B2W1

Week 1

Fructosuria

Deficiency in fructokinase

asymptomatic: eliminate fructose in urine

Elevated fructose in blood and urine

Mono/Disaccharide

Hereditary Fructose Intolerance

Deficiency in Aldolase B

Hypoglycemia, vomiting, and liver damage

No sorbitol consumption

Mono/Disaccharide

Galactokinase Deficiency

Less symptomatic

Can lead to increased galactositol (cataract formation)

Mono/Disaccharide

Galactosuria

Uridydyltransferase deficiency

Phosphate trapping so more severe; galactose and galactositol buildup

mental retardation, cataracts, liver

Mono/Disaccharide

Fructose of Glucose Metabolism Speed

Fructose is quicker because it skips the rate limiting step (PFK-1)

Mono/Disaccharide

Polyol Pathway

Relevant to why those with hereditary fructose intolerance cannot consume sorbitol

1. Aldose Reductase: galactose to galactitol (cataracts) and glucose to sorbitol (cataracts and osmotic pressure)

2. Sorbitol Dehydrogenase: Sorbitol to fructose (liver problems)

Mono/Disaccharide

Pentose Phosphate Pathway (OX)

Glucose-6P:G6PDH:6-phosphogluconate:ribulose-5P

Pentose Phosphate Pathway

G6PDH

Committed Step

Regulated by NADPH/NADP+

Pentose Phosphate Pathway

G6PDH Deficiency

Hemolytic anemia, decreased NADPH causes increased ROS leading to the formation of heinz bodies and bite cells

High H2O2, GSSG, NADP+

Aggravated by Fava beans and antimalarials drugs

Pentose Phosphate Pathway

Pentose Phosphate Pathway (NON-OX)

1. Transketolase (2C utilizing thiamine B1) and Transaldolase (3C)

2. Generate ribose groups for nucleotide synthesis and glycolytic intermediates

Pentose Phosphate Pathway

Why is the PPP integral to RBCs?

1. NADPH keeps glutathione reduced (GSH) to maintain membrane and eliminate ROS

2. Around 10% of glucose to PPP in RBCs

Pentose Phosphate Pathway

Why is PPP integral to cancer cells? 

1. NADPH for ribose towards DNA/RNA, oxidative defense, and FA synthesis

2. Glycolytic intermediates to maintain ATP and amino acids

Pentose Phosphate Pathway

What coenzyme is required by transketolase?

Vitamin B1 (thiamine)

Pentose Phosphate Pathway

Fructokinase vs. Hexokinase?

Fuctokinase is liver specific and generates F1P

Hexokinase is present in most other cells and generates F6P

Mono/Disaccharide

What happens to excess carbohydrates and proteins?

Fatty Acid Synthesis!

How does excess glucose oxidation (glucose to pyruvate) impact the cell?

Pyruvate high in the mitochondria leads to acetyl coA buildup — convert to citrate to leave mitochondria — citrate into OAA and AcoA for FA synthesis and cholesterol synthesis

Lipid Synthesis

What is the coenzyme of AcoA carboxylase and what step does AcoA carboxylase act on? What activates it? Inhibits it? 

ALL carboxylases use biotin (B7) and it converts acetyl-coA to malonyl-coA using ATP and CO2

Activated by citrate (polymerize ACC promoters) and Insulin (dephosphorylates)

Inhibited by palmitoyl-coA and glucagon/epinephrine (phosphorylates)

Lipid Synthesis

Fatty Acid Synthesis General Steps

1. Initiation: acetyl-coA starter

2. Elongation: add 1 malonyl-CoA (2 carbons) in the ER using transferases

3. Condensation

4. Reduction

5. Dehydration

6. Reduction

7. Repeat 6 times to get 16 FA group

8. Hydrolysis of ester to release palmitate

9. Carbon contribution: 2 carbons per elongation and 1 lost as CO2 during addition

10. Final Product: palmitate

Need NADPH for reduction steps; cytoplasm of liver and adipose tissue cells in the fed state

Lipid Synthesis

(Lipid Synthesis) Where does desaturation occur? What enzyme? What positions can it go up to?

ER, fatty acty-coA desaturase, position 9

Lipid Synthesis

What inhibits CPT-1? 

malonyl-coA

Lipid Synthesis/Breakdown

Where does fatty acid elongation occur?

ER

Lipid Synthesis

What is a key difference between TAG synthesis in the liver compared to adipocytes?

Liver can synthesize TAG using glycerol while adipocytes cannot.

