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Flashcards in Protein Metabolism Deck (18)
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Protein metabolism in the liver

- 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


What is protein used for?

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


Amino acids

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


Are amino acids weak or strong?

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


Amino acid entry into cells

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


Amino acid fate inside cells

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


What does the body prefer for its main energy sources?

CHO and fats
- spares proteins for other body functions
- after starvation, AA are used primarily for energy


Growth hormone and insulin ______ formation of tissue protein




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



Increases protein deposition in tissues, especially muscles



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


Protein functions

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


Plasma protiens

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


Essential AA

Cannot be synthesized and therefore must be in diet
- arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
- cats also require taurine


Non-essential AA

Synthesized by body from keto-acids, which are precursors to the AA
- process known as transamination
- aminotransferase enzymes are derived from vitamin B6, pyridoxine



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


Protein breakdown products are used for

Gluconeogenesis or ketogenesis


Urea cycle

Occurs in the liver
- controls ammonia levels in the body
- ammonia (NH3) from deamination --> urea --> renal excretion
- prevent hepatic encephalopathy