Regulation

Question 1

Adipose hormones (Adipokines) act as sensors

Describe the role of Adiponectin

Answer 1

• Affects metabolism of faIy acids and carbohydrates
  
  • ↑ rate of FA β-oxidation in muscle
  • ↓ FA synthesis and gluconeogenesis in liver
  • ↑ glucose uptake in muscle
  • ↑ insulin sensitivity
• Effects of adiponectin are indirect as adiponectin acts through AMP dependent kinase (AMPK)
  
  • AMPK is a fuel sensor regulating metabolic activity

Question 2

Absense of adiponectin causes desensitization to what?

Answer 2

Insulin

Poor glucose tolerance; ingestion of dietary carbohydrates results in long-lasting rise in blood glucose, similar to type 2 diabetes.

Question 3

Compare and contrast Leptin and Adiponectin

Answer 3

• Leptin
  
  • Controled at the level of CNS
  • Regulation of food intake and appetite
  • Levels are higher in obesity
  • Leptin resistance prevents anti-obesity effects
• Adiponectin
  
  • Acts through tissue metabolism
    
    • (less so through CNS)
  • Levels are lower in obesity
  • Hypoadiponectinemia + type 2 diabetes.

Question 4

Ghrelin

Answer 4

• Produced when hungry and about to eat a meal
• Peptide hormone produced in cells lining the stomach
• Acts on hypothalamus
• Stimulates appetite via plasma
• Levels peak just before a meal and drops after a meal
• Exceptionally high levels in Prader-Willi syndrome

Question 5

PYY

Answer 5

• Peptide hormone produced by endocrine cells in the lining of the small intestines.
• Acts on hypothalamus to signal satiety
• Appetite suppresant, plasma levels peak after a meal

Question 6

Orexigenic vs Anorexigenic neurons

Answer 6

Leptin acts on anorexigenic cells to release α-MSH, upregulating satiety and metabolism.

Leptin also acts on orexigenic cells to inhibit NPY, which is the “eat more” signal.

Ghrelin stimulates NPY.

PYY inhibits NPY.

Question 7

What are the 2 mechanisms for degrading proteins?

Answer 7

Lysosomal degradation

• Nonselective
  
  • material taken up by endocytosis
  • intracellular constitutents enclosed in vesicles
• Highly selective
  
  • Chaperone-mediated autophagy

Proteasome degradation

• Ubiquitination

Question 8

Chapernone-mediated autophagy (CMA)

Answer 8

Critical during times of starvation in the liver and kidney.

Proteins are selected for degradation based on the KFERQ protein sequence. This sequence is exposed during gradual unfolding and turnover. 30% of liver and kidney proteins have this sequence.

Question 9

What is Lamp2a?

Answer 9

Cytosolic chaperone protein recognizes KFERQ proteins and targets them to LAMP-2A at surface of lysosomes.

• LAMP-2A monomers aggregate to form multimeric complexes
• Protein is moved into the lysosome through the LAMP-2A complex (with assistance from lysosomal chaperone)
• LAMP-2A levels control CMA activity:
  
  • Upregulation of Lamp-2A increases CMA Downregulation of Lamp-2A decreases CMA

Question 10

Proteasome degradation

Answer 10

• Housekeeping
  
  • Eliminates misfolded or damaged proteins
• Regulatory
  
  • rapid system of degradation for proteins involved in regulatory function, like during the cell cycle
• A protein tagged with a chain of at least 4 ubiquitin molecules binds the 19S cap

Question 11

The 1st step in the catabolism of most AAs in the liver

Answer 11

Question 12

2^nd step in the catabolism of most AAs in the liver

Answer 12

Oxidative deamination catalyzed by glutamate dehydrogenase

Question 13

What is transdeamination?

Answer 13

Steps 1 and 2 of Amino Acid catabolism

Question 14

Two nitrogen groups enter the urea cycle.

How do they do so?