Lipid Synthesis

Why can’t adipocytes use glycerol to synthesize TAGs? Why is this important? 

It lacks glycerokinase and can only make glycerol 3P from glycolytic pathway, ensuring FAs are stored only when fed

Lipid Synthesis

Where is the main site of TAG synthesis?

Liver

Lipid Synthesis

Generalized TAG synthesis mechanism?

G3P + 2 acyl CoA — PA — DAG — TAG

Lipid Synthesis

What is the makeup of lipoproteins? 

Surface made of apoproteins on a phsophlipid MONOlayer with the interior filled with cholesterol, TAGs, and cholesteryl esters

Lipoproteins

Describe Chylomicrons

Sacks of DIETARY TGs/fat, originate in the small intestine (enterocytes), transport dietary fat to periphery

Lipoproteins

How does an immature chylomicron become a mature chylomicron?

HDL adds Apo-C2 and Apo-E to the surface

Lipoproteins

What apoprotein activates LPL? 

ApoC2

Lipoproteins

What is the consequence of lipoprotein lipase?

Sucks out the TAGs to make FFA (adipocytes) and glycerol (liver) which yields a chylomicron remnant

Lipoproteins

What is the consequence of Apo-E?

hepatocytes recognize and take up the remnants in the liver

Lipoproteins

Describe VLDLs

made in the liver with Apo-B100 (nascent) and are also sacks of fat (TAGs)

Lipoproteins

How do nascent VLDLs become mature?

HDL adds Apo-C2 and Apo-E

Lipoproteins

What is the consequence of LPL activity on VLDLs and beyond?

They become IDLs (less TAGs) but still have all the apoproteins until LPL sucks out more TAGs to create LDLs which are taken up by the periphery cells that contain LDLRs for Apo-B100

LDLs are high in cholesterol

Lipoproteins

What happens when intracellular cholesterol is high? 

LDLR synthesis is decreased, which increases plasma LDL

Lipoproteins

What is PCSK9?

Degrades the LDLR receptor

Lipoproteins

Which lipoproteins are present during fasting periods?

Liver derived lipoproteins: VLDL, IDL, and LDLs

Lipoproteins

Which lipoproteins are present during periods of nonfasting? 

ALL: Chylomicrons, VLDLs, LDLs, HDLs, IDLs

Lipoproteins

Why is HDL “good cholesterol”? What is involved in the process?

It can deliver cholesterol back to the liver from the periphery to remove excess (LCAT matures HDL)

CETP mediates transfer of cholesterol to other lipoproteins

Lipoproteins

How are lipoproteins cleared from circulation?

Receptor mediated endocytosis

Lipoproteins

Sensitivity

+Test/+Disease

Ability to correctly designate a subject with the disease as positive

EBM

Specificity

-Test/-Disease

Tests ability to correctly designate a subject without the disease as negative

EBM

Positive Predictive Value

True+/All positive

Among all patients w/ positive test result, what proportion will actually have the disease

EBM

Negative Predictive Value

True-/All negative

Among all patients with a negative test result, what proportion will not have a disease

EBM

Prevalance

TP + FN/(Total population)

Fraction of subjects in the population under study that have the disease

EBM

What is the relationship between positive predictive value and prevalence?

PPV will increase with increasing prevalence (Direct)

EBM

What is the relationship between negative predictive value and prevalence?

NPV will decrease with increasing prevalence (indirect)

EBM

What are the characteristic symptoms of urea cycle disorders?

High ammonia, vomiting, seizures, lethargy, respiratory alkalosis

Urea Cycle

Order of substrates in the Urea Cycle

Ornithine, Citrulline, Citrulline (cytoplasm), Aspartate, Arginosuccinate, Fumarate, Arginine, Urea, Ornithine

Urea Cycle

Where are the nitrogen entry points in the urea cycle?

NH4+ and Aspartate

Urea Cycle

Describe ALT and AST

ALT uses alanine and aKG to make glutamate (LIVER**)

AST uses glutamate and OAA to form aspartate

Urea Cycle

What activates carbamoyl phosphate synthetase I (CPS-I)? 

N-acetylglutamate (NAG) is an alosteric activator formed from glutamate and acetyl-coA

Urea Cycle

What activates NAG synthesis? What inhibits it?

Arginine in the urea cycle activates NAG synthase. It is inhibited by organic acid buildup (MMA, PA)

Urea Cycle

What is NAG made of? Where do these constituents come from?

