Peptides - covalent bond links AA together. This is called a peptide bond - between the COOH of AA1 and the NH3 of AA2

di- = 2 AA linked together

peptide = usually 3 - 8 AA long

poly- = > 8 AA long

Function: dependent on AA composition

Disulfide Bond: important for maintaining protein structure

Terminal Ends

Glycine = Amino terminal AA

Cysteine = Carboxyl terminal AA

Structure

1^o = the AA sequence of a protein

2^o = involves bonding with disulfide bonds

3^o = 3-D shape - caused by:

1. Hydrophobic AA (Neutral & Aromatic AA) go to inside

2. Hydrophilic AA (Acid/Base) go to outside

¶ Protein function dependant on 3-D structure, 3-D structure dependent on AA sequence, Þ AA determines function

 Denature = irreversible unfolding of a protein

¯ 3-D structure Þ ¯ activity

Methods:

1. Heat

2. Chemical

3. ¯ pH (HCl)

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!!