Protein Digestion in the Monogastric

Goal: break protein down to AA for absorption

Stomach

HCl = parietal cells = ¯ pH Þ denature protein; activates pepsinogen to pepsin.

Pepsinogen = chief cells - proteolytic enzyme - endopeptidase (cleaves peptide bond inside AA sequence).

Duodenum

Pancreatic enzymes - Trypsin & Chymotrypsin (Endo) and Carboxypeptidase (Exo)

Duodenal enzymes (mucosal cells) - enterokinase amino peptidase, dipeptidase

Once Endo- enzymes cleave to poly-peptides, they "stop" and Exo starts.

¶ Enterkinase activates Trypsin

Absorption

Carrier system! There are at least 5. These systems are specific for certain AA or types of AA (ie. Basic, Acidic) and are used to get AA across the mucosal cell membrane.

If Basic is a carrier system, then both ARG and LYS compete for absorption, so if a diet is high in ARG and LYS is adequate, LYS may not be absorbed at a rate which is required, and although the diet shows enough LYS, the body is not absorbing adequate amounts.

Protein Synthesis

Two Processes

Transcription

DNA (nucleus) ® mRNA (cytoplasm)

Translation (Ribosomes in the Endoplasmic reticulum)

mRNA ® protein AA sequence

 mRNA is divided into 3 base pairs (codon) that code for a specific AA.

tRNA carries specific AA to Ribosomes for protein synthesis & recognizes specific codons on mRNA.

MRNA + tRNA-AA1 + tRNA-AA2 + tRNA-AA3 = AA1-AA2-AA3

4 ATP needed to add one AA to chain (work)

 Turnover = constant protein turnover and it is energy demanding (4 ATP) This is part of the maintenance energy requirement.

AA availability - Protein synthesis stops if a specific AA is not available when the ribosome needs it.

¶ All AA have to be available for protein synthesis to occur, it will not skip a missing AA. Maximum protein synthesis occurs when all Essential AA are at or above requirement

Limiting AA concept

Deficiency of on essential AA stops the entire process!!