Answer 14

1 enters as carbamoyl phosphate

1 enters as aspartate

*Liver mitochondria

Question 15

Enzyme that forms Carbamoyl Phosphate

Answer 15

Carbamoyl phosphate synthetase:

couples HCO₃ with NH₄ to form carbamoyl phosphate

Question 16

Where does the Urea Cycle take place?

Answer 16

• Mitochondrial
  
  • Glutamate dehydrogenase
  • Aspartate aminotransferase
  • Carbamoyl phosphate synthetase
  • 1 st step of Urea cycle
• Cytosolic
  
  • Aminotransferases

Question 17

What happens to NH₄ from AA breakdown in extrahepatic tissue where there is no urea cycle?

Answer 17

Extrahepatic tissue transports NH₄ through the bloodstream in the form of Glutamine. Once in the liver, NH₄⁺ is removed from glutamine and is processed in the urea cycle.

• Glutamine Synthetase extrahepatically.
• followed by Glutaminase in the liver.

For muscle, Glucose-alanine cycle.

Question 18

How is the Urea Cycle regulated?

Answer 18

• Long-term regulation
  
  • upregulation of urea cycle enzymes
    
    • prolonged starvation
    • high protein diets
• Short-term regulation
  
  • Allosteric activation of carbamoyl phsophate synthetase by N-acetylglutamate (a glutamate derivative whose concentration is proportional to glutamate).

Question 19

Glucose-alanine cylce

Answer 19

• Amino groups are transferred to alanine.
• Alanine transports amino groups to liver for the urea cycle.
• Carbon skeleton is used for gluconeogenesis.
• Glucose is essentially forming pyruvate, which picks up the nitrogen via transamination.

Question 20

Ketogenic vs. Glucogenic amino acids

Answer 20

Glucogenic AA’s can all result in net synthesis of glucose:

• Pyruvate
• α-ketoglutarate
• Succinyl-CoA
• Fumarate
• Oxaloacetate

Ketogenic AA’s → acetyl CoA ⇒ ketone bodies or Fatty Acids.

Question 21

Formyl tratrahydrofolate

Answer 21

derived from Folic Acid, important donor of single carbons in biological systems.

Question 22

De novo synthesis of purines

Answer 22

Production of IMP

Ribose Pentose Phosphate Pathway makes PRPP, which is the precursor of IMP (Inosine monophosphate).

PRPP makes purines and pyramidines, and some amino acids.

First committed step to purine synthesis is glutamine-PRPP aminotransferase, to make Phosphoribosylamine.

Question 23

Amino acid synthesis

carbon-skeleton precursors

Answer 23

Glycolysis, CAC, and PPP

Question 24

In amino acid synthesis,

Where are amino groups derived from?

Answer 24

Glutamate and Glutamine

Question 25

Regulation of Purine Synthesis

Answer 25

AMP (adenine monophosphate) and GMP (guanine monophosphate) don't really get stockpiled, but IMP does. **When IMP builds up, it blocks its own formation.** AMP and GMP independently regulate their own production. AMP also regulates **PRPP synthetase** (the very first step). **PRPP is a feedforward allosteric activator**. A lot of PRPP accelerates flux through the pathway, whereas IMP causes feedback inhibition.

Question 26

Regulation of _pyrimidine synthesis_ in animals

Answer 26

*Carbamoyl Phosphate Synthetase II* is in the cytosol and has nothing to do with the Urea Cycle. Carbamoyl phosphate + Aspartate makes N-Carbamoylaspartate, which eventually reacts with PRPP, ending up as **uradine triphosphate** and then **cytidine triphosphate**.

Question 27

Purine Synthesis

Answer 27

Question 28

If PRPP builds up, what happens?

Answer 28

Generation of more Purines and more Pyrimidines

Question 29

How are deoxynucleotides made?

Answer 29

Ribonucleotide Reductase Disregulation in the levels of individual deoxynucleosides can increase the chance of mutations; a lack can be lethal.

Question 30

Ribonucleotide reductase enzyme mechanism

Answer 30

The _ribonucleotide reductase_ enzyme utilizes a free radical, which is transfered from outside to inside the active side, where the free raidical is used for enzyme catalysis.

