DSA: Heme, Iron and Bilirubin Metabolism

5 questions from this DSA will appear on the exam <3

What are the two major components of hemoglobin

1. Heme → approximately 3% of the molecule

2. Globin proteins

Hemoglobin has an approximate molecular weight of 64 kDa

What is the basic structure of a heme group

A porphyrin ring containing one iron atom chelated in the center by 4 nitrogen atoms

Where does oxygen reversibly attach to hemoglobin

at the iron-containing heme group

What globin chains make up normal adult hemoglobin A1

2 alpha + 2 beta

Hoe are the globin chains arranged in hemoglobin

The two pairs of globin chains are twisted together so that the heme groups are exposed on the outside of the molecule

How many heme groups are present in one hemoglobin molecule

4 heme groups

there is one heme attached to each of the four globin chains

How many oxygen molecules can one hemoglobin molecule carry at max

4 O2 molecules → each heme group can carry one O2

What is the relationship between heme groups and oxygen-carrying capacity

1 heme → 1 O2

therefore: 4 hemes → maximum of 4 O2 molecules

Where foes hemoglobin synthesis occur

in immature red blood cells in the bone marrow

what three things are necessary for normal hemoglobin synthesis

1. adequate iron

2. normal heme synthesis

3. normal globin synthesis

How is iron transported to the bone marrow for hemoglobin synthesis

in plasma bound to transferrin

Where is heme synthesized

mitochondria

Where are globin chains synthesized

cytoplasmic ribosomes

Where do heme and globin finally joint

heme leaves the mitochondria and joints the globin chains in the: cytoplasm

Give the compartmental organization of hemoglobin synthesis

Heme → mitochondria

Globin → cytoplasmic ribosomes

then:

heme + globin → cytoplasm → hemoglobin

What oxidation state of iron must hemoglobin contain to bind oxygen

Fe 2+ - ferrous iron

What happens when F2 2+ in hemoglobin becomes oxidized to Fe 3+

it forms Methemoglobin

methemoglobin is incapable of binding oxygen

What is methemoglobin

Hemoglobin containing Fe3+ rather then functional Fe2+

Fe3+ cannot bind O2

Which form of iron should you associate with functional hemoglobin

Fe2+ = functional O2 binding

Fe TWO → binds O TWO

Which form of iron should you associate with methemoglobin

Fe3+ → methemoglobin → cannot bind O2

What drugs are specifically listed as potential causes of Fe2+ oxidation to Fe3+

Nitrites

Sulfonamides

What RBC enzyme protects against methemoglobin accumulation

methemoglobin reductase

What does methemoglobin reductase do

converts methemoglobin back to functional hemoglobin

a drug oxidizes hemoglobin iron from Fe2+ to Fe3+. What happens to oxygen binding

oxygen binding decreases because methemoglobin is formed

What are the two major types of dietary iron

1. Heme iron

2. Nonheme iron

What is the source of heme iron

• myoglobin

• hemoglobin

Where are heme and nonheme iron absorbed

Duodenum

However, the two forms are absorbed by different mechanisms

How is heme iron absorbed into duodenal epithelial cells

binding or endocytosis

what enzyme acts on heme inside the enterocyte

heme oxygenase

What does heme oxygenase release from heme in the enterocyte

Free Fe3+

CO

Biliverdin

What happens to biliverdin after heme iron is processed

biliverdin → bilirubin

What happens to Fe3+ from heme within the enterocyte

it is converted:

Fe3+ → Fe2+

What happens to heme-derived iron after is has been converted to Fe2+

It is handled exactly like nonheme iron

In which oxidative states can dietary nonheme iron exist

Fe3+ - ferric

Fe2+ - ferrous

Which form of nonheme iron is more readily absorbed by GI mucosa

Fe2+ - ferrous iron

Where is dietary nonheme iron absorption restricted

duodenal mucosa

what happens to dietary Fe3+ before it can be absorbed

it must be reduced: Fe3+ → Fe 2+

What enzyme reduces dietary Fe3+ to Fe2+ at the apical membrane

Ferric reductase → Dcytb

What transporter carries Fe2+ into the enterocytes

DMT1

What does DMT1 cotransport with Fe2+

H+

the lecture describes DMT1 as an Fe2+/H+ cotransporter

What intracellular protein does Fe2+ bind

Mobilferrin

What happens to Fe2+ after it exits the enterocyte

it is converted back to Fe3+ and binds transferrin for transport

What is the major plasma transport protein for iron

transferrin

in what oxidation state does iron bind transferrin

Fe3+

Where is circulating iron primarily deposited for storage

liver

reticuloendothelial system (RES)