Acetyl coA and glutamate

AcoA from glucose when fed, FA oxidation when fasting

Glutamate produced by glutamine which increases NH4+

Urea Cycle

Ornithine Transcarbamoylase Deficiency

X-linked Recessive

Vomiting, stunted growth, drowsy, enlarged liver

High glutamine and uracil, high ammonia

HIGH orotate, LOW citrulline, LOW arginine (all in urine)

The excess carbamoyl phosphate is shunted to pyrimidine biosynthesis

Urea Cycle Disorder

Ornithine Transcarbamoylase Deficiency

Urea Cycle Disorder

What do these patients have? 

CPS-I, Argininosuccinate synthetase, arginase, ornithine transcarbamoylase, argininosuccinate lyase

Urea Cycle Disorder

Carbamoyl Phosphate Synthetase-I Deficiency (CPS-I)

HIGH ammonia, LOW arginine, LOW citrulline

Urea Cycle Disorder

Arginosuccinate Synthetase Deficiency

HIGH citrulline, LOW arginine

Urea Cycle Disorder

Arginase Deficiency

HIGH arginine, moderate ammonia

Urea Cycle Disorder

Ornithine transcarbamoylase Deficiency

HIGH ammonia, LOW citrulline, LOW arginine, HIGH orotate

Urea Cycle Disorder

Argininosuccinate lyase deficiency

HIGH citrulline, LOW arginine, HIGH ammonia

Urea Cycle Disorder

What can hyperammonemia cause?

Disruption of glutamate/glutamine homeostasis: cerebral edema, disrupted signaling, seizures, coma

NH4 will pull AKG to Glutamate then Glutamine, leading to brain swelling

Urea Cycle

How can MCAD cause increased ammonia?

Acetyl-CoA is needed to make NAG, which activates urea cycle

Which step of the urea cycle is activated by NAG? 

CPS-I

Urea Cycle

How does pyruvate carboxylase defects increase ammonia?

If PC isn’t active, then there is no OAA production, meaning asparate is not being produced by AST - aspartate is a substrate in the urea cycle and without aspartate it will slow down

Urea Cycle

What is cystinuria? 

disorder of the proximal tubes reabsorption of filtered cystine and dibasic amino acids (COAL: cystine, ornithine, arginine, lysine), causes cystine crystals in the urine

Essential and Nonessential AA

Know These

Conditional Amino Acids to Know

Arginine (urea cycle)

Tyrosine (phenylalanine can’t be metabolized)

aspartate (PC not functional)

Methionine

Maple Syrup Urine Disorder

Elevated Valine, Leucine, Isoleucine (LIV)

Defective branched-chain alpha-ketoacid dehydrogenase

The Dehydrogenase uses the FIVE enzymes (TPP, lipoic, CoA, FAD, NAD)

Physical and mental retardation

Homocystinuria

Left eye lens dislocated, intellectual disability, myocardial infarction, creates a huge amount of acid, elevated homocysteine

Cystathionine Beta Synthase Deficiency (uses B6)

Methionine elevation

How can homocysteine be elevated? 

1. cystathionine synthase

2. methionine synthase

3. B12/Folate (as related to 2)

Methylmalonyl CoA mutase Deficiency

Methylmalonic acidemia - vomiting, hypotonia, metabolic acidosis, elevated ammonia

Restrit VOMIT: valine, odd chained FAs, methionine, isoleucine, and threonine

Odd chained fatty acid produce propionyl-coA

Phenylketonuria (PKU) Disorder

Musty mouse urine odor, tremors, twitching movements

Restrict Phenylalanine, supplement tyrosine

Albinism, IQ reduction

Defect in phenylalanine hydroxylase (BH4 coenzyme, regenerated by dihydrobiopterin reductase)

Inable to make tyrosine, hence no melanin and catecholamines

Type II tyrosinemia

Tyrosine aminotransferase (TAT) uses B6 as a coenzyme

tyrosine accumulation (no 4-hydroxyphenylpyruvate)

Corneal ulcers and hyperkeratosis

Alkaptonuria

Elevated homogentisate, black urine, black spots in sclera and ear, arthritis when older

Not Life Threatening

Defect in homogentisate 1,2-dioxygenase

Tyrosinemia Type 1

Elevated tyrosine and succinylacetone, elevated bilirubin and liver enzymes

Cabbage like odor in urine (from succinylacetone), jaundice, bleeding disorders, kidneys

Defect in fumarylacetoacetate hydrolase