Question 31

Synthesis of Thymine

Answer 31

Reduction depends on tetrahydrofolate, suscpetible to anti-folate drugs.

Question 32

Purine catabolism

Answer 32

Relies on **Xanthine Oxidase** to convert purines to Uric Acid. AMP → IMP → Xanthine → Uric Acid. Guanine → Xanthine → Uric Acid.

Question 33

Gout

Answer 33

Impaired uric acid excretion or overproduction of uric acid, which is non-soluble and builds up in joints. Purine rich foods should be avoided.

Question 34

Thymidine monophosphate (TMP or Thymidylate) snythesis

Answer 34

**_Thymidylate synthetase_** is an enzyme that catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). The cycle depends on **folate,** a target for chemotherapy.

Question 35

Purine salvage pathways

Answer 35

Recycling the nitrogen base instead of making it from scratch Enzymes are HGPRT and APRT If something goes wrong with the enzymes, PRPP accumulates, activating the production of IMP, whose degradation causes an increase in Uric Acid and therefore gout.

Question 36

Pyrimidine Catabolism

Answer 36

Source of NH₄⁺ and Urea Source of malonyl-CoA (FA synthesis) Source of Succinyl-CoA (CAC intermediate)

Question 37

How are TAGs broken down?

Answer 37

Triacylglycerols (TAGs) are broken down by lipases Lipases are located in dfferent locations to serve distinct functions. * Intestinal lumen (pancreatic lipase) * Absorption of FA from diet into intestine * Capillary walls (lipoprotein lipase) * Absorption from chylomicrons and VLDLs into tissue * Intracellular (hormone-sensitive lipase) * Break down cellular fat stores in adipose tissue

Question 38

What is Bile?

Answer 38

derivatives of cholesterol that act as detergents to solubilize dietary fats

Question 39

Characteristics of Lipoproteins

Answer 39

* TAG + cholesterol, cholesteryl esters, and specific proteins. * Phospholipid head group on outside. * Apolipoproteins coat the surface. * Apolipoproteins determine the fate of the particle. * high affinity ligands for lipoprotein receptors

Question 40

*Exogenous* lipoprotein metabolism

Answer 40

* Dietary TAGs absorbed in the intestines are packaged with dietary cholesterol into Chylomicrons * transport of dietary fat to tissue through lymphatic system * Once in tissue, lipoprotein lipases (LPL) in the capillaries hydrolyze TAGs into FAs and glycerol * Muscle FAs are oxidized for energy * Adipose FAs are re-esterified as TAGs for storage * Remnants of Chylomicrons (depleted of most TAGs but containing cholesterol and apolipoproteins) are taken up in the liver

Question 41

Different Apolipoproteins

Answer 41

* ApoB48 * Intestine (Chylomicrons) * ApoB100 * Liver (VLDLs) * ApoE * IDL but not LDL * ApoCII * acquired from HDL

Question 42

Roles of Apolipoproteins

Answer 42

* Chylomicrons that drain into the blood interact with HDL * HDL gives Chylomicrons ApoCII * ApoCII interacts with Lipoprotein Lipase on capillaries * Once Chylomicrons become small and dense enough, they exchange ApoCII and gain ApoE from HDL * As VLDL particles are metabolized, they become more dense, becoming IDL particles * 50% of IDLs exchange ApoCII for ApoE * ApoE is high affinity substrate for internalization * So, these IDLs are taken up by the liver * 50% of IDL become LDL, losing ApoCII * LDLs stay in the plasma * LDLs bind to LDL-R (receptors)

Question 43

Lipoproteins in order by size

Answer 43

Question 44

Chylomicrons vs VLDL

Answer 44

Chylomicrons and VLDLs are coated with ApoB lipoproteins. ApoB is not exchangeable.

Question 45

HDL

Answer 45

* HDL serves as a reservoir for the exchangeable of apolipoproteins. * HDL particles extract excess cholesterol from cell-surface membranes as a cholesterol scavanger * Transports the cholesterol to the liver where it is converted to bile salt

Question 46

What is HMG-CoA reductase?