What protein binds iron to form ferritin

Apoferritin

iron + apoferritin → ferritin

What is ferritin’s major role

iron storage

Why is apoferritin called the “iron buffer system”

it can

• store excess circulating iron

• release iron when circulating levels fall

this helps maintain relatively constant serum iron levels

Transferrin vs ferritin: what is the easiest distinction

transferrin = transport

ferritin = storage

what is the primary negative regulator of iron absorption

hepcidin

What gene encodes hepcidin

HAMP

Where is hepcidin produced

hepatocytes

released as circulating peptide

what protein does hepcidin bind

ferroportin

what happens when hepcidin binds ferroportin

ferroportin is internalized and degraded

What is the overall effect of hepcidin on dietary iron absorption

decreased iron absorption

because hepcidin reduces functional ferroportin

What happens when hepcidin expression is lost or decreased

severe iron overload

What 3 cell types are emphasized as controlling iron homeostasis

1. enterocytes

2. macrophages

3. hepatocytes

DMT1 vs ferroportin - what direction does each move iron

DMT1

→ brings Fe2+ INTO the enterocyte

Ferroportin

→ allows iron to leave/export the cell

High hepcidin → high or low ferroportin

LOW ferroportin

because hepcidin causes its internalization/ degradation

High hepcidin ultimately causes high or low iron absorption

LOW iron absorption

Low hepcidin ultimately favors iron retention or iron overload

iron overload

handout specifically states that loss/decreased hepcidin expression can cause severe iron overload

Trace dietary nonheme Fe³+ from the intestinal lumen to plasma transferrin

Fe³+ → Dcytb → Fe²+ → DMT1 → enterocyte → export → Fe³+ → transferrin

What is the key oxidation-state sequence during nonheme iron absorption

Fe³+ → Fe²+ → Fe³+

Fe is reduced for intestinal uptake, then oxidized again after export for transferrin transport

What is bilirubin

A breakdown product of hemoglobin liberated from dead RBCs by the reticuloendothelial system

What system initially processes dead/aged RBCs

Reticuloendothelial system (RES)

Which cells degrade hemoglobin from RBCs

macrophages

What does macrophage degradation initially split hemoglobin into

Heme-iron complex + globin chain

What happens to the heme moiety during bilirubin production

Heme → biliverdin + Fe

What happens to the iron liberated during heme breakdown

reabsorbed and recycled for new RBC formation

What is biliverdin converted into

unconjugated bilirubin

Is unconjugated bilirubin water-soluble

NO - it is water- insoluble

What protein transports unconjugated bilirubin in blood

Albumin

Why does unconjugated bilirubin require albumin for transport

because unconjugated bilirubin is water-insoluble

How does unconjugated bilirubin enter hepatocytes

1. passive diffusion

2. receptor-mediated endocytosis

Where does unconjugated bilirubin travel after entering the hepatocyte

smooth endoplasmic reticulum (SER)

Where does bilirubin conjugation occur

smooth ER of the hepatocyte

What enzyme conjugates bilirubin

Uridine 5’ - diphosphate glucouronyl transferase - UDPGT

What does UDPGT add to bilirubin

glucuronide

primarily two glucuronide molecules are added

what major bilirubin molecule results from GDPGT activity

bilirubin diglucuronide

Is conjugated bilirubin water-soluble

yes

What is the critical biochemical purpose of conjugating bilirubin

convert water-insoluble bilirubin into water-soluble conjugated bilirubin so it can be excreted

Conjugated vs unconjugated bilirubin - what is the key solubility distinction

unconjugated = water INSOLUBLE

conjugated = water SOLUBLE

Where does conjugated bilirubin go after formation in the hepatocyte

it travels to the opposite side of the hepatocyte and enters:

bile canaliculi

is secretion of conjugated bilirubin into bile active or passive

active secretion

where does conjugated bilirubin travel after secretion into bile

intestinal tract

what happens to bilirubin after entering the intestinal tract

intestinal bacteria degrade it into

• urobilinogen

• urobilin

What bilirubin-derived pigment gives stool its color

sterobilin

what happens to the majority of urobilinogen

reabsorbed by the gut and re-excreted by the liver

what happens to a small amount of urobilinogen

it is excreted in the urine

what happens to urinary urobilinogen in hyperbilirubinemia

increased urinary urobilinogen

Why can conjugated bilirubin be excreted in urin

it is water-soluble → can be filtered by the glomerulus

increased urinary bilirubin indicates an increase in which serum bilirubin fraction

conjugated bilirubin

why isnt unconjugated bilirubin readily filtered into urine

water-insoluble

transported tightly bound to albumin

what is jaundice in this context

a clinical manifestation of bilirubin accumulation in serum

What happens to bilirubin excretion in hepatocellular or biliary obstructive disease

bilirubin may not be adequately excreted, leading to serum accumulation → jaundice

What bilirubin fraction is elevated in physiologic jaundice to the newborn

unconjugated bilirubin

What enzyme activity is inadequate in physiologic neonatal jaundice

UDPGT activity

Why does inadequate UDPGT cause unconjugated hyperbilirubinemia

bilirubin cannot be efficiently converted from unconjugated form to conjugated bilirubin

What treatment does this lecture emphasize for physiologic neonatal jaundice

phototherapy with a “bili light”

What does phototherapy do to unconjugated bilirubin

light converts it from the trans form → cis form