Protein Metabolism

Question 1

Protein metabolism in the liver

Answer 1

• breakdown of amino acids
• formation of urea for removal of ammonia from body fluids
• formation of virtually all plasma proteins
• manipulation of amino acids and synthesis of other “stuff” from amino acids

Question 2

What is protein used for?

Answer 2

• energy
• gluconeogenesis/ketogenesis
• enzymes
• transport proteins
• structural support
• muscles

Question 3

Amino acids

Answer 3

Building blocks of proteins

• all have an acidic group (-COOH) and a nitrogen atom attached (NH2)
• approx 20 AA in the body
• arranged into chains via peptide linkages to form proteins
• average protein size is 400 AA

Question 4

Are amino acids weak or strong?

Answer 4

All are weak acids

• protein digestion in GIT takes 2-3 hrs = final breakdown is AA
• once in blood, cells absorb AA within 5-10 min to prevent build up of circulating AA

Question 5

Amino acid entry into cells

Answer 5

AA are too large for diffusion

• requires facilitated diffusion or active transport
• AA that make it to the kidneys are mostly resorbed by proximal convoluted tubule

Question 6

Amino acid fate inside cells

Answer 6

Turnover is rapid

• AA are quickly used up to make proteins inside the cell
• most intracellular proteins can be degraded rapidly to yield AA
• constant cycle of AA –> protein –> AA –> protein
• not much is stored, any excess proteins are used for energy or converted to fat or glycogen

Question 7

What does the body prefer for its main energy sources?

Answer 7

CHO and fats

• spares proteins for other body functions
• after starvation, AA are used primarily for energy

Question 8

Growth hormone and insulin ______ formation of tissue protein

Answer 8

Increase

Question 9

Glucocorticoids

Answer 9

Increase plasma concentrations of AA

• break down extrahepatic proteins to provide AA for energy
• increase protein production in the liver
• oral steroids can cause muscle wasting

Question 10

Testosterone

Answer 10

Increases protein deposition in tissues, especially muscles

Question 11

Thyroxine

Answer 11

Increase rate of protein synthesis, or increase the rate of protein degradation for energy

Question 12

Protein functions

Answer 12

• plasma proteins
• essential and non-essential amino acids
• source of energy

Question 13

Plasma protiens

Answer 13

Albumin: provides oncotic pressure, transporter, involved in healing
Globulins: immunity and enzyme functions
- 50-80% made in the liver
Fibrinogen: polymerizes into fibrin to form blood clots
- mostly made in liver

Question 14

Essential AA

Answer 14

Cannot be synthesized and therefore must be in diet

• arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
• cats also require taurine

Question 15

Non-essential AA

Answer 15

Synthesized by body from keto-acids, which are precursors to the AA

• process known as transamination
• aminotransferase enzymes are derived from vitamin B6, pyridoxine

Question 16

Degradation

Answer 16

Occurs in liver via deamination (removal of amino group via aminotransferase)

• amino group is transferred to another molecule or released in form of ammonia (NH3)
• yields NADH + H + NH3
• yields a keto-acid –> TCA cycle

Question 17

Protein breakdown products are used for

Answer 17

Gluconeogenesis or ketogenesis

Question 18

Urea cycle

Answer 18

Occurs in the liver

• controls ammonia levels in the body
• ammonia (NH3) from deamination –> urea –> renal excretion
• prevent hepatic encephalopathy