Answer 46

HMG-CoA reductase is the key regulatory step in cholesterol biosynthesis.

Question 47

Process of LDL endocytosis

Answer 47

* Tissue receptors specifically bind ApoB-100 * LDL is internalized in vesicles which fuse with lysosomes * Lysosomes hydrolyze cholesteryl esters * Cholesterol and FA are released into cytosol * LDL receptor is recycled to surface * Free cholesterol goes to ER

Question 48

LDL absorption and subsequent regulation

Answer 48

↑ [cholesterol] in Endoplasmic Reticulum ↓ HMG-CoA reductase ↓ the rate of cholesterol biosynthesis ↓ synthesis of LDL receptors ↑ ACAT

Question 49

What is ACAT?

Answer 49

ACAT is a liver enzyme that converts cholesterol to cholesteryl esters. It replaces the hydrophilic hydroxyl group with a hydrophobic lipid, so that it can be packaged up inside a VLDL.

Question 50

Serum Albumin

Answer 50

Free Fatty Acids are hydrophobic; they're mobilized through the blood on Serum Albumin.

Question 51

Role of Hormone sensitive lipase in TAG metabolism

Answer 51

Question 52

How and where are fatty acids are activated and transported for β oxidation?

Answer 52

**Activation of FA** occurs on the cytosolic side of the outer mitochondrial membrane. Activation requires ATP, and the product is **Fatty Acyl-CoA**. Fatty Acyl CoA runs through a Carnitine shuttle into the matrix. _β oxidation occurs in the mitochondria._

Question 53

How is fatty acyl CoA transported into mitochondria?

Answer 53

Carnitine Shuttle 1. Fatiy acyl group of activated FA is transferred to carnitine 2. Acyl carnitine enters the mito matrix through a carnitine transporter 3. Carnitine acyltransferase in the mito matrix transfers a fatty acyl group to a CoA 4. Carnitine returns to the cytosol through the transporter

Question 54

β oxidation

Answer 54

Oxidation of acyl-CoA (fatty acid aattached to a Coenzyme A) * Each round of β oxidation yields * 1 FADH2 * 1 NADH * 1 acetyl-CoA for oxidation by citric acid cycle, which yields: * 3 NADH * 1 FADH2 * 1 GTP

Question 55

β oxidation in odd-chain fatty acids

Answer 55

Odd-chain fatty acid β oxidation produces a bunch of Acetyl-CoAs plus one additional Propionyl-CoA, which is a CAC intermediate and feeds directly into it, increasing the amount of intermediates.

Question 56

Regulation of fatty acid oxidation

Answer 56

* Regulation by Malonyl CoA of carnitine shuttle * Regulation by energy state of the cell * High levels of NADH/NAD+ inhibit β-oxidation enzymes * Hormonal regulation of Hormone sensitive lipase * ↑ Glucagon and epinephrine * ↑ cAMP * ↑ cAMP-dependent phosphorylation of **perilipin** and hormone sensitive lipase * ↑ activity of hormone sensitive lipase * ↑ free fatty acids * ↑ β-oxidaEon * Transcriptional regulation of FA oxidation enzymes * PPAR family of nuclear receptors with fatty acid ligands * Act as transcriptional factors * Triggered with increased energy demand

Question 57

Perilipin in TAG mobilization

Answer 57

The protein coating that protects a lipid droplet is called **perilipin**, and when perilipin is phosphorylated, activated hormone sensitive lipase can gain access to the lipid and break it down to individual fatty acids.

Question 58

Medium Chain acyl-Coa dehydrogenase deficiency

Answer 58

Inability to break down some medium-sized fatty acids. Symptoms: hypoketonic hypoglycemia (low ketone bodies, low blood sugar).

Question 59

FA synthesis vs. degradation

Answer 59

Degradation is an **oxidative** process (which needs NAD+), whereas **synthesis** is a reductive process (utilizing NADPH).

