Lecture 2- Globular Proteins

Question 1

Globular vs Fibrous Proteins

Answer 1

Globular: -protein domains are folded into spherical shape
-generally water soluble
-ex. transport proteins, enzymes, antibodies
Fibrous: -Form protein filaments
-generally water insoluble
-Ex. collagen, elastin, fibrillin

Question 2

Myoglobin

Answer 2

• Stores/carries O2 in muscle
• Muscle destruction can lead to excretion of myoglobin in urine (and discoloration)
• Primary: AA sequence
• Secondary: 8 a-helices
• Tertiary: 8 a-helices form 3D spher shape w binding pocket for heme

Question 3

Heme

Answer 3

• Contains chelated Fe2+ ion in porphyrin ring
• Fe2+ non-covalently interacts w histidine of protein and an O2 molecule
• Fe3+ cannot bind O2

Question 4

Hemoglobin (Hb)

Answer 4

• O2 transporter in RBC
• Quaternary: Formed from 4 subunits (2 a and 2 B) that have similar structure to myoglobin
• 4 heme groups = can bind to 4 O2
• O2 binding is cooperative (binding one O2 enhances binding of other)

Question 5

O2 binding in Myoglobin and Hemoglobin

Answer 5

• O2 saturation curve of myoglobin is hyperbolic (no cooperativity)
• O2 saturation curve of hemoglobin is sigmoid (cooperativity)
• In muscle (low O2 pressure), O2 has stronger affinity to myoglobin than hemoglobin, important bc hemoglobin must release O2 so myoglobin can receive it

Question 6

Regulation of O2 binding of Hemoglobin

Answer 6

Allosteric Regulator: binds to hemogobin and changes O2 binding affinity

• O2: increases affinity
• H+: decreases affinity
• CO2: decreases affinity
• 2,3-biphospho-glycerate:decreases affinity

Question 7

2,3-biphospho-glycerate and O2 affinity

Answer 7

• 2,3-biphospho-glycerate is glycolysis intermediate abundantly present in RBCs
• Binds to pocket in Hb B chains and stabilizes deoxy (taut) form of Hb
• Shifts O2 dissociation curve to right, (lowers affinity)
• In hypoxemia, 2,3-biphospho-glycerate increase to facilitate O2 unloading in tissue

Question 8

pH and O2 affinity (Bohr effect)

Answer 8

• H+ bind better to and stabilize deoxy form of Hb, which shifts curve to right/lowers affinity
• High [H+]/ low pH= lower affinity
• Metabollically active tissues produce more H+ -> capillaries have lower pH-> easier to release O2 from Hb

Question 9

CO2 and O2 affinity

Answer 9

• Dual role
  1. High [CO2] -> High [H+]/ low pH (bc most of CO2 -> bicarb and H+) -and low pH lowers affinity
  2. Direct binding of CO2 to Hb stabilizes deoxy form -deoxy form= lower affinity
• CO2 doesnt bind heme, forms carbamte w terminal AA group of protein subunits

Question 10

O2/CO2 transport via Hemoglobin

Answer 10

In Lungs: High pO2 favors O2 affinity and will override any mechanism that favors O2 unloading -CO2 unloaded from Hb and O2 loadd
In Tissue: Low pO2, high H+, and high CO2 conditions -O2 unloaded and CO2 is loaded to Hb

Question 11

CO poisoning

Answer 11

• CO binds 220 times more tightly to heme iron than O2
• CO binding reduces O2 content of hemoglobin
• binding of one CO to heme group causes other heme groups to bind O2 w higher affinity (curve shifts left), Hb cannot deliver O2 efficiently to tissues

Question 12

Fetal Hemoglobin Changes

Answer 12

a chains: -Zeta and a chains initially -> convert to all a chains before birth
B chains: Epsilon and Gamma mostly, some B chains
-Some gamma and delta exist after birth

Question 13

Hemoglobin Distribution in Adults

Answer 13

HbA-a2B2-90%
HbF-a2G2-<2%
Hba2-a2D2-2-5%
HbA1c-a2B2-glucose-3-9%

Question 14

Hemoglobin F (fetal hemoglobin)

Answer 14

• Higher O2 affinity than HbA (bc has lower affinity to 2,3-BPG)
• Higher O2 affinity in HbF needed for effcient O2 unloading from mother’s Hb
• some individuals have hereditary persistance of HbF as adults, can be beneficial in sickle cell disease or B-thalassemia

Question 15

Hemoglobin A1c

Answer 15

• Glycated hemoglobin by glucose, doesnt require enzyme
• Amount of HbA1c is proportional to BGL
• HbA1c used to monitor glucose levels in diabetic pts and check compliance w meds, RBCs live for ~120 days

Question 16

Common Hb B chain mutations

Answer 16

-Everyone has 2 Hb B chain genes inherited from parents
-both genes have to be mutated to have disease
-mutations change overall charge of B chain
Normal: Glu6 has negative charge
Sickle Cell: point mutation causes Glu6->Val, neutral charge
Hemoglobin C disease: point mutation causes Glu6->Lys, positive charge

Question 17

Identification of Hb mutations based on charge differences

Answer 17

Gel electrophoresis can show mutations due to charge differences in AA 
Normal Hb (HbA) will travel farthest bc has negative charge 
Negative Charge Density: HbC (Hemoglobin C Bchain) < HbS(Sickle Cell)

Question 18

Sickle Cell Disease

Answer 18

Deoxy HbS polymerizes RBCs and deforms them to sickle shape

• Inflexible RBCs block capillaries -> insufficient O2 to tissues and severe pain
• Defored RBCs get destroyed much faster than regular RBCs (20 days vs 120 days), causes hemolytic anemia

Question 19

Methemoglobinemia

Answer 19

-Hb Fe2+ converted to Met-Hb Fe3+(Methemogloulin, cant bind O2)
-Caused by oxidative drugs or nitrates, or hereitary mutation
Symptoms: chocolate colored blood, cyanosis, SOB, headache, seizure

Question 20

a Thalassemias

Answer 20

• Decreased amount or absence of a chain (depends on genes)
• G (fetal) and B (adult) chain synthesis is unaffected
• Not enough chains, excess G and B chains form homotetramers (Hb Bart’s and HbH) which have high O2 affinity and cant release O2 to tissue

Question 21

B Thalassemias

Answer 21

• Deceased amount or absence of B chain (depends on genes)
• a chain synthesis is unaffected
• increased HbF and HbA2 levels (% increase and more G chain production)
• Excess chains precipitate in erythyroid precurors, causes insufficient RBV (no reticulocyte release)