Innermost cells of kidney medulla
  • Lens of the eye
2
Q

Why are stable blood glucose levels important?

How is this done?

A
  • Essential for normal brain function
  • Glycogen stores glucose to enable blood glucose to be kept at required levels
3
Q

How is glycogen stored?

A

Glycogen is stored as granules

4
Q

Where is glycogen stored?

A
  • Muscle glycogen – stored as granules in and between myofibrils
  • Liver glycogen – stored as granules in hepatocyte
5
Q

Describe the molecular structure of glycogen

A
  • Glycogen is a polymer consisting of chains of glucose residues
  • Glucose residues linked by α-1-4 glycosidic bonds with α-1-6 glycosidic bonds
6
Q

Glycogenesis is the process of synthesising glycogen. It requires energy.

In four steps, describe this process

A

⇒ Glucose + ATP → Glucose 6-P + ADP (hexokinase/glucokinase)

⇒ Glucose 6-P → Glucose 1-P (phosphoglucomutase)

⇒ Glucose 1-P + UTP + H2O → UDP-Glucose + PPi

⇒ Glycogen(n residues) + UDP-Glucose → Glycogen(n + residues) + UDP (glycogen synthase / branching enzyme)

7
Q

Glycogenolysis is the process of glycogen degradation.

In three steps, describe how this process is not a simple reversal of glycogenesis

A

⇒ Glycogen(n residues) + Pi → Glucose 1-P + Glycogen(n-1 residues) (glycogen phosphorylase / debranching enzyme)

⇒ Glucose 1-P → Glucose 6-P (phosphoglucomutase)

⇒ Glucose 6-P → released into blood as glucose (liver) / used for glycolysis (muscle)

8
Q

Illustrate and explain how glycogen stores serve different functions in liver and muscle

A
  • Liver: G6P converted to glucose and exported to blood (buffers blood glucose levels)
  • Muscle: G6P enters glycolysis for energy production (no glucose-6-phosphatase)
9
Q

Illustrate the relationship between glycogen synthesis and degradation

A
10
Q

What are the rate limiting enzymes for glycogen metabolism?

A
  • Glycogen synthesis – glycogen synthase
  • Glycogen degradation – glycogen phosphorylase
11
Q

Illustrate how the hormonal regulation of glucose metabolism occurs in a reciprocal fashion

A
12
Q

Explain how the hormonal regulation of muscle glycogen stores differs from that of the liver

A
  • Glucagon has no effect on muscle glycogen stores
  • AMP is an allosteric activator of muscle glycogen phosphorylase but not of the liver form of enzyme
13
Q

Glycogen storage diseases are inborn errors of metabolism (inherited diseases).

How do they occur?

A

Glycogen storage disease arise from deficiency or dysfunction of enzymes of glycogen metabolism

14
Q

Describe three consequences of glycogen storage diseases

A
  • Liver and/or muscle can be affected
  • Excess glycogen storage can lead to tissue damage
  • Diminished glycogen stores can lead to hypoglycaemia & poor exercise tolerance
15
Q

Identify and describe two examples of glycogen storage diseases

A
  • McArdle disease – muscle glycogen phosphorylase deficiency
  • von Gierke’s disease – glucose-6-phosphatase deficiency
16
Q

What is gluconeogenesis and when does it occur?

A
  • Gluconeogenesis is the production of new glucose
  • Occurs beyond ~ 8 hours of fasting as liver glycogen stores start to deplete and an alternative source of glucose is require
17
Q

Where does gluconeogenesis occur?

A

Occurs in liver and to lesser extent in kidney cortex

18
Q

Identify and describe the three major precursors for gluconeogenesis

A
  • Lactate – from anaerobic glycolysis in exercising muscle and red blood cells (Cori cycle)
  • Glycerol – released from adipose tissue breakdown of triglycerides
  • Amino acids – mainly alanine
19
Q

Describe the Cori Cycle

A
20
Q

Why can glucose not be synthesised from acetyl-CoA?

A

Acetyl-CoA cannot be converted into pyruvate as PDH reaction is irreversible so there is no net synthesis of glucose from acetyl-CoA

21
Q

Identify the three key enzymes in gluconeogenesis

A

Steps 1 & 2 are the major control sites of pathway

22
Q

Which enzymes are used in the hormonal regulation of gluconeogenesis?

A
  • Phosphoenolpyruvate carboxykinase (PEPCK)
  • Fructose 1,6-bisphosphatase
23
Q

Gluconeogenesis is regulated by hormones in response to which physiological states?

A
  • Starvation/fasting
  • Prolonged exercise
  • Stress
24
Q

Illustrate how the hormonal regulation of gluconeogenesis is dependent on the enzymes PEPCK and Fructose 1,6-bisphosphatase

A
25
Q

Outline the time course of glucose utilisation

A
26
Q

How and when are lipids stored?

A
  • TAG is storaged when lipid intake is in excess of requirements
  • TAGs are hydrophobic and therefore stored in an anhydrous form in adipose tissue
27
Q

The storage & mobilisation of TAGs is under hormonal control.

When are TAGs utilised?

A
  • Prolonged exercise
  • Stress
  • Starvation
  • Pregnancy
28
Q

TAGs are stored in adipocytes in adipose tissue.

Describe the structure and contents of these cells

A
29
Q

Provide a brief overview of dietary triacylglycerol metabolism

A
30
Q

In six steps, outline the mechanism of lipogenesis

A

⇒ Glucose → pyruvate in cytoplasm (glycolysis)

⇒ Pyruvate enters mitochondria and forms acetyl-CoA & OAA

⇒ Acetyl-CoA and OAA condense to form citrate

⇒ Citrate enters cytoplasm and cleaved back Acetyl-CoA & OAA

Acetyl-CoA carboxylase produces malonyl-CoA from Acetyl-CoA

Fatty acid synthase complex builds fatty acids by sequential addition of 2C by malonyl-CoA

31
Q

What is the key regulatory enzyme in lipogenesis?

A

Acetyl-CoA carboxylase

32
Q

Describe the hormonal regulation of Acetyl CoA carboxylase

A
  • Insulin (covalent de-phosphorylation), & citrate (allosteric) increase activity
  • Glucagon/adrenaline (covalent phosphorylation) & AMP (allosteric) decrease activity
33
Q

Compare and contrast fatty acid synthesis and β oxidation

A
34
Q

Fat mobilisation occurs in the adipose tissue and is under hormonal regulation by lipase.

Illustrate the mechanism of lipolysis

A