Question 60

Starting fatty acid synthesis

Answer 60

Acetyl-CoA in mitochondria is transported to the cytoplasm for FA synthesis by the Tricarboxylate transport system, which involves the citrate transporter. *Mitochondrial acetyl CoA cannot exit to the cytosol for FA biosynthesis.* 1. Oxaloacetate + Acetyl-CoA makes Citrate. 2. Citrate can be transported out of the mitochondrial matrix and into the cytosol. 3. There, the citrate is converted back to Oxaloacetate and Acetyl-CoA. 4. Oxaloacetate can return to the matrix.

Question 61

What is the regulatory step for fatty acid synthesis?

Answer 61

The 1st step of fatty acid synthesis is rate-controlling. It's catalyzed by acetyl-CoA carboxylase (ACC). Bicarbonate Carboxyl group + acetyl CoA → Malonyl-CoA

Question 62

Palmitate formation by **Fatty Acid Synthase**

Answer 62

* FAS is “charged” with **acetyl** (from acetyl CoA) and **malonyl** (from malonyl CoA) * CoAs are released * The fatty acyl chain grows by 2 carbon units donated by malonyl-CoA * Loss of CO 2 at each step * Reductions at each step (2 NADPH) * [NADPH is from the PPP]

Question 63

Energy requirements of FA synthesis

Answer 63

Acetyl-CoA Used in the iniEal loading of acetyl CoA (1 per palmitate) Used in the production of every Malonyl-CoA (7 per palmitate) ATP Used in the production of Malonyl-CoA (7 per palmitate) NADPH Reducing power is required by 2 active sites of FAS (14 per palmitate) Overal reaction for 1 molecule of palmitate: **8** *Acetyl-CoA* + **7** *ATP* + **14** *NADPH* + **14** *H⁺* → *_palmitate_* + **8** *CoA* + **7** *ADP* + **7** *P_i* + **14** *NADP* + **6** *H₂O* Additionally, ATP is used for movement of acetyl-CoA into the cytosol

Question 64

Synthesis of unsaturated FAs

Answer 64

elongation and desaturation Mammals cannot introduce double-bonds beyond the 9th carbon.

Question 65

Regulation of FA synthesis and degradation through acetyl-CoA carboxylase (ACC)

Answer 65

Allosteric regulation * upregulation by Citrate (signalling high acetyl CoA and ATP) * citrate also inhibits PFK-1 and downregulates glycolysis * downregulation by Palmitoyl CoA (signal of excess of FA) Covalent modification * phosphorylation/dephosphorylation triggered by hormones Acetyl-CoA carboxylase also inhibits FA oxidation through Malonyl-CoA * Malonyl-CoA inhibits carnitine acyltransferase (the carnitine shuttle) * When FA synthesis is underway, there's no transport of fatty acyl groups into the mitochondria for β oxidation.

Question 66

Overview of fatty acid metabolism

Answer 66

Question 67

Insulin and glucagon's effect on Acetyl-CoA carboxylase

Answer 67

Glucagon - phosphorylation Insulin - dephosphorilation

Question 68

Making triacylglycerides (TGAs)

Answer 68

Question 69

Cholesterol Biosynthesis

Answer 69

2 Acetyl-CoA ⇨ HMG-CoA ⇨ Mevalonate **HMG-CoA Reductase -** a membrane protein of the smooth ER - is the key regulatory step of cholesterol production. Subsequent steps as follows.

Question 70

Regulation of Cholesterol Synthesis

Answer 70

**_HMG-CoA reductase:_** rate determining step of cholesterol synthesis **_Sterol regulatory element-binding proteins (SREBPs)_** **SCAP** *(SREBP cleavage-activating protein)* is a **sterol sensor**. * High* [cholesterol] ➠ transcriptionally active SREBP bound to SCAP. * Low* [cholesterol] ➠ change in SCAP frees SREBP's active domain.

Question 71

HMG-CoA reductase regulation overview

Answer 71

Question 72

Hypercholesterolemia and Statins

Answer 72

Statins ≋ competitive inhibitors of HMG-CoA reductase Reduction of endogenous cholesterol synthesis (inhibition of HMG-CoA reductase) Increase in LDL receptors ⬇︎ levels of blood LDL-cholesterol

Question 73

Summary of FA metabolism

Answer 73

Question 74

There are three different ketone bodies, what are they? When are they formed?

Answer 74

* Acetone * exhaled * Acetoacetate * transported to extrahepatic tissues * β-hydroxybutyrate * transported to extrahepatic tissues Ketone bodies are formed during starvation when FAs are being heavily used - increased lipolysis + inadequate glucose uptake. When Acetyl-CoA can’t enter CAC because of low OAA (depleted for gluconeogenesis), Acetyl-CoA accumulates and forms ketone bodies.

Question 75

Leptin

Answer 75

Leptin is released by adipose in response to rising adipose mass. Acts on hypothalamus of the brain to suppress appetite * Promotes ⬇︎ in fuel intake * Signals brain to release α-MSH - neuronal signal to eat less * Inhibits the release of NPY - neuronal signal to eat more * Promotes ⬆︎ in expenditure of energy * ⬆︎ blood pressure and heart rate

Question 76

Glucose 6-phosphate, Pyruvate, and Acetyl-CoA Key junctions in metabolism

Answer 76

Question 77

Primary oxidative fuel in the liver

Answer 77

FAs are oxidative fuel for the liver FA synthesis with ample fuel supply Synthesized FAs are released in VLDLs During starvation, FAs are converted to ketone bodies.

Question 78

dietary amino acids and the liver

Answer 78

The liver absorbs majority of dietary amino acids Protein synthesis or amino acid catabolism depending on fuel needs Urea cycle to remove nitrogen

Question 79

Metabolic cycles in the muscle

Answer 79

Regenerating NAD⁺ Disposing of NH₃

Question 80

Metabolic activity of cardiac muscle

Answer 80

The heart can use lactate produced by active muscle directly Functions highly aerobically (high density of mitochondria) Very limited glycogen reserves Fatty Acids are the main fuel source Also glucose, ketone bodies, and lactate

Question 81

What organs can do gluconeogenesis?

Answer 81

Liver and Kidney

Question 82

Fuel metabolism by different tissues

Answer 82

Question 83

Hormonal effects on Fuel Metabolism

Answer 83

Question 84

AMP-dependent kinase

Answer 84

Activated by high AMP:ATP ratio (not net concentrations) Phosphorylates enzymes: * turns off ATP consuming anabolic pathways * turns on ATP generating catabolic pathways Example targets: FA synthase, acetyl-CoA carboxylase, hormone sensitive lipase, HMG-CoA reductase, glycogen synthase, GLUT4

Question 85

AMPK target enzymes

Answer 85

* FA synthase * acetyl-CoA carboxylase * hormone sensitive lipase * HMG-CoA reductase * glycogen synthase

Question 86

What are PPARs?

Answer 86

_Peroxisome proliferator-activated receptors_ ## Footnote A nuclear receptor involved in transcriptional regulation of fat metabolism: essentially, lipid sensors. They bind hydrophobic ligands, like FAs. PPARs bind to another nuclear receptor RXR prior to binding DNA and then act as transcription factors.

Question 87

What and where are the three types of PPARs?

Answer 87

* **PPARα:** *(alpha)* * **​**Starvation response * **PPARγ:** *(gamma)* * *​*Master regulator of adipogenesis * instructs a cell to become an adipocyte * **PPARδ:** *(delta)* * Senses dietary lipid.

Question 88

NPY

Answer 88

neuronal signal to eat more

Question 89

α-MSH

Answer 89

neuronal signal to eat less

Question 90

What is parabiosis?

Answer 90

Parabiosis experiments to study a continuous, low-volume blood exchange between living animals.

Question 91

Where are Ketone Bodies made?

Answer 91

In the liver.

Question 92

Sources of glucose during starvation

Answer 92

Question 93

Pharmacological Treatments for Type 2 diabetes

Answer 93